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Construction Surveys

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Chapter 10 Construction Surveys

Table of Contents

10.1 General ............................................................................ 1 10.1.1 Survey Methods ................................................ 2 10.1.2 Planning ............................................................. 2

10.2 Control ............................................................................. 2 10.3 Staking Books ................................................................. 3 10.4 Staking Items ................................................................... 3

10.4.1 Slope Stakes ........................................................ 3 10.4.2 Bluetops .............................................................. 13 10.4.3 Pavement Lines ................................................. 15 10.4.4 Major Structures ................................................ 17 10.4.5 Minor Structures and Pipes ............................. 20 10.4.6 Signs .................................................................... 23

10.5 Volume Determination ................................................. 26 10.5.1 Topsoil, Overburden, and Borrow Areas…. 26 10.5.2 Measurement Methods ..................................... 26 10.5.2.1 Cross-sectioning .............................................. 26 10.5.2.2 Digital Terrain Modeling................................ 26

10.6 Standards .......................................................................... 27 10.7 Real Time Kinematic (RTK) GPS Requirements...... 28 10.7.1 Introduction ......................................................... 28 10.7.2 Method ................................................................. 28 10.7.3 Equipment Requirements .................................. 28 10.7.4 Survey Design ...................................................... 29

10.7.4.1 Local Datum ..................................................... 29 10.7.4.2 Map Projection ................................................. 29 10.7.4.3 Geoid Model ..................................................... 30 10.7.4.4 Site Localization ............................................... 30

10.7.5 Field Procedures ........................................................... 32 10.7.5.1 Third Order RTK Survey Requirements...... 35 10.7.5.2 General Order RTK Survey Requirements.. 36

10.7.6 Office Procedures .......................................................... 37 10.7.7 Deliverables ..................................................................... 37

10.1 General

Construction staking establishes basic line and grade control, delineates working areas, and serves as a base for verification of location and quantities of completed work. Normally, Photogrammetry & Surveys provides the preliminary survey and essential control to accomplish establishment of lines and grades. Construction personnel are responsible for construction staking — see [Operating Policy 18-16]. In some cases, special contract provisions may limit the amount of control that will be set by WYDOT.


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10.1.1 Survey Methods

The increased use of computers and specialized design equipment has brought about many advances in the entire highway development process. These advances mandate the need for the preliminary survey to be based on a three-dimensional coordinate system.

This chapter provides various methods for performing construction surveys. The method used should be the one that is most appropriate for the project.

10.1.2 Planning

Planning and maximizing the use of personnel and facilities are essential in providing the most efficient survey. Planning — such as layout of horizontal and vertical control — should have taken place during the preliminary survey. Do not hesitate to establish additional control for the construction staking phase. If it is apparent that any control monument will be destroyed during construction, establish additional control. Prior to setting additional monumentation, a utility locate is required. If monumentation is not in the right-of-way, permission to survey will need to be obtained.

Review the plans and all design output information for items that may aid in planning for the survey, such as horizontal and vertical control. Prior to preparing staking notes or data collector files, review and verify all computer listings to assure that the output is based on the same design as shown in the construction plans. Inspect the condition of and re-flag all control points prior to use. Check any points that may be questionable.

10.2 Control

Uniform horizontal and vertical control is an important part of all engineering projects. The project control is seldom accomplished in one survey, but is a culmination of several control surveys beginning with the preliminary control survey.

Establish additional control so it is 3D, not just horizontal or vertical. Use the same datum for all control on a project. Control from one project may not be suitable as an extension on another project due to a different DAF1.

The requirements, including survey accuracy for control surveys and extendable control, are set forth in the WYDOT Survey Manual. Point names shall adhere to the conventions set forth in the WYDOT Survey Manual. Any control points required to be moved to a new location shall be renamed.

1[Definition: Datum Adjustment Factor] A combined scale and elevation factor used to raise the coordinate values of a point from the state plane datum to that of the modified or surface plane.


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10.3 Staking Books

The survey crew chief shall maintain notes and data in a professional manner, ensuring easy readability and interpretation by any other individual in the future. Use an appropriate source document for the intended need of the data.

Include the following information in all staking books:

• Title page • Index • Page headings • Page numbers

Those pages which show a specific part of the survey — such as centerline, structure, grade, fence, or sign staking — shall include the following information:

• Date(s) • Name and function of all personnel on the crew • Equipment used • Weather conditions

Fill in all required information on each page of preprinted staking books, such as grading and bluetop books. Record the date on the first and last page of notes for each day.

The staking books document the location, dimensions, and materials incorporated in the project. Clear and concise sketches are an aid for those who read the notes and compute quantities. The sketches do not have to be drawn to scale, but they should show actual field measurements.

10.4 Staking Items

10.4.1 Slope Stakes

General. It is intended that slope stakes are surveyed prior to the project award. It may be necessary to survey them at the initial stages of the construction project. They should be set at the positions specified by the slope stake report generated by the computer design software or additional locations as required by field conditions. Slope stakes are typically set on 100 ft (30 m) stations, however in urban areas or in places where tighter control is necessary, 50 ft (20 m) or less spacing may be required.

