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GPS Applications

in Engineering Surveying

LSGI337/3571 - Engineering Surveying I

By Xiaoli Ding

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What Can GPS Do

in Engineering Surveying

Control survey

Setting out

Topographic survey

Deformation monitoring

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GPS Control Surveys Site reconnaissance

GPS control network design Field survey

Data processing and quality control

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Example of Engineering Control Networks Road Construction

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Example of Engineering Control Networks Bridge Construction

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Example of Engineering Control Networks Tunnel Construction

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Site Reconnaissance Purpose

Determine if a site is suitable for GPS survey

Carry out some preliminary measurements

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Preparation before reconnaissance Find existing maps and plans of the area

Find existing control points near-by and get their coordinates

Tentative design of network

Useful tools Maps or plans of the area

Pen & notebook

Information on existing control points (site description, coordinates ofpoints)

Total station & prism pole, etc.

Compass & tilter meter

Shovel

Pegs & stakes

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What to do in reconnaissance

Site stability

Visibility, get a visibility diagram if necessary

Any structures that may cause multipath errors

Get approximate position of new control points

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Visibility diagram

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Sokkia

Compasses

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Sky plot

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Design of Network and

Survey Schedule Try to measure between adjacent sites

How many baselines to measure to a point?

One is th

e minimum Two provide one redundant link

Three or more provide more redundancy

The shape of a GPS network does not affect theaccuracy

Name and code each site unambiguously

Select suitable time period if necessary

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DOP values as a measure of satellite geometry:

DOP dilution of precision

hVDOP W!

22ENHDOP WW !

222222

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r

dtTDOP W!

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r

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dthdtEdtNdt

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WWWW

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Field Survey How long to survey on a baseline?

Depending on length of line, number of

satellites observed and satellite geometry, e.g.,under reasonable visibility and geometry,

1-10 km baseline: 30-60 mins.

10-20 km baseline: 60-120 mins.

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Planning the logistics of survey and preparation

Sch

edule observation time for each

site Allow enough time to pack up the equipment, travelbetween sites, and set up the equipment

Familiarize with sites that field parties have never beento

Ch

eck completeness and conditions of all equipment Check memory of receivers and memory cards anddownloaded existing files

Check and charge batteries

Coordinates of reference station

Communication between survey parties Test surveys if it is first time survey

Minimum two sets of receivers, more would increasethe efficiency significantly

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GPS survey sessions

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Things to take to site when doing GPSsurvey A survey schedule

A spare battery or cigarette-lighter adapter for

vehicle use Spare cables

A maps, aerial photos

Compass

Tape measure Field notebook

Range pole and external antenna

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What to do on a site

Preparing receiver

Check available memory and download data if necessary

Enter name of station

Observation mode

Data sampling rate

Time zones

Satellites (enabled/disabled)

Minimum number of satellites to be tracked

Minimum elevation of satellites to be tracked

Minimum DOP values

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Antenna orientations should be the same

Measure and enter antenna height

measurement, before and after the survey Vertical height method

Slope height method

Avoid cycle slips and multipath errors

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Example: Leicas Instruction

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Field notes

Project

Point ID

Session number

Names of observers

Receiver type

Receiver S/N

Antenna S/N

Antenna height

Antenna orientation

Scheduled start time

Actual start time

Scheduled stop time

Actual stop time

Observation mode

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File name

Comments

Sky obstruction diagram, if necessary

Plan view of site

Survey mark diagram

Chronology of events

Satellites enabled/disabled

Sampling rate

Elevation mask angle

L1 and/or L2 observations

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Data Processing

Download data to computer

Reduce GPS raw data to baselines

Least squares adjustment of baseline network

Appropriate quality control

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Pros and Cons of GPS

Control Survey Main advantages

No need for line-of-sight between stations

24-hour, all weather condition survey Highly automatic and less manual observations

Long baselines can be observed easily

If accuracy requirement is not very high (a few cm),

RTK method can be used to get real-time results High accuracy

High efficiency

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Is the all-weather system really good?

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Main disadvantages

Requires good sky visibility. This often is

difficult to get in HK Affected by multipath errors

GPS height is ellipsoidal height. Orthometric

height (RL) is required in most engineering

projects.

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GPS in Setting Out Surveys In setting out surveys, real-time positioning

is required. Only RTK method can be used

for this purpose

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RTK System Configuration One or more reference base stations

GPS receiver set up on a known station

Computation facility to determine in near real-

time the corrections (either to the coordinates of

the point or to GPS observations)

Broadcasting facility to broadcast th

ecorrections

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One or more rover stations

A GPS receiver

A radio receiver to receive broadcastcorrections

Computation facility to compute thecoordinates of roving station based on GPS

observations and received corrections A controller that can allow data input and

displaying of roving station and set out points

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Example:

Reference base

station of Leicas

GPS500 system

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Example:

Roving station of Leicas GPS500 system

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Procedure of using RTK GPS

in setting out surveys

Set up reference base station

Prepare the roving receiver Establish data link between the two receivers

Initialise roving receiver (ambiguity resolution) Start from a known point

Start from any point with the On-the-Fly (OTF) ambiguity resolutiontechnique

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Setting out points with reference to the

displayed information

If loss of lock occurs, the roving receiver

needs re-initialised

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Schematic View of Using GPS for Setting Out Surveys

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Heigh

t Reference System for GPS GPS provide geodetic height

Setting out usually requires orthometric

height

Conversion can be done with the knowledge

of local gravity field

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