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SURVEYING AND MONITORING SYSTEMS Polo Regionale di Lecco POLITECNICO DI MILANO September 20 TH , 2010 ABERA MAMO JALETA - 749383

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Page 1: Surveying Final

SURVEYING AND MONITORING SYSTEMS

Polo Regionale di Lecco

POLITECNICO DI MILANO

September 20TH, 2010

ABERA MAMO JALETA - 749383

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Surveying and Monitoring SystemsCarolina Cubides / Shashank Mishra

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Contents

1. Introduction

2. Total station

3. Global positioning system(GPS)

INTRODUCTION

This project involves the surveying of the landslide relief structure near the Cortenova. Land relief

structures were built to protect the village from landslides on the downside of the mountains which

usually occur in the mountainous region in rainy season.

On May 26th, 2010, a survey of a retaining wall and a channel’s bank was performed on Cortenova, LC.

Figure-2

Figure 1

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Different surveying and monitoring instruments like Total station, GPS and Terrestrial Laser Scanner

were used in the project to give the complete demonstration of the working of these instruments. The data

was analyzed and results were obtained. Figure 1 shows the retaining wall (on the left side of the

channel) and the right side channel’s bank. Figure 2 shows the landslide in the mountain above

Cortenova. This report describes the working principle and the discussion of the results obtained after the

analysis of data.

TOTAL STATION

A total station is a terrestrial sensor used to obtain measurements of well defined points, being

natural points or signalized points, and not well defined points describing a regular or irregular

surface. In principle, the total station is an electronic theodolite with an electronic distance

meter. It is mostly used for surveying and monitoring in small areas where GNSS (Global

Navigation Satellite System) cannot be used because of limited sky visibility or high accuracy

required, i.e. under 5mm. A total station is a reliable and accurate instrument that has the

capability of storing data, for example measurements, linear and areal features, point labels and

point attributes. It is also capable of a precise repositioning on scheduled points, making it

suitable for monitoring tasks. This automatic fine pointing does not require focusing and has a

constant accuracy independent of the observer. The total station requires a single operator, as it

has a remote control connected via radio modem; nonetheless, the single operator must be skilled

and knowledgeable. Additionally, it has an integrated GNSS sensor.

The direct measurements obtained from a total station are azimuth directions (horizontal),

zenith angles (vertical) and tilted ranges. The measured angles have a precision of 0.15 to 5

mgon, while the range precision depends on the range-finder used. The phase-shift measurement

range-finder required a reflecting prism or target tape and has a maximum range of up to 2km;

the time of flight (ToF) measurement phase-finder does not require a reflecting prism and has a

maximum range of up to 350 m, depending on surface reflectivity. Moreover, the derived

measurements consist of three dimensional point coordinates, in which a 2,500m range has a

precision of approximately 10mm; differences in elevation, horizontal distances, and angular

variations. More precise results may be obtained by performing error propagation computations.

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The possible errors encountered using a total station include repositioning the tripods,

atmospheric refraction, earth’s curvature and residual vertical error. Good weather conditions

are required for precise measurements and visibility between the instrument and the surveyed

point is strictly necessary.

The total station is placed on a tribrach with either three or two vertical screws. Three vertical

screws allow for height adjustment at each positioning, while the two vertical screws provide a

fixed instrumental height, making it less practical. The tribrach is then placed either on a tripod

or on a pillar that can be permanent or removable.

Operational Workflow

Epoch 0

Obtain control points, measured and stationing

points

For ea stationing point: TS setup, georeferencing

and measurement

3D coordinates

Comparison to other epochs

Download data to PCObservation/Processing and computation of 3D

coordinates

Comparison to other epochs

Other Epochs

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Procedure and Results

During the field day at Cortenova, the total station was set up at points 100, 200, 300 and 400.

