If I were to subcontract a surveyor who is using RTK GPS on a virtual reference network, is there any way that he can prove to me his shots are accurate? How can I know that obstructions did not introduce errors?
Thanks in advance.
Multiple occupations for different numbers of epochs at different times of day.
Then compare and see for yourself.
Frequent users will have more advice, but for starters:
You can require that he get a reading on a known point, such as a National Geodetic Survey HARN or CBN station. Comparing versus the data sheet will check settings such as antenna height and Datum.
Requiring critical points to be measured multiple times at least a couple hours apart will give some indication of repeatability. A significant obstruction or reflective surface will probably make the results vary more than other stations, although the readings could be off more in one direction than the other.
Kinda depends on what you mean by accurate.
I am always leery of anyone that will not provide me with the raw data and original data files so I can review how the information was acquired and run the processing myself.
0.02
> If I were to subcontract a surveyor who is using RTK GPS on a virtual reference network, is there any way that he can prove to me his shots are accurate? How can I know that obstructions did not introduce errors?
I use GPS with a Network all the time, mostly for setting up my control. I always have some sort of redundant check; whether it's a comparison to a record dimension; a check with my conventional gear; or, as others have said, a repeat observation on the point at a different time. When I set control to use with my conventional gear, I always set at least 3 points; 1 to occupy, 1 to back sight and 1 to check.
My Leica gear also tells me the estimated error of the position, his equipment might be different.
No more than anyone can prove any other form of data collection is accurate, without repeating all of the shots.
You could ask him to hit the same control point every hour and compare those shots to see if they wander.
Bruce is correct, without 100% redundant measurements on every point that you want to prove, you only have smoke and mirrors and statistics, but you have proved nothing.
The only thing that checking into existing points at the beginning and end of day proves is those two shots, it says nothing about the shots in between. It would prove that at least you likely had good procedure on those two points and hopefully used the same good procedure the rest of the day.
Because of the limitations of RTK in building redundancy, if it is important, I do repeat measurements and if it is even more important, I have interconnecting ties AND I post process it all my self, in other words RTK of any kind isn't the best tool for the tightest work.
I guess it all depends on what you are trying to accomplish or prove that your contractor accomplished, different procedures for different levels of accuracy and confidence.
SHG
Generally RTN data only gives a coordinate file, no raw data. You can set the DC to collect raw data but its usually 10-30 seconds.
Tom
I know that on the Leica stuff you can publish a report with the accuracy of each point. While we still do redundant measurements on important points we have found the reports to be very accurate.
On a side note...we just did a 3 mile long topo, ran three sets of static azimuth pairs. We went back and used the NYS RTN Network on them to compare. We took 30 second observations. The worst one was just under 0.03 off of the static data. We then proceed the TPS traverse holding the RTN shots as control and ended up with a 1:70,000 closure. Not so bad for three minutes of collection.
I use redundant data. I usually take three (three-minute) shots at different times on different days. Then I put my raw coordinates into a spreadsheet I created (with a little help from a NSPS example) that test the horizontal and positional accuracy of each point. North Carolina horizontal positional accuracy standards are 0.10' for urban-class surveys and I never have any trouble meeting that requirement.
If he is using TopSURV you can set the receiver up on a point and do a "Auto Topo Survey". I would assume most software packages have a similar feature. Leave it to collect data on the same point for several hours and save a point every 30 seconds or whatever frequency you want. Bring the coordinate file into a spreadsheet and compare how much they vary. This will give you a good idea of the error. Then try it around obstructions and see what happens.
> How can I know that obstructions did not introduce errors?
As others, including Fatiretom and Gavin have said, the vector data will contain vector quality data which will yield an accuracy estimate. I typically stroke my RTK data through StarNet to get such a report. It's not infallible, but its something.
Here is an idea. I'm assuming you are a surveyor subcontracting work out. Reconnoiter the site and put several random marks down in obscured areas. Tie (some of) these points to your satisfaction. Tell your sub to tie them all as part of his work (without telling him what coordinates you got, of course). In other words, achieve partial redundancy by spot checking the subs results.
Definitely insist on more than just a dump of coordinates.
Norman,
I use StarNet for my conventional work and I dont have the GPS upgrade, but I would love to start throwing my RTK data into StarNet. What kind of software do you use to export the RTK vectors? I am using Trimble Survey Controller, do you know if I can export the vectors with TSC?
> What kind of software do you use to export the RTK vectors?
You will need StarNet Pro to work GPS vectors. But you don't export anything too special from your dc. At various times I have used data from Lieca, Trimble, and Topcon. StarNet accepts data from most major data collectors, including TSC and Access.
> I am using Trimble Survey Controller, do you know if I can export the vectors with TSC?
You don't export anything other than the native .jxl or .job file from TSC. StarNet Pro will import that format, along with many others.
If you are importing Total Station data you will need one of the file converters Microsurvey sells for the purpose to create a .dat file. If you are using StarNet and don't already have one of those, you need one.
It's been a couple of years since I last used TSC. There is a setting in there (QC1, QC2, QC3 ???) that has to be set right for the vector quality control data to be included in the raw data.
I don't have a controller with me but I guess that for TSC there is an option to set in the survey style to store the results "as positions" or "as vectors"
Chr.