SURVNET –A LSurvNET comes bundled with Carlson Survey (both IntelliCAD and AutoCAD OEM ... • A...

SURVNET – A LEAST SQUARES ADJUSTMENT BY…

IntroductionMeasurement Errors

• Types of Errors• Error propagation

Adjustments• Averaging• Traditional Adjustments• Least Squares Adjustment• Advantages of Least Squares Adjustment

Creating a Project in SurvNETSample Project in Carlson SurvNET

• Traverse Network• GPS Vectors• Combination of Traverse/GPS data• Control File

Notes from Programming

SURVNET – INTRODUCTION

SurvNET is a least squares adjustment program that allows you to perform a mathematically rigorous adjustment and statistical analysis of a network of traverses.

SurvNET comes bundled with Carlson Survey (both IntelliCAD and AutoCAD OEM platforms) and can also be purchased separately as an independent Windows application.

SurvNET allows you to process GPS vectors, Traverse data, Trig‐level and Differential level data, or any combination of the above at the same time. Additionally, you can process multiple raw data files at the same time. You do not need to merge the raw data files before processing. Control points can be in a separate raw data or coordinate file. You can also have multiple GPS vector files.

A common question… Does SurvNET “do” GPS post‐processing?No. Carlson’s GNSS post‐processing is handled by SurveyGNSS.

Least Squares is the BEST adjustment possible: By definition, a least squares adjustment will adjust the measured data the least amount possible to fit the desired control (the sum of the square of the residuals is a minimum).

SURVNET – MEASUREMENT ERRORS

Surveyors Make Measurements:• One aspect of a land surveyor’s job is to make measurements.• There are no perfect measurements, all measurements have errors.• Errors are inherent in the instrumentation we use.

A 5” instrument measures angles to ± 5”. An EDM may measures distances to ± 0.01’ and 3ppm.

• GNSS receivers by their very nature will have errors.

There are three classifications of errors:1. Blunders2. Systematic Errors3. Random Errors

SURVNET – BLUNDERS

Blunders are mistakes and cannot be adjusted.Blunders MUST be removed from the data set prior to adjustment.

Examples of Blunders:• Failure to level the instrument.• Failure to put the instrument or target over the point.• Assigning the wrong point number to a point.• Error measuring the instrument height.• Recording the wrong measurement.

SURVNET – SYSTEMATIC ERRORS

Systematic Errors are predictable errors and should be removed from the dataset prior to adjustment.

Examples of Systematic Errors:• Atmospheric corrections – temperature, and pressure.• Curvature and Refraction corrections.• Wrong prism constant

SURVNET – RANDOM ERRORS

Random errors are errors inherent in the instrumentation used to make the measurement. Random errors can be adjusted out of the traverse!

Examples of Random Errors:• A theodolite may measure to a ±5” precision.• An EDM may measure to 0.01’ plus 3 ppm• You can only set your instrument over a point to a ± 0.01’ accuracy.• You can only measure from the point to the height of instrument to a ± 0.01’

accuracy.

SURVNET – ERROR PROPAGATION

• Coordinates are indirectly calculated from direct measurements (angles, distances, GPS vectors).

• Since the direct measurements contain errors, so do the coordinates calculated from them.

• This is known as error propagation and there are statistical equations that allow us to predict the probable error for each point.

• Prior to a least squares adjustments, each measurement is assigned an initial standard error based on the equipment specifications.

• These standard errors are used to predict the error throughout the traverse and weight the measurements.

• As you traverse further from known control points, the less you know your position.

• The next slide shows the error ellipses of a traverse with one control point.

SURVNET – ERROR PROPAGATION – 1 CONTROL


• Error ellipses increase as you get further from a known point.• The next slide shows the error ellipses of the same traverse with two known

points.

SURVNET – ERROR PROPAGATION – 2 CONTROL


• Additional control points and redundant measurements add strength to the traverse and make the statistics more meaningful.

• GPS can be used to add additional control.• Cross traverses and measurements to points from different locations can add

redundancy to the traverse.• Triangulation can also be useful in adding redundancy.