From these four setups, a total of 224 points were surveyed. Figure 3 shows the total station

network, where point 100 was the fixed control point:

Figure 1

The following is information was obtained from the total station network adjustment report

provided in class:

Stations

Number of (partly) known stations: 2

Number of unknown stations: 222

Total: 224

Observations

Directions: 234 (including 221 free observations)

Distances: 236 (including 217 free observations)

Zenith angles: 236 (including 217 free observations)

Known coordinates: 6

Total: 712 (including 655 free observations)

Unknowns

Coordinates: 672

Orientations: 7

Total: 679

Degrees of freedom: 33

Testing

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Alfa (multi dimensional): 0.4974

Alfa 0 (one dimensional): 5.0 %

Beta: 80.0 %

Sigma a-priori (GPS): 10.0

Critical value W-test: 1.96

Critical value T-test (2-dimensional): 2.42

Critical value T-test (3-dimensional): 1.89

Critical value F-test: 0.98

Results of total station measurements were adjusted using least square adjustment method.

Results for each point are in the form of relative distances from the fixed point 100 and of

heights. Figure 4 shows the landslide relief structure; it was obtained positioning the observed

points in GIS and then drawn in CAD.

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TERRESTRIAL LASER SCANNER

Terrestrial laser scanners (TLS) were developed 10 years ago. The instrument allows for the

automatic measurements of points describing a surface. In principle, it is similar to a robotic

total station, as it is capable of measuring horizontal and vertical angles and ranges. Its

acquisition rate is approximately 12,000 points per second, making it the simplest and fastest

technique for data acquisition. The TLS processes data by registering the geometry of each

acquired point cloud into a given reference system or with respect to a previous scan.

TLS are either window scanners or panoramic scanners. Normally, one scan is not enough to collect data covering the entire object of interest. For measuring one point on the object surface, three observations are made, being the range and two angles: the horizontal angle α and the vertical angle β. In the sensor coordinate system the coordinates x, y, and z of the point are obtained by a conversion from the spherical to the Cartesian coordinate system. If only the object itself is of interest, it is sufficient to determine the relative orientation of the scans. If the object has to be placed in a superior coordinate system, then absolute orientation becomes necessary. Terrestrial laser scanning differs from airborne laser scanning principally in that, during the scanning process, the scanner is not moved. The stationary Terrestrial laser scanner therefore requires deflection mechanism for pointing in two directions.

The pulsed laser beam is sent from range finder electronic unit (1) and meets the polygonal mirror element (3) which rotates at relatively high speed.

Figure 2

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The laser beam is (2) is reflected off the mirror surface such that it is Scand through the vertical angle Ϛ. After Ϛ has been recorded, the upper part of the instrument rotates through small angle α in order to sample the neighboring β profile, and so on until a full horizontal circle has been covered.

In terrestrial lesser scanning, the object is Scand from several measurement stations such that there is only a small overlap between the point clouds generated at each station. GPS can well serve the propose of location this station. From one station one can obtains the polar coordinates Ϛ, α and s. The date recorded from one station is, in terrestrial laser scanning normally called a scan.

Scanners are classified by their method of range measurements. The three methods are the time

of flight (ToF), phase measurement and triangulation based.

The ToF method is used in environmental and civil engineering fields. It is a fast method for

data acquisition, for measurements of up to 2km. The precision on the range measurements are

from 5mm to few cm. The advantages include: measurements are not affected by environmental

parameter changes, works well with reflecting surfaces and requires a low energy supply as it

uses a pulse signal instead of a continuous one.

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The phase measurement method is used in the architectural field. It is a slow data acquisition

method with a range precision of a few mm. Lastly, the triangular based method is used in the

industrial and medical fields, for close range measurements. It is a very fast data acquisition

method with a high precision of less than 0.1mm.

Procedure and Results

Georeferancing

The lesser scanner position (X0 , Y0 ,Z0) can be determined with help of GPS in Static mode, and angular orientation (w, y, K) has been determined. From these parameters, and the polar coordinate β, α, s and X, Y, Z coordinates in a global coordinate system can be derived.

= + Ϛ cosϚ sinϚWhere: Ϛ… … … zenith angle with zero direction along axis

α…………Horizontal angle with zero direction defined by zero direction for rotation

During the field day at Cortenova, the TLS was set up at points 100, 200, 300 and 400 and the

retaining wall was scanned. The data was transferred to a computer for the analysis. First, the

TLS was setup at the control point. Then, five ground control points (GCP) were scanned and

then the retaining wall was scanned. After all the scans had been fused, triangularization was

used to transform the point clouds into surfaces. The horizontal, vertical and tilted cross sections

were then extracted and an orthoimage and 3D model were generated for further mathematical

and structural analysis. Figure 5 shows the TLS used at Cortenova.