Marking Stakes. It is extremely important that the information shown on the construction stake is concise, readable, and clearly understood by the contractor. Since the contractor may have projects in other parts of the State, consistency among the various survey crews is an aid to the contractor's understanding of the information being conveyed. The required information should be neatly written with a permanent magic marker or stake pencil, and the stake painted and set. Slope stakes should be tilted away from the survey centerline at approximately 30° from perpendicular, with the printed face directed toward centerline. The station identification should be printed on the back side. If a large


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quantity of information is required, two slope stakes should be used. Place a half lath or full lath beside the stake for better visibility.

Staking. Slope stakes and recross data shall be measured, set, and recorded to the accuracy specified in Table 10.1, “Accuracy Table for Construction Points”. The slope staking process may be accomplished by either using pre-printed slope stake books, or by using electronic information stored in the survey equipment which matches the design information.

Slope Staking Procedure Using Slope Stake Book

This procedure uses the printed slope stake books generated by the computer design software.

• Measure the actual ground elevation at the station and offset shown in the slope stake book.

• If the difference between the actual ground elevation and the design elevation (as shown in the slope stake book) is less than 0.1 ft, the slope stake is a “catch”.

a. Place the stake at the offset shown in the slope stake book.

b. The cut/fill shown on the stake is as shown in the slope stake book.

c. Use the design slope when writing the stake.

d. In the slope stake book, document that the stake was set with no changes.

• If the difference between the actual ground elevation and the design elevation (as shown in the slope stake book) is greater than or equal to 0.1 ft , the slope stake is not a “catch”.

a. Place the slope stake. If the slope stake is on a constant cut/fill line, do not change the offset of the slope stake — place it at the offset shown in the slope stake book. If the slope stake is not on a constant cut/fill line, move the slope stake to the point where the design slope intersects the actual ground. Take care when changing a slope stake to ensure that it matches the adjacent slopes, ditches, pipes, and transition sections, and that it provides for proper drainage.

b. The cut/fill shown on the stake is computed by using the actual ground elevation.

c. If the slope stake is on a constant cut/fill line, compute the new slope to be written on the stake. Otherwise, use the design slope.

d. If the difference measured earlier between the actual ground elevation and the design elevation at the design slope stake point was greater than 0.5 ft, then measure the actual ground elevation of the terrain points on each side of the slope stake.

• If the difference between the actual ground elevation and the design elevation for the terrain point is less than or equal to 0.5 ft for both terrain points, no recross of the station is necessary.

• If the difference between the actual ground elevation and the design elevation for the terrain point is greater than 0.5 ft for either terrain point, a recross of the


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station is necessary. Recross the station from the new slope stake location to the PGL2.

e. In the slope stake book, document the changes made to the slope stake information on the slope stake, and also any recross information.

Slope Staking Procedure Using Electronic Data

This procedure uses all-electronic data — a road model, an original ground terrain model, and a list of design slope stake locations. The design slope stake locations may be either 3D points or stored in the road model.

• Measure the actual ground elevation at the design slope stake location.

• If the difference between the actual ground elevation and the elevation of the design slope stake point is less than 0.1 ft, the slope stake is a “catch”.

a. Place the stake at the design slope stake location.

b. The cut/fill shown on the stake is computed by using the road model.

c. Use the design slope when writing the stake.

d. The as-staked information shall be recorded in the data collector.

• If the difference between the actual ground elevation and the design elevation (as shown in the slope stake book) is greater than or equal to 0.1 ft , the slope stake is not a “catch”.

a. Place the slope stake. If the slope stake is on a constant cut/fill line, do not change the offset of the slope stake — place it at the offset shown in the slope stake book. If the slope stake is not on a constant cut/fill line, move the slope stake to the point where the design slope intersects the actual ground. Take care when changing a slope stake to ensure that it matches the adjacent slopes, ditches, pipes, and transition sections, and that it provides for proper drainage.

b. The cut/fill shown on the stake is computed by using the road model and the actual ground elevation.

c. If the slope stake is on a constant cut/fill line, compute the new slope to be written on the stake. Otherwise, use the design slope.

d. If the difference measured earlier between the actual ground elevation and the design elevation at the design slope stake point was greater than 0.5 ft, then starting at the as-staked slope stake location, measure the ground at each grade break or every 25 ft (10 m), whichever is less. Continue moving to the next break point and measuring, until the PGL2 for the station is reached, or the difference between the measured ground elevation and the elevation derived from the original ground terrain model is less than or equal to 0.5 ft.

2[Definition: Profile Grade Line] On a two-lane highway, almost always the centerline. On a divided highway, generally the inside edge of travel-way.


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e. The as-staked and any recross information shall be recorded in the data collector.

Fill Stakes. Fill stakes are placed at the fill catch point (toe of slope) and show the fill height(s) and distance(s) to some referenced point of the roadway (such as centerline, shoulder, hinge point, etc.). See Figure 10.1, “Slope Stake for Fill and Flat Bottom Ditch Section” for an example of a fill stake. The computer design software determines the slope of fill slopes in accordance with the fill height chart. If the fill height is borderline between two different slopes, this may result in cases where the slope varies back and forth between stations — e.g. between 2:1 and 4:1. In most cases, the designer will correct the situation by providing a consistent slope through a fill area. The surveyor should review such situations and make adjustments where necessary to provide the most pleasing appearance. Care must be taken in transition sections and pipe sections, to insure proper drainage of the ditch. See Figure 10.2, “Geopak Slopestake Report (Fill and Flat Bottom Ditch Section)” for an example of a computer-generated slope staking book for a fill section.