SURVNET – ADJUSTMENTS

Why do we adjust traverses?• All traverses have errors; they do not close exactly on the terminal point.• If we do not adjust the traverse, all the error is placed in the last leg of the traverse

which is not a valid assumption.• Error adjustments are important to future work on a project. Placing all the error in

one measurement can prove problematic for both project design and layout.

AveragingAveraging is a type of adjustment. We average SETS of angles measured in both direct and reverse faces:

• For additional precision(Sm = S / √N) Standard deviation of the mean equals the standard deviation of the single measurement divided by the square root of the number of measurements.

• As a check against blunders• To help remove instrument errorsWe average distances and zenith angles measured in both faces and in both directions:• For added precision• To help remove instrument errors• To mitigate curvature and refraction errors• To improve vertical closures.

SURVNET – TRADITIONAL ADJUSTMENTS

Prior to the advent of high powered computers, when only calculators or hand calculations were used, there were three popular adjustments:

• Crandall’s Rule• Transit Rule• Compass Rule

Typically angles were balanced prior to adjusting the traverse with these methods.

SURVNET – CRANDALL’S RULE

• Crandall’ Rule is a “special case” least squares adjustment.• Crandall’s Rule assumes there is NO error in the angles – angles/directions are

assigned an infinite weight.• Therefore, adjustments are made only to distances.• This adjustment was typically used to match bearings with previous surveys but

under certain conditions can give unexpected results.• The assumption that angles contain no error is not a valid assumption.

SURVNET – TRANSIT RULE

• Transit Rule adjusts both angles and distances but makes the assumption that angles are measured with a higher precision than distances.

• This assumption may have been valid in the past when using a 10” theodolite for angles while pulling a chain for distances, but it is not valid for today’s measuring equipment.

SURVNET – COMPASS RULE

• The Compass Rule assumes both angles and distances are measured with equal precision.

• Of the traditional adjustments, this assumption is most valid for today’s measuring equipment.

• The Compass Rule remains a very popular form of adjustment by surveyors but has distinct disadvantages when compared to Least Squares Adjustments.

SURVNET – WHAT IS LEAST SQUARES?

• Least Squares is a statistical method used to compute a best‐fit solution for a mathematical model when there are excess measurements of certain variables making up the mathematical model.

• Least Squares requires a mathematical model, a system of equations. It requires redundant measurements of one or more variables (the known variables). Lastly it requires variables that are unknown that are being solved for.

• The Least Squares criteria is reached when the sum of the squares of the residuals have been minimized.

• Least Squares Adjustment (LSA) applies the least possible amount of correction when adjusting the measurements which arguably makes it the best adjustment.

SURVNET – ADVANTAGES OF LSA

• It allows the simultaneous adjustment of a network of traverses. Traditional adjustments can only adjust one traverse at a time.

• It allows the combined adjustment of traverse data, GPS data and level data –1D, 2D or 3D adjustments: 1D: Horizontal or Vertical 2D: Horizontal and Vertical, independently 3D: Horizontal and Vertical, simultaneously

• It allows complete control of the adjustment process. Measurements are weighted based on the equipment used and the number of measurements made.

• It allows processing data of different precision – weights can be applied to individual measurements or groups of measurements.

• It allows flexible control. The control points can be anywhere in the traverse, you don’t need to start on a known point. Control points do not have to be contiguous and they can be side‐shots.

• It can handle resection data (measurements from an unknown point to known points), triangulation (angle‐only measurements) and trilateration (distance‐only measurements).

SURVNET – ADVANTAGES OF LSA (CONT.)

• Automatic reduction to State Plane Coordinate systems if desired.• Extensive data analysis provides more information for evaluation of traverse

networks.• Enhanced blunder detection tools.• Allows flexible field procedures; the data does not have to be in any specific

order.• Provides statistics for ALTA or State survey certifications.

SURVNET – COMMON MISCONCEPTIONS OF LSA

Least Squares Adjustments are just too complicated.• Reports are intuitive and easily understood• Flexibility of LSA makes processing of difficult data sets easy.Least Squares Adjustments are only necessary for very precise surveys.• LSA can and should be used for any type survey.• LSA can be used for simple loop traverses as well as complex traverse networks.

