Using GPS Networks
Ellis R. Veatch II, PS

Why a network?

The buzzword of GPS these days is RTK. RTK is inherently a method that produces side shots. Checks are not built into the method. When doing RTK, we must develop and utilize procedures to provide checks, such as re-occupying our RTK points using a different base station at a different time. The resulting averaged points will have improved accuracy. However there will be no direct measurements between the points. This is fine for photo control, topo, alignment stakeout and slope staking, etc., but may not meet the needs of tasks such as section break-down, or other boundary surveys where direct measurements between points may be required. These tasks may be better done using rapid static post-processed techniques, which have built in redundancy and point-to-point measurements. The network approach provides us with pretty reliable indications as to the accuracy of the points in relation to each other, not just to a base station.

 

This article is going to present a simplified method of designing a network for GPS surveys. It is not a scientific treatise on networks or least squares adjustments. It is intended to be a basic guide for the average surveyor that has projects that would be better done with a network approach rather than with RTK.

 

The basic rules

The basic rules of simple network design are:

 

 

We want to be able to arrive at accurate points. Therefore we want to make sure that we control the distribution of errors as best we can. Following these two simple rules will provide a good distribution of errors.

 

To do this reasonably efficiently, we need to use at least 3, and preferably 4, receivers. In all projects I bid when I was really a surveyor, it was always more cost efficient to use 4 receivers than 3 receivers. Using only two receivers to attempt a network would be very inefficient.

 

The following is a possible set of 8 sessions, using 4 receivers, for the network of points shown. It follows both rule number one and rule number two. Generally, rule number one will provide also for rule number two. Note that not every pair of points is directly connected, e.g. between URBAN and LOCKMAN, but the point spacing in that area is fairly homogeneous and this structure should provide for a good distribution of the residual errors.

 

The design session occupations are as follows:

 

THAPA              54102               LOCKMAN        BURTCH

BENSON           54102               URBAN             TTMPS

BENSON           ADVANTAGE    BARTLETT        JERSEY

KIEFT                ADVANTAGE    NCS                 HOLLAND

JERSEY            GIFFLES           NORM              HOLLAND

SPENCER         GIFFLES           LEICA               CARL

BULLDOG          TTMPS             BURTCH           CARL

JERSEY            TTMPS             METCO             LEICA

 

Note: One station is always used as a hinge between each set of occupations. Sometimes two stations may hinge, but in general it is most efficient to try to hinge on only one point and not to double measure lines. This of course does not apply to high accuracy network procedure requirements. Follow the procedures specified by your contract.

 

 

 

Use all vectors or only the “independent” ones?

Much has been written about the use of “trivial” or dependent vectors in a network. If you are doing session processing (where all the data between all the points in a session is considered at the same time), you will only use the resulting independent vectors in the adjustment. However most, if not all, of the manufacturer’s software today does not use session processing. Generally, the manufacturer’s software processes vector by vector independently. When processing this way, you can use all the vector solutions in your adjustment. Some surveyors will use only the “independent” vectors from a session even though it was processed vector by vector.

 

Should you use only the “independent” vectors, or all the vectors available? In reality, it doesn’t really matter. However, choose your method and stick to it. Don’t mix and match

 

Below is the same set of points as shown above, measured in 10 sessions (an increase of 20%), where only three of the six possible vectors per session were used. That is, only the “independent” vectors (3 out of 6) of each session were used. The processing time was about 20 minutes because of having to select individual vectors for processing. The resulting adjusted positions (minimally constrained with Bulldog held as control) are listed below the network plan.

 

The observed sessions are as follows:

 

LOCKMAN         54102               BURTCH           THAPA

LOCKMAN         URBAN             BENSON          METCO

ADVANTAGE     BARTLETT        KIEFT               METCO

JERSEY            NCS                 KIEFT               METCO

JERSEY            HOLLAND         NORM              LEICA

NCS                  HOLLAND         NORM              GIFFLES

SPENCER         BULLDOG         CARL                GIFFLES

TTMPS              LEICA               CARL                SPENCER

TTMPS              URBAN             BURTCH           BULLDOG

THAPA              54102               BENSON          ADVANTAGE

 

 

Station               Northing            Easting             Ht.        3D Q

54102

243192.896

3909955.942

267.165

0.011

ADVANTAGE

243660.760

3909962.938

262.233

0.012

BARTLETT

243659.299

3910078.328

258.230

0.012

BENSON

243454.060

3909959.680

259.668

0.010

BULLDOG

243173.009

3910290.332

263.815

0.000

BURTCH

243082.187

3910114.009

269.131

0.009

CARL

243201.204

3910309.850

263.539

0.010

GIFFLES

243606.086

3910450.799

256.381

0.012

HOLLAND

243703.537

3910372.339

259.706

0.012

JERSEY

243646.446

3910192.358

257.137

0.011

KIEFT

243793.129

3910186.574

261.475

0.013

LEICA

243453.274

3910243.590

260.047

0.010

LOCKMAN

243172.883

3910071.392

269.651

0.010

METCO

243537.360

3910169.008

260.240

0.011

NCS

243777.405

3910249.478

260.818

0.012

NORM

243666.117

3910430.788

259.201

0.011

SPENCER

243488.887

3910401.127

250.848

0.012

THAPA

243003.731

3909965.451

274.251

0.011

TTMPS

243415.485

3910121.972

264.365

0.009

URBAN

243321.266

3910063.340

266.921

0.010

 

Below, I show the same network using all the vectors contained in the 10 sessions. The processing took 3 minutes instead of 20. The resulting adjusted positions (minimally constrained with Bulldog held as control) are listed below the network plan.

 

 

Station               Northing            Easting             Ht.        3D Q

54102

243192.899

3909955.940

267.159

0.006

ADVANTAGE

243660.761

3909962.938

262.242

0.006

BARTLETT

243659.301

3910078.327

258.234

0.007

BENSON

243454.060

3909959.679

259.672

0.006

BULLDOG

243173.009

3910290.332

263.815

0.000

BURTCH

243082.186

3910114.009

269.125

0.005

CARL

243201.206

3910309.850

263.532

0.006

GIFFLES

243606.087

3910450.798

256.381

0.005

HOLLAND

243703.541

3910372.339

259.698

0.006

JERSEY

243646.448

3910192.357

257.140

0.006

KIEFT

243793.133

3910186.572

261.476

0.006

LEICA

243453.276

3910243.589

260.047

0.006

LOCKMAN

243172.885

3910071.391

269.646

0.006

METCO

243537.361

3910169.008

260.243

0.005

NCS

243777.407

3910249.475

260.820

0.006

NORM

243666.118

3910430.786

259.203

0.006

SPENCER

243488.888

3910401.127 </