Cut Stakes. Cut slope stakes are marked to the “Bottom of Ditch” or “Back of Ditch”, which is marked on the stake as BD. In normal cut sections, this is the lowest elevation and the start of the

bottom (or toe) of the backslope. The cut stake is marked with the cut depth to ditch bottom, the distance from centerline, the ditch elevation below shoulder and distance from centerline, and width of ditch indicated by elevation below shoulder and distance from centerline. See Figure 10.3, “Slope Stake for V–Ditch Section” for an example of a cut stake. Care must be taken in transition sections and pipe sections, to insure proper drainage of the ditch. See Figure 10.4, “Geopak Slopestake Report (V–Ditch Section)” for an example of a computer-generated slope staking book for a cut section.

Daylight Stakes. Daylight stakes are used in areas where a traditional cut or fill stake will not meet the criteria of the design slopes. They may be used to remove a small knoll, fill a small depression, provide drainage from a cut section, or in areas of transition from cut to fill to provide for better drainage. Their location usually does not match the line of slope stakes which are adjacent to them. See Figure 10.5, “Slope Stake for Daylight Section” for an example of a daylight stake. Care must be taken in transition sections and pipe sections, to insure proper drainage of the ditch. See Figure 10.6, “Geopak Slopestake Report (Daylight Section)” for an example of a computer-generated slope staking book for a daylight section.

Documentation (Using Slope Stake Book). All as-staked slope stake information and recross data collected shall be recorded in the slope stake book. Any changes made to the design information shall be plotted on the cross-sections.

Documentation (Using Electronic Data). The electronic job file that contains the collected data shall be retained with the project files. A report which details all the as-staked information shall be generated and put in the project files. All as-staked slope stake information, as well as any recross data, shall be plotted on the cross-sections.


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Figure 10.1 Slope Stake for Fill and Flat Bottom Ditch Section


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Figure 10.2 Geopak Slopestake Report (Fill and Flat Bottom Ditch Section)


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Figure 10.3 Slope Stake for V–Ditch Section


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Figure 10.4 Geopak Slopestake Report (V–Ditch Section)


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Figure 10.5 Slope Stake for Daylight Section


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Figure 10.6 Geopak Slopestake Report (Daylight Section)


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10.4.2 Bluetops

General. Grade stakes (bluetops) are generally 1″ × 1″ stakes driven to grade along specific roadway template lines from which the contractor performs the fine grading. The bluetop line is also used by the contractor to define template limits. Surfacing layers above or below the subgrade may also be bluetopped under certain circumstances. The “bluetop” description simply describes the stake as being driven to some specified grade. The top may be marked in any color that makes it easy to see in contrast to the surface being graded. “Feathers” are typically attached to the stake to aid in locating the bluetop. Bluetops can be spaced at any distance required for the type of finishing equipment used by the contractor. However, they are seldom spaced at less than 25 ft (10 m) or more than 100 ft (30 m) apart.

Staking. Bluetops shall be set to the accuracy specified in Table 10.1, “Accuracy Table for Construction Points”. The bluetopping process may be accomplished by either using pre-printed bluetop books, or by using electronic information stored in the survey equipment which matches the design information.

Documentation (Using Bluetop Book). Bluetop staking books can be produced by the computer design software for any stations for which a cross-section exists in the design software. Stake locations can be generated for any or all lines used in producing the cross-section, for example: centerline, traveled way, shoulder, or curb & gutter. If a specific line of bluetops is desired, other than those normally used, contact the designer as far in advance as possible to have those line(s) included in the earthwork run. Check the bluetop books against the final design before staking. See Figure 10.7, “Geopak Bluetop Report” for an example of a computer-generated bluetop book.


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Figure 10.7 Geopak Bluetop Report


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10.4.3 Pavement Lines

General. Pavement lines are stakes that give the contractor line and grade. Examples of pavement lines are Curb & Gutter stakes and hublines for paving. Pavement lines are generally set using a hub, with a grade stake set behind the hub. They are typically set on 50 ft (20 m) stations on tangents, and 25 ft (10 m) stations on curves (depending on the curvature). The spacing may need to be reduced for areas where drainage is critical. Curb & Gutter stakes are typically set on 25 ft (10 m) stations, at an offset determined by the contractor. During the construction process the contractor prepares the embankment and the base to grade. After this is complete, the pavement lines are set. After the line(s) are set, the contractor will run a stringline or wireline between the stakes.

Staking. All “critical” points must be staked, including angle points, PC, PT, SC, CS, PCC, PRC, station equations, and any other point which controls the line. When staking pavement lines for a roadway, check the distance between the lines to ensure that the width of the roadway and all offset distances are included. The accuracy standards for pavement lines are found in Table 10.1, “Accuracy Table for Construction Points”. The stakes should be written with the grades to the pavement and the offset distance on the front, and the station on the back side. Paint the stakes orange. See Figure 10.8, “Concrete Paving Grade Stake” for an example of pavement line staking, and Figure 10.9, “Curb & Gutter Grade Stake” for an example of Curb & Gutter staking.