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GLOBAL POSITIONING SYSTEM (GPS)

GPS is a type of Global Navigation Satellite System (GNSS). GNSS refers to the use of existing

and future systems for positioning men and vehicles on the earth based on signals emitted from

artificial satellites orbiting around the globe. GPS uses the precise distances from the satellites to

the receivers that are determined from timing and signal information, enabling receiver positions

to be computed.

Global Positioning System (GPS) is currently becoming one of the major techniques in

surveying. It can be used for several applications such as real time navigation, geo-referencing

and high precision relative positioning.

In this context, GPS has several advantages over the traditional land surveying methods. Among

these advantages are the following:

The line of sight between stations and target is no longer a requirement;

GPS observations (baselines) can span large distances;

Observations have a high precision;

The measurement process is quick and efficient;

Figure 3

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Fig.6 Position and Time from GPS Satellite Signals

It is able to collect individual points, lines and areas in any combination necessary for

mapping, and create complex data dictionaries.

In this project, a small control network was performed by using GPS. The results of these points

will be used for other more detailed surveying activities by total station and laser scanning.

Principles of GPS and project description

GPS is a system which employs 24 satellites in 20,200

km circular orbits inclined at 55 degrees. These

satellites are placed in 6 orbit planes with 4 operational

satellites in each plane. A GPS receiver on ground can

calculate its position by measuring the distance between

itself and three or more GPS satellites. Measuring the

time delay between transmission and reception of each GPS microwave signal gives the distance

to each satellite, since the signal travels at a known

speed. The signals also carry information about the

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satellites' location. If the position and the distance of satellites are known, a GPS receiver can

calculate its own position. The important part is there should be at least 3 satellites for that

calculation. But, in fact the clocks of receivers are not definitely correct. To correct the receiver’s

clock error, one or more clocks are needed. So the amount of satellites enough for positioning is

at least 4 (Fig.6).

The differential GPS refers to the contemporary use of a pair of receivers. The master (or base)

station is stationed in permanent way over a point whose coordinates are precisely known from

previous surveying, and the other receiver (or receivers) is setup at stations whose positions are

unknown. Since both stations are relative close to each other (usually within10-20 km) the errors

affecting signals will have approximately the same magnitude. After computing the corrections

for each visible satellite at the master station, they can be applied to the roving receiver, so that

several of the errors can be substantially reduced or eliminated. Usually differential positioning

gives real-time coordinates of rover station, because corrections are transmitted from the master

via a radio-modem or via a cellular phone (GPRS or UMTS modem, currently). This technique

can be applied to the positioning based on carriers which is called Real-Time Kinematic (RTK),

and provides centimeter-level positional accuracies.

The RTK-GPS surveying requires the simultaneous use of two or more receivers. Signals must

be contemporarily collected by all receivers from at least four of the same satellites through the

entire measurement period. The RTK technique exploits the fact that the master station’s

coordinates are known.

During our field work, the movable receivers, Leica GPS1200, were set up in 5 stations, named

[100], [200], [300], [400],[5000].( station [5000] was named [200] in our field surveying work, y

cvxSOND also collected GPS data there continuously.

During the data processing hours, all the data acquired by movable receivers and the permanent

station was processed by using the software “Leica Geo Office 5.0”. It has four primary

functions: preplanning, post-processing correction of the raw satellite data, display/editing of the

data, and converting/exporting of the data. Preplanning includes determining satellite availability

for a particular place, the time and the preparation of data dictionaries for a particular job.

Correction involves the use of a base station file to apply corrections to the raw data collected

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from the satellites, greatly improving accuracy.

By adopting COMO, LECC, PORA, SOND as control station, the coordinates of master station

[5000] can be computed by static positioning. Once we know the coordinates of master station,

we can compute the corrections for each visible satellite at the master station, then they can be

applied to the roving receivers [100],[300],[400].