SURVNET – TYPICAL PROJECT WORK-FLOW

1. Choose a coordinate system via Coordinate System tab2. Select data file(s) to be processed including control points (known points) via

Input Files tab3. Enter/review pre‐processing settings via Preprocessing tab4. Enter/review preliminary standard errors via Standard Errors tab5. Enter/review adjustment parameters via Adjustment tab6. Specify output options via Output Options tab7. Process the data8. Analyze report(s), examine and loop through options 2 through 7 until

satisfactory results are obtained

SURVNET – THE “NELSON” PROJECT

SURVNET – THE “NELSON” PROJECT PLAN

Equipment Used• Carlson RT3 Ruggedized Tablet with SurvPC 6.x• Carlson BRx6+ Multi‐constellation GNSS Network Rover• Carlson CR5+ Robotic Total Station

“The Plan”• Utilize Iowa Low‐Distortion Projection (LDP) Coordinate System Projection• Utilize Iowa Real‐time Network (IaRTN) for RTK GNSS Corrections (nearest

base)** to establish two known positions for robot work to follow• Utilize Wisconsin Virtual Reference Station (VRS) Network (WisCORS) for RTK

GNSS Corrections (nearest base)** as redundancy to IaRTN readings** ‐ both IaRTN and WisCORS were observed with 300 epoch readings in morning, early afternoon and late afternoon.

• Applicable IaRTN and WisCORS nearest base station positions recorded and treated as “control”

• Robotic total station traverse data collected when convenient between GNSS observations

• Turned sets for traverse points collected to minimize instrumentation error• Readings to a previously measured points were “stored to raw only”• Sample data at https://www.dropbox.com/s/1rrcl2cz1pzllp5/SURVNET‐

Nelson.zip?dl=0

SURVNET – SURVCE/SURVPC COORDINATE SYSTEM

SurvCE/SurvPC – Job Settings• Consistently specified in each “job” used for clarity• “CTRL” job was first, then readings from WisCORS Network• Then GPS readings from IaRTN Network with RTS Readings

SURVNET – SURVCE/SURVPC GPS ROVER

SurvCE/SurvPC – GPS Rover (Data Collector Internet)• Specify “SingleBase” Option so Vector Information Can Be Recorded

SURVNET – SURVCE/SURVPC MONITOR/SKYPLOT

SurvCE/SurvPC – Monitor/Skyplot• Getting the Base Position of the Iowa Network

SURVNET – SURVCE/SURVPC MONITOR/SKYPLOT

SurvCE/SurvPC – Monitor/Skyplot• Getting the Base Position of the Wisconsin Network

SURVNET – SURVCE/SURVPC STORE POINTS

SurvCE/SurvPC – Store Points• Recommended Procedure is to Re‐Use Point Numbers Previously Measured!• THEN…

SURVNET – SURVCE/SURVPC POINT PROTECT

SurvCE/SurvPC – Point Protect• Store the data into the Raw File for SurvNET Analysis!

SURVNET – LET’S BEGIN!

SurvNET by Carlson Software• Found in Carlson Survey OEM (Embedded AutoCAD)• Found in Carlson Survey with IntelliCAD/AutoCAD Add‐on• Runs as Stand‐alone

SURVNET – SOME USER-DEFINED PROJECTIONS…

These Low Distortion Projections (LDP’s) Are Around or Near the Project Site:

Iowa• https://iowadot.gov/rtn/iowa‐regional‐coordinate‐system‐iarcs/iarcs• https://www.iowadot.gov/rtn/pdfs/IaRCS/IaRCS_11_Dubuque_Davenport.pdf

Wisconsin• https://www.sco.wisc.edu/• https://www.sco.wisc.edu/wp‐

content/uploads/2017/07/WisCoordRefSys_June2015.pdf (p. 45)

SURVNET – UDP EXAMPLE (IOWA RCS DATA)…

Iowa• https://www.iowadot.gov/rtn/pdfs/IaRCS/IaRCS_11_Dubuque_Davenport.pdf

SURVNET – UDP EXAMPLE (IOWA RCS DATA)…

Iowa• https://www.iowadot.gov/rtn/pdfs/IaRCS/IaRCS_11_Dubuque_Davenport.pdf

(Grid Origin Northing/Easting specified in Meters unless noted)

