Your GNSS Vectors Match Your EDM Slope Measurements – In Case You Didn’t Know :)
If you wonder about things like grid-ground and you aren’t sure why you might be seeing differences between your GPS measurements and your total station measurements, here is a little exercise you can run through to firm up your understanding and increase your confidence in your approach to combining these 2 measurement technologies in your daily surveying practice.
Try going to your local EDM CBL nearest to your town to measure it with your GPS. Note that the published lengths are NOT projected to the state plane grid or computed as ellipsoid distances. They are purely slope distances measured by NGS using high precision instruments on 2 separate days. They reduced their slope measurements to the mark to mark published lengths and the horizontal lengths. Note the accuracy is published at tenths of millimeters.
See here for an example –
Texas EDM CBLs
https://www.ngs.noaa.gov/CBLINES/BASELINES/txExample – BASE LINE DESIGNATION: MIDLAND CBL TEXAS
You can compare your EDM slope measurements with the published mark to mark lengths to make sure your instrument is working properly. In some states, in order to qualify to perform work for the DOT or other agencies, you are required to measure an NGS EDM CBL using your GPS to verify your equipment, methods and procedures are all working properly.
Using 2, 3, or 4 GNSS receivers, log data occupying all the marks to obtain simultaneous data sets between each pair of marks. Then post-process all the baselines between all the marks. Note, you do NOT have to select a grid coordinate system to process that raw GPS data. That is because the raw GPS data is NOT referenced to the state plane coordinate system OR the ellipsoid as some may assume.
So, all you need to do is open a new project to start a post processing session and select US Survey Feet as the units. Then, process the raw data to solve all the baselines which will be displayed as delta X, Y, and Z between all the points and the vector lengths will match the published EDM CBL mark to mark lengths AND your EDM slope measurements. EDM slope length = GNSS vector length. [ SQRT(dX^2 + dY^2 + dZ^2)= straight vector between the 2 points) ]
And by the way, you can do the the same thing using RTK instead of post-processing. Just place your Base receiver on a mark and observe all the other marks with your Rover. For a full data set on that EDM CBL, move your Base to each mark and repeat the whole process. RTN and VRS are not well suited to make this direct comparison between GNSS vectors and EDM slope distances.
In this old article linked below, originally printed as a series of 3 installments for Professional Surveyor magazine, I purposely chose to use an example located near sea level in order to make an important point about scale factors. The EDM CBL comparison with GNSS vectors example is in this paper too. Go out and try it for yourself. Any uncertainty you had about grid-ground will disappear after you enjoy a day at your local EDM CBL performing this exercise. Why bother understanding any of this? Stay tuned for the next step, combining GNSS vectors and total station measurements in a least squares adjustment.
GPS & EDM Measurements ?? Why Don??t They Match?
http://www.sawj.org/files/drupal4/GPS_Vs_EDM.pdfEDMI Calibration Base Line Program
https://www.ngs.noaa.gov/CBLINES/calibration.shtml
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