RTK for Boundary Surveying
I remember as a kid going to the park with friends and playing on the merry-go-round. We’d spin it as fast as we could then run and try to jump on. I was reminded of this reading the thread https://surveyorconnect.com/community/threads/javad-integrates-magnetic-locator-into-the-rover-pole.327651/ The thread was a collaborative exercise in attention deficit disorder. It was difficult to find a place to jump in, so I thought it best to start a new thread.
I started my own business a few months ago. I live in an area known as the Piney Woods of East Texas. Pine trees are difficult on precision GNSS results. Because of this and other considerations there are times that GNSS is not the right tool for the job at hand. Having said that, I’ve been continually amazed at the results I’ve obtained from RTK. I’ll remind readers at this point that I provide sales and support for Javad GNSS, but this thread is meant to discuss RTK in general and not intended to be brand specific.
I’ve been using RTK steadily for four years. Prior to that, I have dabbled with RTK since around eight years. I’ve been using post processed GPS (static and kinematic) for 16 years as well as sub-meter GPS for about 14 years. Early experience with post processed GPS and conventional (total station) surveying revealed that least squares adjustments provided the best method for blending the results. Least squares adjustments provided a rigorous process that could apply the statistical strengths and weaknesses of each measuring technology to arrive at the best estimation of the position of points surveyed in a network. Often times the task of using least squares adjustments was arduous as the preparation of the data required no small degree of effort, however the results always proved to be very good when tested with additional cross ties not used in the adjustment (for example surveying a 100 acre tract, adjusting the survey measurements with least squares and then comparing a diagonal GPS vector that was not initially included in the adjustment to find agreement within hundredths of a foot).
Collecting data in the field for post processing was always done with an eye toward insuring good results with extra time in the field. The idea of a return trip due to insufficient data was typically unacceptable logistically and economically, so we would collect more data than necessary. This was particularly true in marginal environments as there were no standards on observation time near obstructions or under canopy.
Starting in 2008 I had the opportunity to provide product reviews for American Surveyor Magazine. I tested several brands of RTK systems. At the time, there was a noticeable difference in precision between some of the brands. Two stood out as remarkable from the others: Altus APS-3 and Javad Triumph-1. Others I had tested at the time were Leica 1200, Spectra Precision Epoch 25 (which was based as I recall on the Trimble 4800 internals), and ProMark 3 and ProMark 500 from Ashtech. The Altus and Javad systems produced precisions comparable to my experience with static GPS results. This ultimately led me to convince my Dad to invest in an Altus APS-3 system when we made our decision to buy RTK. In 2014 I tested a Champion GNSS system which used a Trimble board. This receiver also produced results that were comparable to my experience with static GPS. When I say results were comparable, I mean that we would usually set pairs of points that we would then occupy with a total station. We would always check the backsight (horizontal and vertical) and the results would typically be less than 0.02 foot horizontal and less than 0.04 foot vertical, which was also true for static GPS pairs. The advantage of RTK over static was that I knew the coordinates immediately in the field and could perform COGO on the spot. This did indeed speed the process of collecting field data immensely.
Fast forward to the last few years. I’ve been consulting for Javad which has caused me to step up my understanding immensely. I’ve performed numerous tests in many environments with his equipment. I was prepared for some of this from my days providing technical reviews for American Surveyor, but the testing has only grown from those experiences. One of the tests I performed last year was an analysis of accuracy. I should note that in my observations, similar to the accuracy stated for OPUS, time on site plays a key role in expected accuracy. The longer the time on site, the better the precision of the results. Similar to OPUS, the improvement eventually comes to a point of diminishing returns, but I am persuaded that 4 minutes of RTK can produce an accuracy of:
5mm+0.7ppm at 1 sigma and 10mm+0.7ppm for the Vertical Precision at 1 sigma.
I based this on tests at three ranges:
at 13560′ BL: HSD (estimated) 0.025′, VSD (estimated) 0.041′
at 41930′ BL: HSD (estimated) 0.045′, VSD (estimated) 0.061′
at 50100′ BL: HSD (estimated) 0.051′, VSD (estimated) 0.067′To determine the 2 sigma error estimate of the horizontal, we would multiply the 1 sigma value by 1.6, which would give 8mm+1.1ppm, close enough to call it 1cm+1ppm. For the vertical, we would multiply by 1.96 to get the 2 sigma estimate, which would give 20mm+1.4ppm, close enough to call it 2cm+1.5ppm.
NOTE that these error estimates are based on 240 epoch observations. “Outliers” have been seen at around 3x the standard deviation in these tests.
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