Figure 10.8 Concrete Paving Grade Stake


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Figure 10.9 Curb & Gutter Grade Stake


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10.4.4 Major Structures

General. Stakes set to control the location and elevation of structures serve several purposes:

• They ensure that the structure will be constructed to the lines and grades shown in the plans, and are compatible with the adjacent roadway.

• They provide the contractor with accessible and understandable reference points and working lines from which the contractor can line up or measure without the use of sophisticated surveying equipment.

• The staking notes are the documentation used by WYDOT of quantities involved in the structure, as well as the dates, times, and manner in which the various operations were started and completed.

Documentation. A structure book should be set up for major structures and maintained. Information for setting up staking diagrams and sketches should be obtained from the detail sheets in the plans. Separate pages should be used to show the overall staking system and detail drawings of the various structural components. Do not try to crowd too much information on one page.

Staking. In general, all major working lines for abutments, footings, columns, and centerline should be referenced with two intersecting lines of stakes. At a minimum, the two stakes nearest the component should be on-line with, and at a measured distance from the component. All reference points should be double-guarded and lathed. Each guard stake should be marked with the station, offset, cut or fill, and description as appropriate. All stakes set shall be verified by an independent survey crew. See Figure 10.10, “Reference Stake” for an example of a reference stake. See Figure 10.11, “Staking a Structure” for an example of structure staking.


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Figure 10.10 Reference Stake


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Figure 10.11 Staking a Structure


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10.4.5 Minor Structures and Pipes

General. Minor structures consist of reinforced concrete boxes, multi-plate metal pipes, and cattleguards. Examples of pipes include reinforced concrete pipe (RCP) and corrugated metal pipe (CMP). The same general procedures for staking and documentation of the structures apply as described in Section 10.4.4, “Major Structures”.

Documentation. Minor structures should be put in the Structure book(s). Pipes should be put in the Pipe book(s). Books are generally set up in the office using the plans and revised pipe list. Separate pages should be used for each separate installation.

Reference Points. Reference points shall be a monument of a size and type to provide sufficient stability. See Figure 10.10, “Reference Stake” for an example of a reference stake. See Figure 10.12, “Staking a Box Culvert” for an example of staking a box culvert. See Figure 10.13, “Pipe Stake” for an example of pipe staking. Information on the guard stake for the nearest-to-pipe reference point should include:

• The station and offset distance on the back side • Type and size of pipe • Type of end finish (e.g. flared end) • Length and grade of pipe • Cut or fill to flow line on the back side • Any other information required for a specific installation


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Figure 10.12 Staking a Box Culvert


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Figure 10.13 Pipe Stake


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10.4.6 Signs

Check the “Sign Summary” in the plans for station, location, lateral clearance, etc., and detail sheets for the specific sign and post spacing for centerline of drilled shaft. Also check the Standard Plans for any other pertinent information. A utility locate shall be conducted at each proposed sign location before staking is performed.

Locate the station on the ground and measure the lateral clearance to the centerline of the first post/drilled shaft foundation from the designated measure point as shown on the Standard Plans. Then determine the location of the remaining posts/drilled shafts relative to the first post location as per the post spacing from the plans. Set a hub & tack flush with the ground at the centerline of each post/drilled shaft. See Figure 10.14, “Staking a Sign” for an example of how to stake a sign.

Set offset hubs & tacks flush with the ground ahead or behind the centerline of post/drilled shaft locations; 5–10 ft (2–3 m) works well or at the contractor's request. See Figure 10.15, “Offset Staking a Sign” for an example of offset stakes for a sign.

Determine the elevation at the Edge of Traveled Way and at each hub & tack at the centerline of post/drilled shaft locations and the corresponding hub & tack at the offset locations. Cuts or fills to the top of the drilled shaft shall be computed and recorded in the field book. The elevation of the top of the drilled shaft foundation should be at or slightly higher than the surrounding ground.

Place guard stakes with the station, sign number and post number at the centerline of the post/drilled shaft locations. Also, place guard stakes with the station, sign number, post number, offset, and cut/fill to the top of the drilled shaft at the offset hub location. If necessary, place a lath along each hub & tack for better visibility.

Documentation. Information in the field book will be used for computing the steel and/or wood sign post lengths for fabrication and installation. Record the following information in the field book:

• Sign number and location (station and offset) • All measurements (e.g. level notes) • Elevations and offsets of each each post location • Elevation, offset, and cut/fill of each stake set • Sketch of sign and location


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Figure 10.14 Staking a Sign


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Figure 10.15 Offset Staking a Sign


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10.5 Volume Determination

10.5.1 Topsoil, Overburden, and Borrow Areas

Topsoil and overburden is measured and documented as the volume stored in and placed from topsoil and overburden piles. The volume removed from borrow areas is determined by measuring the ground surface before and after each stage of the removal of material.

If the ground is uniformly sloped underneath the pile location, it is not necessary to measure the ground surface before placing the pile. The ground surface underneath the pile is determined from the shots taken on the perimeter of the pile. When measuring volumes in this manner, the field notes must contain a statement that the ground surface underneath the pile is uniformly sloped.

If the ground is not uniform underneath the pile location, the ground surface must be measured before the pile is put there.

