surveying ii week 01

Week 08

Assist. Prof. Dr. Himmet KARAMAN

Topics

ISTANBUL TECHNICAL UNIVERSITY - DEPARTMENT OF GEOMATICS ENGINEERING

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GPS Leveling

Global Positioning System

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The GPS is a U.S. space-based global navigation

satellite system.

It provides reliable positioning, navigation, and timing

services to worldwide users on a continuous basis in

all weather, day and night, anywhere on or near the

Earth which has an unobstructed view of four or

more GPS satellites.


Basic Concepts of GPS


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A GPS receiver calculates its position by precisely

timing the signals sent by the GPS satellites high

above the Earth.

Each satellite continually transmits messages which

include;

the time the message was transmitted

precise orbital information (the ephemeris)

the general system health and rough orbits of all GPS

satellites (the almanac)

GPS Surveying Techniques


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Static survey

Rapid static survey

Stop and go survey

Continuous kinematics survey

Real-time kinematic (RTK) survey

Static Survey


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Stable platforms or pillars

Long distances (10 km to thousands of kilometres)

Long occupation time (hours to days)

Control surveys

Simultaneous recording at several stations

Observation rates varying from 5 to 30 seconds

Reducing multipath effects

Post-processing required

Rapid Static Survey


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Shorter distances (up to 10 km)

Shorter occupation time (10

minutes)

Densification of control networks

Observation rates varying from a

second to a few seconds

Post-processing required

2 reference receivers required

Reference

receiver 1

Reference

receiver 2

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2

3

4

Stop-and-Go Survey


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Distances less than 1 km

1 minute occupation time

Observation rates of seconds

Initialisation required

Repeat initialisation when less than 4

satellites are being tracked

Reference receiverinitialisation

Continuous Kinematic Survey


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Initialization required

Non-stop occupation

Observation rates of 1 second

Real-Time Kinematic Survey


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receiverreceiver

radio

radio

antennaantenna

GPS


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The impact of the GPS is undeniable.

In the span of just a few years GPS has became the

leading positioning technology.

Generally, coordinate determination from GPS

measurements uses the known positions of satellites

and the measured distance between satellites and

the known points.

It is commonly considered as a three-

dimensional system (Latitude, Longitude, and

ellipsoidal height).

GPS Heights


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The heights obtained from GPS are typically heights

above an ellipsoid model of the earth.

It is a geodetic height which purely geometrical

quantity, and represents the length of the normal

to the reference ellipsoid between the ellipsoid

and the point of interest.

These GPS ellipsoidal heights are not

consistence with leveled heights above mean

sea level (MSL), often known as orthometric

height.

GPS Leveling (1)


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In determination of orthometric heights with GPS, the

accuracy of vertical positioning changes depending

on the application type and quality of the project.

The efforts and the attempts for using the system

practically and rentable especially engineering aimed

geodetic application have been increased for the last

decades.

It is known that the surveyor especially to constitute

and densification of the vertical geodetic networks,

traversing and similar applications use several

methods and technique.

GPS Leveling (2)


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To achieve cm height accuracy, surveyors have been

examined alternative methods instead of static GPS

surveying methods with long session duration and

professionally processing and analyzing of the

collected data for long years.

As the result of the development in fast static, stop

and go, kinematic and other surveying methods,

GPS receivers, IGS and other regional and local

permanent GPS networks, it can be seen that it is

possible to obtain desired accuracy in very short

time interval by using single GPS receiver.

GPS Leveling (3)


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Many countries throughout the world can make use

of active RTK networks that have been developed in

recent years for this purpose and resolve the vertical

positioning problem very economically, cheaply and

practically by means of multi-reference RTK

approach.

GPS derived ellipsoidal heights have geometric

meanings in practical engineering survey,

geophysics and in other applications, and they bear

no physical meanings.

GPS Leveling (4)


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However, in many surveying and engineering

applications, orthometric heights are required.

To have ellipsoidal heights converted into

orthometric heights, precise geoid heights are

required.

The most effective technique used in practice

particularly for the determination of orthometric

heights is the GPS-leveling technique.

GPS Leveling (5)


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With the GPS-leveling method, it is possible to

determine orthometric heights with cm accuracy.