SURVNET – UDP EXAMPLE (WISCRS DATA)…

Wisconsin• https://www.sco.wisc.edu/wp‐content/uploads/2017/07/WisCoordRefSys_June2015.pdf

SURVNET – UDP EXAMPLE (WISCRS DATA)…

Wisconsin• https://www.sco.wisc.edu/wp‐content/uploads/2017/07/WisCoordRefSys_June2015.pdf

(Grid Origin Northing/Easting specified in Meters unless noted)

SURVNET – REVIEW COORDINATE SYSTEM TAB

SURVNET – SPECIFY INPUT FILE(S)

SURVNET – NOTE SUPPORT OF OTHERS!

SURVNET – SPECIFY PREPROCESSING SETTINGS

SURVNET – SPECIFY STANDARD ERROR SETTINGS

SURVNET – SPECIFY ADJUSTMENT SETTINGS

SURVNET – SPECIFY OUTPUT OPTIONS

SURVNET – SPECIFY DATA FOR ALTA ANALYSIS

SURVNET – CHOOSING DATA GRAPHICALLY IS EASY!

SURVNET – PROCESS THE DATA

SURVNET – SOME PERTINENT RESULTS

Relative Error and ALTA Tolerances==================================

SPECIFIC CONNECTIONS: Tolerance of 0.070 + 50 PPM. at the 95% CI.

Actual Allowable RatioSta. Sta. Dist. Semi Major Semi Major Actual/Allowable Semi Minor Max. Err. Az.18 19 295.2567 0.0713 0.0848 0.8408 0.0488 S 88-56'29.93"E

*** All Specific Connections passed ***

ALL CONNECTIONS: Tolerance of 0.070 + 50 PPM. at the 95% CI.

All possible connections between the following points were checked:11,12,13,14,15,16,18

21 connections tested, the 20 largest relative error ellipses will be shown:

Actual Allowable RatioSta. Sta. Dist. Semi Major Semi Major Actual/Allowable Semi Minor Max. Err. Az.14 18 282.2136 0.0676 0.0841 0.8034 0.0405 S 39-10'12.65"E15 18 218.3045 0.0597 0.0809 0.7373 0.0398 N 08-47'12.28"E15 16 192.7965 0.0552 0.0796 0.6927 0.0330 N 22-55'40.55"E12 15 251.4279 0.0569 0.0826 0.6897 0.0297 N 53-51'35.39"E11 14 246.7638 0.0548 0.0823 0.6658 0.0311 S 51-24'08.41"E. . .11 13 135.3810 0.0348 0.0768 0.4530 0.0286 S 23-29'48.36"E11 18 71.0288 0.0332 0.0736 0.4508 0.0277 N 11-16'50.94"E13 16 85.4797 0.0326 0.0743 0.4394 0.0242 N 10-28'45.68"E

*** All connection combinations passed ***

. . .

Reference Variance:0.986Standard Error Unit Weight: (+/-)0.993Passed the Chi-Square test at the 95.00 significance level28.366 <= 46.732 <= 65.410

SURVNET – BLUNDER DETECTION

SURVNET – SOME NOTES FROM PROGRAMMING

• One of the biggest support issues has to do with defining the control. Control consists of the known points that will constrain the adjustment. Make sure your control is defined properly and assigned the desired standard errors. When you process the data, the first part of the "Unadjusted Observations" section will show the control and the standard errors assigned to them. It is good to look at this to make sure the control is correct.1. All point records (SP) in an RW5 file will be treated as control. Remove any PT

records that are not control.2. Control can be defined in either the RW5 file (PT records) or in a Supplemental

Control File. The advantage of defining them in the RW5 file is that you can assign different standard errors to each point. For example, you can hold some of them fixed and let others move a little. If the control is from a coordinate file, the standard errors set in the project settings will be applied to all the control points.

3. If you have GPS vector records in the RW5 file and which to use them in the adjustment, you have the option of using the BP records as control. You may or may not want to do that which is why it is optional.