Once the pile has been measured, place crossed lath on top of the pile to show that it has been measured and is not to be disturbed.

Documentation. A separate field book should be used to document topsoil and overburden volumes.

10.5.2 Measurement Methods

10.5.2.1 Cross-Sectioning

Whenever practicable, elevations should be taken from a project benchmark. A temporary benchmark can also be established where it will not be disturbed in case the volumes calculated are later questioned.

Establish a straight baseline which is referenced to the highway centerline, right-of-way line, or an independent base. The baseline should be in close proximity to the pile.

Start with 0+00 at a point on natural ground on the outside edge of the pile. Proceed to the nearest break point on the pile for the next station, and cross section the pile at this location to natural ground on each side of the pile. Measure the ground at each grade break or every 25 ft (10 m), whichever is less. Continue moving to the next break point and cross-sectioning, until the end of the pile is reached. Record the ending station at the end of the pile.

Distances and elevations shall be measured and recorded to the accuracy specified in Table 10.1, “Accuracy Table for Construction Points”.

Documentation. Keep the survey notes in a separate field book with no more than two sections per page. If project control or a temporary benchmark was used as an elevation reference then record this information as well.


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10.5.2.2 Digital Terrain Modeling

This involves building terrain models of the ground surfaces from field measurements, and using computer software to determine the volume contained between the models. This method is more accurate than cross-sectioning, and is significantly faster for larger areas.

Measurements. Project control should always be used as the basis of coordinates whenever possible. Otherwise, establish a minimum of 2 temporary control points as references. Measure the natural ground at the perimeter of the pile or borrow area. Inside the pile or borrow area, measure the ground at each break point, and use breaklines when applicable during data collection to produce a more accurate model. Measurement accuracy shall meet that specified by Table 10.1, “Accuracy Table for Construction Points”.

Processing. Visually check the terrain model that it properly represents the features measured. Check that the boundary of the terrain model does not extend beyond the area measured. Determine the volume between the terrain models by prismoidal method.

Documentation. Keep the survey notes in a separate field book with no more than one area per page. The notes shall comply with the requirements in [Notekeeping, Section 2, Survey Manual]. The hardcopy volume report from processing the terrain models shall be put in the field book, or kept in a separate folder and referenced in the field book. The electronic files containing the data collection measurements and terrain models shall also be saved and archived with the project documentation.

10.6 Standards

Table 10.1 Accuracy Table for Construction Points

Item Horizontal Accuracy (ft)a Vertical Accuracy (ft)a

Supplemental Construction Control

0.08 0.02

Slope Stakes b 0.30 0.10

Earth/Subgrade Bluetop 0.15 0.06

Base Course Bluetop 0.10 0.04

Pavement Hub Line 0.05 0.02

Bridges and Major Structures c 0.05 0.02

Minor Structures and Pipes 0.10 0.05

Structural Steel Sign Posts 0.05 0.05

aAllowable error radius relative to the nearest control point. bAccuracy shown is for actual staked point. cAccuracy shown is the allowable error radius relative to all other points associated with a specific structure, as well as to the nearest control point.


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10.7 Real-Time Kinematic (RTK) GPS Requirements

10.7.1 Introduction

RTK GPS survey equipment and procedures will only yield a certain level of accuracy. Care shall be taken to understand the accuracy of the equipment and the procedures utilized, to insure that the necessary survey tolerances are met. During the early stages of project development it should be discussed and agreed upon as to what information can be obtained using RTK GPS equipment, and what should be obtained using other surveying techniques and/or equipment. Data being collected for the Photogrammetry and Surveys Section will be collected using the procedures and requirements established by the Photogrammetry and Surveys Section. Construction surveys, including Contractor Surveying and field design surveys, may use these RTK GPS requirements.

RTK GPS surveying is an evolving technology. In the future, these requirements may need to be changed or removed, and new requirements may need to be developed. Not all requirements may apply equally or be appropriate for different models of RTK GPS equipment. If it is questionable whether a particular model of RTK GPS equipment will meet these requirements, its use shall be discussed and agreed upon before it is used on a project. Where these requirements are vague or silent on a subject, refer to the manufacturer's recommendations for use of a specific model of RTK GPS equipment.

New procedures not covered or not conforming to these requirements will not be used without prior approval from the engineer.

10.7.2 Method

Real-Time Kinematic (RTK) positioning is performed using two or more GPS receivers — a base (reference) receiver and one or more roving receivers. The base receiver takes measurements from the satellites in view and transmits those measurements, along with the position of the base receiver, in real-time to the roving receiver(s). The roving receiver(s) also take measurements from the satellites in view, and process them in real-time with the measurement data and location from the base receiver. The result is measurement vectors in the WGS84 datum from the base receiver to the rover receiver. Using these measurement vectors, coordinates for the points occupied by the rover can be computed.

10.7.3 Equipment Requirements

Use only compatible RTK GPS equipment. Mixing RTK GPS systems or antennas from different manufacturers may result in degraded performance, and is not allowed unless the software can accommodate the differences in the equipment. The equipment used, including the GPS receiver, antenna, data link, and data collector, shall be suitable for the type of survey being performed as determined from the manufacturer's specifications and recommendations. All equipment shall be checked and adjusted regularly, especially range poles, tribrachs, and tripods. Fixed height tripods are recommended, but not required for base receivers. Rover receiver antennas must be mounted on fixed height poles (not adjustable poles), or on a tripod.