Procedures of field surveying practice

The field surveying practice was carried on 26/05/2010. The permanent receivers at COMO,

LECC, PORA, and SOND had been working continuously to collect GPS data as well. In the

field practice, 1 GPS receiver was set up in station [5000] for collecting data. 4 baselines (Fig.7)

were measured as follows:

1. 5000 - SOND

2. 5000 - PORA

3. 5000 - LECC

4. 5000 - COMO

Fig.7 baseline of the first step

Once we have known the coordinate of station [5000], we can use it as master station to calculate

other points [100], [300], and [400].(figure 8)

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Fig. 8 baseline of the second step

The GPS observation procedures for station [5000] were as follows:

1. Locate the position of the station and fix a benchmark;

2. Set up the tripod by placing it firmly on the ground. Ensure that it was placed right above

the benchmark;

3. Level the basement until level bubble was in the center;

4. Measure the height between the basement and the ground with a height hook;

5. Install the GPS device on the basement;

6. Turn on the GPS receiver and define a new project [200](named station [5000] when data

is processed);

7. Set the cut-off angle is 15 degree and ensure there were at least 4 to 5 satellites detected ;

8. Start collecting data in a static way. Record the data from satellites every 5 seconds. The

data was saved in a memory card inside the receiver.

The same procedures were also applied for other stations[100],[300],[400].

The RTK-GPS surveying requires the simultaneous use of two or more receivers. Signals must

be contemporarily collected by all receivers from at least four of the same satellites through the

entire measurement period.

6 to 8 visible satellites were detected during observations, which usually depended on the

position of GPS receiver, the atmospheric conditions and the surrounding environment, and this

should be sufficient for the measurement requirement.

Procedures of data processing

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The data observed on the field practice was processed on 1/06/2010. The following procedure

was carried and the final results were obtained:

1. Retrieve data from GPS equipment:

For Leica GPS1200 receivers:

Format Dbx

Form of storage Memory card

Processing software LGO SW (specific software provided by Leica)

For GPS permanent station:

Format Rinex

Form of storage Computer system

Processing software Any compatible software

2. Project set-up and data processing of coordinates of stations:

Software: Leica Geo Office 5.0

Project name: GPS_Cortenova

For GPS1200 receivers ([100],[300],[400] ,[5000]):

Import the GPS data into the project (which also included basic information such as

station ID, time of acquisition, offset of antenna);

Select only the data with sufficient acquisition time and start processing;

The result was in form of coordinates and their respective SD (standard deviations).

For permanent station (COMO, LECC, PORA, SOND):

Extract the data recorded on 26/5/2010, from about 10am to afternoon, from the

computer system;

Unzip the compressed file and start processing;

The result was in form of coordinates (which was regarded as the fixed control point).

3. Data processing of baselines:

Select the acquisition time bar of the two stations and start processing;

Always try to adopt the calculated station as reference one and the unknown as rover

one;

If there were too few common observed satellites between two stations at the time of

acquisition, baselines could not be processed;(for example the baseline between

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station[5000]and the stations [2947-2951]on the mountain);

Results will be in forms of distance (DX, DY, DZ) ;

The 7 possible baselines between 9 stations were processed.

4. Adjustment:

Input the longitude, latitude and height of COMO,LECC,PORA,SOND and adopt

them as control station;

Adjust the coordinates of station[5000] by least square method;

The result was in form of corrections to coordinates of [5000];

Then fix the coordinates of point[5000] to adjust other points [100],[300],[400];

The result was in form of corrections to coordinates of [100],[300],[400].

5. Observation Test:

Check the baseline from station [5000]to target stations COMO,LECC,PORA,SOND;

The result was in form of T-test and W-test.

After processing, these 4 baselines were satisfied. The results of T-test and W-test

were both within the critical value (1.89 and 1.96).

Summary of results:

The table below shows the important results (after adjustments) in this project:

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Discussion of results

The results report printed from the Geo Office software showed several parameters, and

information that will be briefly explained here:

The WGS84 (World Geodetic System 1984) was used as the reference system, and no

transformation to other local systems were done.

During our work we applied differential positioning.The differential GPS refers to the

contemporary use of a pair of receivers. The master (or base) station is stationed in

permanent way over a point whose coordinates are precisely known from previous

surveying, and the other receiver (or receivers) is setup at stations whose positions are

unknown. Since both stations are relative close to each other (usually within10-20 km)

the errors affecting signals will have approximately the same magnitude. After computing

the corrections for each visible satellite at the master station, they can be applied to the

roving receiver, so that several of the errors can be substantially reduced or

eliminated. Usually differential positioning gives real-time coordinates of rover station,

because corrections are transmitted from the master via a radio-modem or via a cellular

phone (GPRS or UMTS modem, currently). This technique can be applied to the

positioning based on carriers which is called Real-Time Kinematic (RTK), and provides

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centimeter-level positional accuracies.