Besides, this method is one that is accepted and

applied by the whole world due to its measurement,

ease of computation and its economic application.

GPS Leveling (6)


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As the result of the observation, measurement and

evaluations, it can be said that Geometric Leveling

and Simultaneous Reciprocal Trigonometric Leveling

are the most accurately leveling techniques among

the all of them.

But these techniques require more surveyor,

instruments and time.

GPS Leveling (7)


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Although Leap Frog Trigonometric Leveling and Unidirectional Trigonometric Leveling more practicability than the Geometric and Simultaneous Reciprocal Trigonometric Leveling, they have systematical effects due to refraction and precision of instruments.

Consequently these methods offer lower accuracy. It can be easily seen that the accuracy are depend on accuracy of the geoid model. Use of local geoid model instead of national or regional can supply an accuracy improvement between percentages 40 to 50.

GPS Leveling (8)


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Orthometric heights obtained from 3 reference GPS

and local geoid model consist with the Geometric

Leveling 2.0cm, consist with Simultaneous

Reciprocal Trigonometric Leveling 2.7cm.

Moreover these methods have better consistency

than the Leap Frog Trigonometric Leveling and

Unidirectional Trigonometric Leveling.

GPS Leveling (9)


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Both inner accuracy and consistency with other

leveling techniques can be satisfy expectations for

routine surveying applications.

On the other hand, GPS Leveling can be performed

by only one surveyor economically and practicability

versus other conventional leveling methods when

considered the surveying team, instruments and

expenses.

Comparison of Leveling Techniques


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Soycan, M., A Cost Effective GPS Leveling Method Versus Conventional

Leveling Methods For Typical Surveying Applications

Advantages & Disadvantages (1)


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One major difference is the network "geometry".

Because of the disadvantageous performance of the

equipments the classical leveling networks must run

along lines with benchmark separation less than 1

km.

The leveling loops are bounding larger areas without

benchmarks, it was not guaranteed that each

settlement got a benchmark.

In contrary the new GPS technology can produce a

network with evenly distributed benchmarks, each

settlement should have at least one point.



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One can put GPS-benchmarks anywhere (if

necessary up to peaks), this was not the case for

leveling.

The GPS-benchmarks must be monumented to

sites, where high quality GPS measurements can be

made any time.

The monumentation should not differ from the

classical, but its stability is much more emphasized!

The main monument has two excenters for security

reasons.



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Known 1st and 2nd order leveling points around the

actual working field also have to be included into the

GPS measurements, they will be the reference

markers for the geoid fit.

The more reference markers are included, the more

reliable will be the fitting procedure.



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The GPS data is processed on the field by a

commercial software checking the daily

measurements, the final GPS solution should be

produced by the a commercial software.

The production and the correct use of the GPS-

gravimetric geoid is a key element of the technology.

The GPS-gravimetric geoid have to be produced

using the best quality data.

If required, additional gravity survey have to be

performed.

GPS Leveling Procedure


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Known: HA: Height of Point A

Measured:

hA and hB: GPS based ellipsoidal heights at point A and B

Observed: NA and NB: GEOID96 Geoid heights at point A and B.

Solution:

Δh = hB - hA (From GPS Measurements)

ΔN = NB – NA (From GEOID96)

ΔH = Δh – ΔN

HB = HA + ΔH



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GPS ile elde edilen yükseklikler, geleneksel yükseklikölçme yöntemleri (nivelman) ile elde edilenyüksekliklerden farklı bir yükseklik sistemindedir. GPS verilerinden elipsoidal yükseklik h kolaylıkla eldeedilebilir.

Bu yükseklik yeryuvarının basit bir elipsoit modelininüstünde yada altındadır ve elipsoidal yükseklik (h) olarak adlandırılır. Nivelman ölçmelerinden ortometrikyükseklikler (H) ortometrik düzeltme getirilerek eldeedilir.

Burada, N geoit yüksekliğidir. Yukarıdaki eşitlik GPS / Nivelman yöntemine göre geoit belirlemeyöntemini ifade eder. Eğer bir noktada ortometrikyükseklik (H) ve elipsoidal yükseklik (h) biliniyorsageoit yüksekliği (N) kolaylıkla bulunabilir.


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surveying ii week 01

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