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10.7.4 Survey Design

The local coordinate system must be established before starting a RTK GPS survey. The following elements are part of the local coordinate system, though not all are required:

Local datum A mathematical surface (ellipsoid) that best represents a certain area of the earth's surface.

Map projection A method used to convert local geodetic coordinates (on the ellipsoid) to local grid coordinates.

Geoid model A model of the differences between the NAD83 ellipsoid and mean sea level, used to obtain more accurate elevations from GPS observations.

Site localization (Site calibration) A coordinate transformation used to adjust projected grid coordinates to fit the local control.

10.7.4.1 Local Datum

Horizontal Datum. Use either the WGS84 datum, or another datum that is directly compatible with the current WGS84 datum used by GPS equipment. The NAD83 datum can be considered equivalent to WGS84, and NAD83 geodetic coordinates are acceptable for direct usage. While it is technically possible to use other datums (such as NAD27) with GPS by means of datum transformations, such usage is not allowed without approval from the engineer.

Vertical Datum. The vertical datum used shall be NAVD88. Other datums, including local arbitrary datums, may be used with approval from the engineer.

All control points must have coordinates of the same datum and epoch as the datum and epoch required for the project. The engineer will provide the appropriate datum and control point coordinates.

10.7.4.2 Map Projection

When working in the NAD83 datum, use the appropriate NAD83 state plane zone and projection for the project area. State plane coordinates may be modified by a scale factor (or DAF) to produce surface coordinates. Local arbitrary coordinate systems can be accommodated by generating a Transverse Mercator projection which has an origin that coincides with the origin and location of the arbitrary coordinate system and scaling the projected grid coordinates to the project height. Most modern GPS equipment is capable of calculating these parameters automatically.


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10.7.4.3 Geoid Model

Use of a geoid model is required for surveys covering an area greater than 10 km (6.2 mi) in any direction. For surveys which cover a smaller area, use of a geoid model is recommended, but not required. The geoid model should be based on the latest available national or regional geoid model published by the National Geodetic Survey. Use only one geoid model for a survey project — do not mix different geoid models in the same project.

Exceptions. The geoid model is not required in the following circumstances:

• Only the horizontal coordinates will be used from the RTK GPS survey.

• The existing geoid model is known to be inaccurate or insufficient.

10.7.4.4 Site Localization

A site localization (or site calibration) is required for all RTK GPS surveys, to account for local variations in the mapping projection and the geoid heights. It normally consists of a translation, rotation, and scale factor for the horizontal coordinates, and an inclined plane or constant shift for the vertical coordinates. If the site localization was not already provided, it shall be completed at the beginning of the survey project, and before staking out any points. It cannot be changed once points have been staked out.

Exceptions. Part or all of the site localization is not required in the following circumstances:

• If the survey project uses a local arbitrary coordinate system and there are no existing control points which can be used, then site localization is not required.

• If only the horizontal coordinates will be used from the RTK GPS survey, then site localization is not required for the vertical component. A site localization for the horizontal component is still required.

Control Points. Control points used for the site localization shall match or exceed the survey accuracy required for the project. The control points shall be evenly distributed to the extent of the project area. Observations shall not be made that extend outside of the area enclosed by the control points. When performing a site localization in the vertical component for a project area with significant elevation change, include vertical control points at both the lowest and the highest elevation of the area.

Observations. The observations used to produce a site localization can be obtained from RTK GPS measurements, or from the static or faststatic GPS observations used to establish the project control points. If RTK GPS observations are used, they shall comply with the requirements listed in Section 10.7.5.1, “Third Order RTK Survey Requirements”, with the exception that repeat observations do not need to be made from different reference stations.

Generating the Site Localization

1. Generate coordinates for all control points for which site localization will be performed, from GPS observations performed in the field. If the observations are networked, for example from a static control network, then perform a minimally constrained adjustment in the WGS84 datum, by holding only one control point fixed. Do not use coordinates generated from a fully


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constrained adjustment. An adjustment is not necessary when observations are not networked together — this is the case for most RTK GPS measurements.

2. Compare the grid coordinates derived from the GPS observations in Step 1 to the published grid coordinates of the control points. The site localization parameters are generated by computing a best-fit coordinate transformation between the two sets of coordinates. Scaling shall not be performed, unless absolutely necessary. If applying a scale factor would produce a significantly better fit to the existing control, thoroughly investigate all possible sources of error before using a scale factor in the site calibration. A scale error may indicate that one of the sets of coordinates was not properly projected to ground (or to the coordinate system used for the project). It may also indicate errors in the measurement or adjustment of the control network.

3. Verify that the results of the site localization are acceptable, and meet the requirements listed in Table 1, “Site Localization Requirements”. An unsuccessful site localization may be caused by a number of factors, including:

• Errors in control point coordinates

• Errors in GPS observations

• Point numbering errors

• Disturbed control points

• Incorrect or mismatched coordinate system

• Errors in the control survey

If the results are still unacceptable after correcting any errors, there may be local distortions in the control network that cannot be removed by site localization. Review, and if necessary, change the area used for site localization.