4. Standard error records placed in the RW5 file apply to all the PT records following them. They over‐ride the control standard errors in the project settings. They are in effect until another standard error record either changes them or puts them back to the project defaults.

• One thing I might mention about control is that SurvNET does not require control points to be contiguous. The minimum is two control points but they can be anywhere in the traverse. For example, you could have an open traverse with a control point near the beginning, one in the middle and one toward end.


• I have found through experience with client’s projects that when using the 3D adjustment model, the zenith angles should be given less weight than the horizontal angles; usually by a factor of 2 or 3. Even though it may be a 3 second instrument, I don't think the zenith angles are within that range because of the error in the vertical compensator. Anyway, you tend to get better results this way. I also changed the total station and target centering and height errors to something more reasonable:


• I have noticed from client's projects that if you use GPS to create a control point every few thousand feet, you generally will have no problem passing the ALTA certification. Without the additional control points, large traverses (more than 20 acres) almost never pass the ALTA certification if it is just a loop traverse.

• If you are going to process GPS vectors with (or without) traverse data, you must select the 3D adjustment model. The 3D adjustment model can only be selected if you are working on a State Plane coordinate system.

• The 2D/1D model separates the data into horizontal and vertical components and then processes them separately. If there are problems with the vertical data, it shows up more in the 3D model.

• In general, you want to set your initial standard errors to reasonable values based on the equipment and field methods you are using. Again, I like to assign higher errors to my zenith angles than my horizontal ones by a factor of 2 or 3.

• If you fail the Chi² test on the low end, that means you had less adjustment than predicted by the initial standard errors and if you lower the standard errors you will increase the critical value (middle value shown).

• If you fail the Chi² test on the high end it means you had more adjustment than predicted and if you raise the standard errors you will lower the critical value. If you fail on the high end by a large margin that usually points to a blunder. Most blunders I deal with are point‐number oriented.


• You can think of the initial standard errors as predictions as to how well you can measure. You want these predictions to be in the ball park or your statistics will be misleading. The Chi² test tells you how well your adjustment matches your predictions. Think of them as weights, the higher the value the less weight is given to the measurement (an angle with a standard error of 10" gets less weight in the adjustment than one of 5".

• Reasonable errors for setups (centering and targeting errors) would be higher for prism poles than tripods. I think 0.01' to 0.005' are reasonable for tripods. I think if you are using a 5" instrument then you should select a 5" reading error for horizontal angles and a 15" to 20" error for vertical angles. If you are in bad viewing conditions, you might want to fudge it a little by entering a pointing error. Most EDM's measure distances to +/‐ 0.01 plus 5 PPM. I have noticed newer equipment is getting lower (+/‐ 2mm with 3PPM). These should be in the instrument specs and assumes the instrument is in good adjustment.

• The more sets you turn, the lower the standard error assigned to those angles will be. For example, if you are using a 10" instrument and turn 2 sets (four angles), the newly calculated standard error for that angle would be 5" (10 divided by the square root of 4). To get a 1" precision from a 10" instrument you would have to turn 100 angles or 50 sets (10 divided by the square root of 100 is 1).


• GPS is a little more tricky. Again, I think a centering error of around 0.01' (3 mm) is reasonable. The vector factor usually depends on the equipment and once established should be the same for future jobs. Zero is no factor (multiply by 1.0). Mostly I have seen values from 0 to 4 (4 is a scale factor of 16, 2 to the forth power). A good way to determine the vector factor would be to shoot a network of vectors with plenty of redundancy and then process them alone (without traverse data). Change the vector factor until your critical value is basically in the middle of the Chi² limits. Based on your vector data I came up with a vector factor of 2 (scale factor of 4).

• The vector factor is necessary because most GPS equipment shows very optimistic variance and covariance values. By raising these values by a factor we are lowering the weight they will have in the adjustment.

SURVNET – QUESTIONS?

• Be gentile!

• For more information, visit SurvNET On‐Line Help or On‐Line Documentation.

THANK YOU for your attention!!

Ladd NelsonMidwest Sales DirectorCarlson Software563‐583‐[email protected]

SURVNET –A LSurvNET comes bundled with Carlson Survey (both IntelliCAD and AutoCAD OEM ... • A...

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