When using a total station to set property corners, my boss wants to compensate for discrepancies in distances between any 2 RTK-derived control points by creating a false backsight point, which will be on the backsight bearing but whose distance will be adjusted. Is this a valid method?
For what possible purpose?
I get that the GPS values probably will never match the total station value, but why say one is better than the other? You could split the difference and adjust both values if that makes you feel good, so total station backsights check, but your still introducing the same error in what your staking.
I do not set property corners from un-checked GPS points. I do shoot all the topo and tie in all the PL offsets and pins I can find and run the calcs to see what fits and where I might need more points but when it gets to filing the recorded maps and setting corners I start over with the ROBOT and re-measure what I need.
All of my jobs have GPS property data on a GPS layer with their own block of point numbers usually in the 200's because my jobs are small. When I start doing work with the ROBOT all the data is on a ROBOT layer with its own block of point numbers usually in the 300's. To start I will pick a GPS point and store it to the ROBOT layer with a 300 number and add the word BASE to the description so I know where I started. Then I backsight a GPS point to get started and the error is copied to the raw file and then I shoot that BS and store it with a ROBOT number and CK.GPS#--- in the description. I also tie in any other GPS property points that are visible from that setup. That way I slowly upgrade all the GPS property data to better ROBOT data and never use any GPS data points again. The end result of 2 layers and different point number blocks is I have a complete record of how good or bad the GPS data was compared to good ROBOT data and the 2 never get mixed together. I learned that one the hard way.
I bought a pair of Stonex S850A GPS's 3 years ago and have been amazed at how close everything has been checking. Usually 0.1' or less horizontal but it does go +/-. Vertical not as good but sometimes I get a surprise.
Valid enough for the purpose which is to establish a bearing basis in the local area for a total station survey. Valid enough because enough use or have used the method to raise it to a practice standard. Is it the best method? No.
I do not set property corners from un-checked GPS points. I do shoot all the topo and tie in all the PL offsets and pins I can find and run the calcs to see what fits and where I might need more points but when it gets to filing the recorded maps and setting corners I start over with the ROBOT and re-measure what I need.
All of my jobs have GPS property data on a GPS layer with their own block of point numbers usually in the 200's because my jobs are small. When I start doing work with the ROBOT all the data is on a ROBOT layer with its own block of point numbers usually in the 300's. To start I will pick a GPS point and store it to the ROBOT layer with a 300 number and add the word BASE to the description so I know where I started. Then I backsight a GPS point to get started and the error is copied to the raw file and then I shoot that BS and store it with a ROBOT number and CK.GPS#--- in the description. I also tie in any other GPS property points that are visible from that setup. That way I slowly upgrade all the GPS property data to better ROBOT data and never use any GPS data points again. The end result of 2 layers and different point number blocks is I have a complete record of how good or bad the GPS data was compared to good ROBOT data and the 2 never get mixed together. I learned that one the hard way.
I bought a pair of Stonex S850A GPS's 3 years ago and have been amazed at how close everything has been checking. Usually 0.1' or less horizontal but it does go +/-. Vertical not as good but sometimes I get a surprise.
Awesome process !
I was on a long drive back from the NC mountains yesterday and I started thinking about what a PLS one hundred years in the future might think about one of my plats. What would they think was quaint vs what would they find value in? My guess would be that absolute accuracy, relative to Earth or the current model of its center, will be of higher value than relative precision. With NGS going in the direction of Earth Centered Earth Fixed with the prioritization of CORS stations over physical markers, it's difficult to imagine that our piers in the future will consider it a negative for surveyors to precisely locate parcels relative to Earth's time and space. Will the future PLS get excited upon seeing a 1:200,000 closure on a parcel tied to a road intersection? Will the same PLS get excited seeing a plat with monuments located 0.10' at the 95% confidence level and State Planed Grid distances and bearings and a couple of coordinate pairs?
As far as procedures go, it's getting difficult for me to view even a single GNSS observation as, "Unchecked", as a poster mentioned above. I've been comparing my GNSS observations against total station work for many years now and see only increased precision in the GNSS and superior blunder detection. I still like to have at least a twenty minute time separation between redundant GNSS locations, but the data I've obtained hints that this is no longer a significant improvement over taking a 30 sec burn, ending survey, walking fifteen or so feet away and reinitializing, then reshooting for 20-30 seconds.
When using a total station to set property corners, my boss wants to compensate for discrepancies in distances between any 2 RTK-derived control points by creating a false backsight point, which will be on the backsight bearing but whose distance will be adjusted. Is this a valid method?
I'm confused. Are you saying your GPS and robot measurements don't agree by some significant amount?
My guess would be that absolute accuracy, relative to Earth or the current model of its center, will be of higher value than relative precision.
In my part of the world, that PLS 100 years in the future will find that my plat coordinates are about 6 feet off.
In my part of the world, that PLS 100 years in the future will find that my plat coordinates are about 6 feet off.
I'm moving slower here, about .15' in twenty years which will be .65' in 100 years, but still way too far to be acceptable. That's the importance of good monumentation, no one cares what the distance is to a plaque in England when they buy a house.
I don’t know what the boss will accept as being within tolerance. Recently we differ by 0.06’ - 0.07’. The boss is concerned about any differences because we’ll be setting property corners for an important project.
I don’t know what the boss will accept as being within tolerance. Recently we differ by 0.06’ - 0.07’. The boss is concerned about any differences because we’ll be setting property corners for an important project.
0.07' is unacceptable for small control projects. I would expect to see nothing over .03' with today's equipment. It's possible mixing GPS and TS has created some sort of sale factor issue.
RTK/RTN 180 seconds per point on 3 monuments that see each other, put the rover in the truck and forget about it.
Gun up, backsite, crank angle as a check.
Slide the backsite corn-nut toward the gun, forget about the check shot.
Translate and rotate record data on over and start hunting for more evidence.
Not saying it is right, but, I have done it under the direction of well respected LS's.
The purpose being to bring the survey onto some datum to meet a municipal requirement. I think the more respected way would be to do something pretty similar but drop perform a more robust adjustment.
@field-dog Hmm, yeah that's a little unusual. What is the typical difference between grid and ground in this area?
When using a total station to set property corners, my boss wants to compensate for discrepancies in distances between any 2 RTK-derived control points by creating a false backsight point, which will be on the backsight bearing but whose distance will be adjusted. Is this a valid method?
By itself to me it's the equivalent of sweeping the error under the carpet. Which one is out? The occupied point or the backsight, or both? I have developed a habit of storing a shot on my back sight if the RTK and TS distances differ by by more than 0.05', but this in itself doesn't really prove anything without running a closed traverse through my RTK points to identify which of the RTK points might be out of tolerance. Over the years the quality of those RTK points has improved greatly with the addition of constellations and better RTK engines, but you can still easily end up with an RTK point that is out of tolerance and how would you know which one it is without either a closed traverse or multiple spaced out RTK observations?
Just because I'm paranoid, doesn't mean they aren't out to get me.
If it's a simple grid-ground issue, the entire process is glitched. You have bigger issues than false points.
I started thinking about what a PLS one hundred years in the future might think about one of my plats. What would they think was quaint vs what would they find value in?
I'm guessing that surveyors of 100 years from now will be most concerned with about the same things that surveyors today are. And that is relative accuracy. They will likely have even greater capacity to deal with the absolute than we have now, but relative will still rule the day.
...what would they find value in?
I think they'd most appreciate a recording law and surveys with narratives.
Not sure of the typical difference between grid and ground in my area. We’re working at
| 28.5831367°, -081.2134467° |
lat, long, Florida SPC zone 901 east.