Table 1 Site Localization Requirements

Requirement Specification

Minimum number of horizontal control points a 3

Minimum number of vertical control points a b 4

Maximum distance between vertical control points 2 km (1.2 mi) c

Maximum horizontal residual for each control point 15 mm (0.049 ft)

Maximum vertical residual for each control point 15 mm (0.049 ft)

aHorizontal control points may be physically different points than vertical control points. bNot required for horizontal–only surveys. cMay be increased to 10 km (6.2 mi) when an acceptable geoid model is used.


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10.7.5 Field Procedures

The following procedures shall be followed for all usages of RTK GPS equipment, regardless of the application or level of accuracy required.

1. Base Receiver.

a. Reference. Use only control points that are or will be part of the site localization, as described in Section 10.7.4.4, “Site Localization”. Also, the current WGS84 coordinates for the control point must be known, and accurate to within 10 meters (33 ft). Inaccurate reference station coordinates will degrade the accuracy of the RTK baseline solutions by approximately 1 ppm for every 10 meters of error in the reference station coordinates. WGS84 coordinates for the control points can be determined by one of the following methods:

• Use published WGS84 coordinates, or coordinates derived from a previous control survey. Coordinates from datums that are directly compatible with (equivalent to) WGS84 are acceptable, such as those from NAD83. However, do not intermix coordinates from different datums (including those from WGS84 and NAD83), different adjustments, or from multiple independent surveys.

• Generate an autonomous GPS position using the base receiver. Only one autonomous position may be used in a survey project. If Wide Area Augmentation System (WAAS) corrections are available, they should be used to improve the accuracy of autonomous positioning. The position generated by autonomous GPS is normally accurate to approximately 10 meters, but if Selective Availability is enabled it may be in error by up to 100 meters. WAAS–corrected GPS positioning is accurate to approximately 5 meters.

b. Location. Selection of a good quality location for the base receiver is crucial to obtaining accurate and reliable GPS observations. Locate the base receiver on points which have a clear, unobstructed view to the horizon. When there are obstacles which block the reception of satellite signals by the base receiver, this also limits the availability of satellites to the rover receivers.

Avoid locations close to large flat surfaces that may reflect satellite signals (multipath), such as buildings or signs. Chain-link fences and vehicles are also significant sources of multipath. Multipath can also be reduced by using an antenna with a ground plane. Locations of high electromagnetic interference or strong radio signals should be avoided, as they may cause degradation of the satellite signal quality, and possible loss of tracking. For example, stations located adjacent to high-power radio antennas and high-voltage electrical transmission lines should be avoided. Unsuppressed or noisy ignition systems on vehicles or other equipment may cause interference, if they are close enough to the receiver. Take special care in urban areas, as there may be many possible sources of multipath, electromagnetic interference, and horizon blockage.

The site chosen should preferably be at or higher than the surrounding area, to facilitate radio broadcast to the rover receivers. Use of radio modem repeater(s) will facilitate radio transmission in rugged terrain, and allow placement of the base receiver in places that are otherwise unsuitable for radio broadcast.


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c. Setup. Set the receiver antenna directly over an established control point. A fixed height tripod is recommended but not required. Set the antenna high enough to avoid obstructions, but low enough to minimize centering errors. Tripod legs shall be set firmly and weighted or spiked to prevent movement.

Measure the antenna height of the base receiver(s) at the beginning and end of every occupation in both meters and feet. Measure all heights to the nearest 1 mm (0.01 ft). The measurements shall all agree to within ±3 mm (±0.01 ft). Check the base receiver(s) periodically during occupations of extended duration and at the end of every occupation to ensure the antenna has been plumb and level over the control point.

2. Rover Receiver. The rover antenna shall be located at a height such that there is no obstruction from the operator. Either a tripod and tribrach, or a fixed height pole may be used for the rover antenna. Check the antenna height in both meters and feet at the beginning of every session and whenever changing rod tips. Measure all heights to the nearest 1 mm (0.01 ft). Orient the rover antenna in the same direction as the base receiver antenna when taking measurements, if recommended by the manufacturer.

3. Data Link. Adhere to FCC requirements for radio frequency transmission where applicable. Follow the equipment manufacturer's recommendations for broadcast format, baud rate, and repeater usage. Be aware of latency issues when using radio repeaters, or other data transmission methods such as cellular or satellite links which may add significant delay to the broadcast message.

4. Observations.

a. All observations must be made with a valid, checked initialization. Each initialization used to collect data or stake points shall be checked, and the check must be recorded in the raw data file.

b. A check shot shall be made to a control point, or other verified point, at the beginning and end of each base receiver session. The maximum allowable residuals for check shots are equal to the residuals listed in Table 1, “Site Localization Requirements”.

c. Use the same caution with the rover receiver as taken with the base receiver, to avoid locations of high multipath and electromagnetic interference.

d. Most weather conditions do not affect GPS measurements. However, do not conduct observations during electrical storms. Effects of the troposphere — the lowest level of the atmosphere where weather phenomena occurs — can be minimized by keeping the base receiver close to the same elevation as the rover receiver(s). Some models of RTK GPS equipment can model and correct for tropospheric bias.

e. Avoid periods of high ionospheric activity — this affects the GPS signal delay, and may degrade solution quality and initialization reliability. Solar radiation storms and solar winds affect the ionosphere, and when strong enough may also affect the GPS satellites.