The RTK-GPS surveying requires the simultaneous use of two or more receivers.

Signals must be contemporarily collected by all receivers from at least four of the same

satellites through the entire measurement period. The RTK technique exploits the fact

that the master station’s coordinates are known.

The symbols DX, DY, and DZ refer to the baseline vectors, and they are considered as

correlated observations as any errors in them could affect the other ones.

The Least Squares Principle is the method used in the observations adjustment, which

aims at minimizing the summation of the squares of the observations residuals.

The redundancy is equal to the number of observations minus the unknowns. The higher

the redundancy of the network, the better ability we have for analysing the observations

with errors. On applying this equation to our network, there are 33 equations (21

coordinate difference from 7 baselines, and 12 known coordinates for the permanent

stations COMO,LECC,PORA,SOND), and 12 unknown coordinates (station[100], [300],

[400], and [5000]). Hence this network has 21 degrees of freedom.

The least square method comprises two models: mathematical model and stochastic

model. The aim of the GPS network is to find the coordinates of the stations (unknowns),

not the observations, hence a proper mathematical model is required to relate the

observations to the required coordinates. The observations (direction, distance or height

differences) are usually described by a range of different values which comprises some

errors. Hence a stochastic model is required to describe the deviations of the

observations and normal distribution was adopted.

A distinction between the notions of the reliability and precision of the observations

should be taken into account; where reliability of the observation is concerned with the

mean value, however the precision of the observations are pertaining to the value of the

standard deviation.

The permanent station COMO, LECC, PORA, and SOND has a constraint to keep its

standard deviation equals to zero, which means its coordinates is absolutely true.

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The reliability of a network can be subdivided into internal and external reliability. The

internal reliability is expressed by the minimal detectable bias MDB presenting the

smallest possible observation error detectable by the statistical tests. The external

reliability is used to measure the effect of an error in the observation on the adjusted

coordinates.

Statistical tests are done to check whether the mathematical models and the stochastic

models (in the least square adjustments) are correct for reality. These mathematical

and stochastic models are based on the following set of assumptions which is known as

null hypothesis.

o There are no gross errors (blunders) presented in the observations

o The mathematical model gives a correct description of the relations between the

observations and the unknown parameters;

o The chosen stochastic model for the observations appropriately describes the

stochastic properties of the observations.

Accordingly the statistical tests check whether these assumptions are valid or not.

Below is briefly mentioned the tests that have been applied by Geo Office software to

check the validity of the null hypothesis to the observations

o F test: This test is called the overall model test because it tests the model in

general

o W Test: The rejection of the F test does not lead to the source of the error,

accordingly the W test is done under the hypothesis that a blunder exists only in

one of the observations, and all the others are correct.

o T test: The W test is a 1 dimensional test, however when we are working on a

GPS baseline, it is not enough to test DX, DY, and DZ separately, it is important

also to test the baseline as a whole, accordingly the T test is used for this purpose

either as a 2 or 3 dimensional test according to the dimension of the element to be

tested.

In case we got a rejected test, the following steps can be taken:

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o Deleting the observation corresponding to the error, which will decrease the

redundancy of the network, and hence the precision and reliability of the network.

o Re-measuring the corresponding observation, which is an expensive procedure.

o Increasing the standard deviation of the corresponding observation.

o Ignore the rejection of the test, under the assumption that there is a possibility to

reject a valid observation which is a risky solution.

Conclusions

In this project, a small control network was performed by using GPS. First the coordinates of

RTK master station [5000] was known by static positioning. Then we can compute the

corrections for each visible satellite at the master station [5000], then they can be applied to the

roving receivers [100],[300],[400]. During the hours of data processing, corrections were made

by using least square adjustments and unreliable baselines were discarded with reference to the

degree of precision .These calculated control points will be used for later surveying activities by

total station and laser scanning.