5. Documentation. The survey documentation consists of the data stored in the electronic job file on the data collector, and original notes contained in separate field survey book(s). The following items must be recorded in the survey documentation:

• Location and type or purpose of survey.


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• Personnel involved.

• Atmospheric conditions, including any unusual weather patterns.

• Equipment used for the survey.

• Setup information, to include the identifier of the control points used as reference points for the base receiver(s), and all antenna heights measured for the base and rover receivers.

• Measurement data, including original and edited values.

• Any pertinent comments and illustrations as needed to clarify and/or preserve the integrity of the survey.


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10.7.5.1 Third Order RTK Survey Requirements

The procedures and requirements specified in Section 10.7.5, “Field Procedures” shall be followed, in addition to the requirements specified here.

Applications. Third-order accuracy is acceptable for the following applications:

• Auxiliary or temporary control points for construction surveys.

• Site localization points.

• Any other usage for which the requirements listed in Table 2, “Third Order RTK Requirements” are acceptable, or as determined during the project development process.

Table 2 Third Order RTK Requirements


Maximum PDOP 6.0

Maximum RMS 40 millicycles

Minimum number of satellites common to base and rover 5

Minimum horizon 10 degrees a

Maximum baseline distance 10 km (6.2 mi)

Horizontal precision of each observation b < 10 mm (0.033 ft)

Vertical precision of each observation b < 15 mm (0.049 ft)

Minimum observation time 60 seconds

Minimum number of epochs of collected data 60

Bipod or tripod used at rover Required

Minimum number of observations to each new point 2

Repeat observations using different reference station Required c

Minimum time between repeat observations to each point 3 hours d

Maximum horizontal residual of each observation from mean 10 mm (0.033 ft)

Maximum vertical residual of each observation from mean 15 mm (0.049 ft)

aOr higher if recommended by the RTK GPS equipment manufacturer. bAs estimated by the RTK GPS equipment. cExcept for site localization points. dRepeat observations shall not be made at times when the satellite constellation is expected to be similar to the first

observation. This can be achieved by observing the second occupation at a time of day that is either 3 hours before or

3 hours after the time of day of the first occupation. If only the horizontal coordinates will be used from the

observations, no separation time is required between repeat observations.


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10.7.5.2 General Order RTK Survey Requirements

The procedures and requirements specified in Section 10.7.5, “Field Procedures” shall be followed, in addition to the requirements specified here.

Applications. General-order accuracy is acceptable for the following applications:

• Construction staking, excluding all structures (e.g. bridges and buildings), and excluding grade stakes which require a higher accuracy1 than provided by this survey method.

• Topographic surveys.

• Quantity surveys for pay items.

• Surveys to document construction activities.

• Any other usage for which the requirements listed in Table 3, “General Order RTK

Requirements” are acceptable, or as determined during the project development process.

Table 3 General Order RTK Requirements


Maximum PDOP 6.0

Maximum RMS 40 millicycles

Minimum number of satellites common to base and rover 5

Minimum horizon 10 degrees a

Maximum baseline distance 10 km (6.2 mi)

Horizontal precision of each observation b < 20 mm (0.066 ft)

Vertical precision of each observation b < 30 mm (0.098 ft)

Minimum number of epochs of collected data 1

Bipod or tripod used at rover Optional

Minimum number of observations to each new point 1

aOr higher if recommended by the RTK GPS equipment manufacturer. bAs estimated by the RTK GPS equipment.

1Increased vertical accuracy may be obtained using other surveying techniques, such as elevating the point by differential

leveling.


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10.7.6 Office Procedures

Perform the following procedures when processing RTK GPS measurements:

1. Review all data files, and edit as necessary to correct all errors.

2. If the site localization was not provided, then perform the site localization in accordance with Section 10.7.4.4, “Site Localization”. If the site localization was already performed in the field, verify that it meets those requirements.

3. Verify point numbers and coordinates of all control points used as reference points by RTK base receivers.

4. Verify all base and rover receiver antenna heights.

5. Verify all check shots, and residuals from multiple observations. For points which have multiple observations, generate the final coordinates of those points by using a weighted average of all acceptable observations.

6. Check for high RMS, high PDOP, poor precision, out-of-tolerance errors, or any other warnings given by the processing software, and correct them.

7. Examine all final coordinates for errors.

10.7.7 Deliverables

When all fieldwork and office processing is complete, the following items shall be archived and when necessary, provided to the engineer on request.

1. Electronic Data Files.

a. All “raw” data files for the project. Include both the original and edited versions, if editing was performed. If editing of the data was not possible, include comprehensive notes describing where editing is required. The format of the files must be the native format used by the equipment, and shall be the same files used in processing.

b. Final coordinates in either a comma-delimited ASCII file format, or LandXML version 1.0 (or later) format.

2. Hardcopy Information.

a. Field book(s), or legible scans or photocopies of all information contained in the original field book(s).

b. A general description of the project, procedures and equipment used, and any problems encountered during the course of the survey.

A listing of all electronic and hardcopy files provided, with a brief explanation of each.

chapter 10 construction surveys...the slope stake book) is greater than or equal to 0.1 ft , the...

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