I recently started working at a company where we put most of our jobs on state plane coords. We use Trimble R6 with a Real Time Network.
The old way they got on state plane was using single frequncy Thales receivers from a published mark and post processing with Thales GNSS solutions.
Now, when we go back to old jobs the Trimble is off from the Thales by about 1.5 ft horizontal but close vertically.
The bearing and distances between points is the same but the coordinates are shifted by 1.5 ft. I believe the Trimble and RTN are working well form checking in to published marks.
The surveyor the used the Thales is no longer with the company and the remaining guys dont know what is up.
Any info would be greatly appreciated.
Thanks
Several possibilities
* The coordinates on the published marks have changed due to a readjustment
* a mix of International vs US Ft units
* the remaining guys dont know what is up.
* its screwed up
I started out my network with Locus receivers (single freq magellan or thales)and later purchased topcon hiper lites. Everything has checked quite well. Now OPUS allows me to put everything on the network without tying and re-adjusting.
It sounds like maybe an itrf shift. That's where I would look.
GPS coordinates are in a constant state of flux. The earth is dynamic. You need to know the projection and time that the original coordinates were generated with. 1.5 feet isn't unusual. Both coordinates may well be correct. Look at a datasheet in the area and you may see the 1.5' shift between new and superseded control values
I don't think you should draw any conclusions from a single old job. Probably better to analyze a cross section of old work and look for consistent patterns. Surveyors tend to be creatures of habit in some respects and this might point to the source of the discrepency if it was something different the departed surveyor was doing from your protocol. All of the others suggestions here seem worth investigating. The NGS adjustments and units being the most plausible to me. I've seen similar shifts in my own work based on work done in various epocs. Like the Mighty Moe said, the system is dynamic and is regularly tweeked. Just got to roll with it baby, change is the only constant.
Did they always use a first order monument. What are the standards or a second order monument? 1/50,000? How far from the monument was the site. 1.5 feet isn't a big deal to me. I have experienced the same issues.
Is it possible someone is using WGS84 lat long and converting to state plane thinking it's the same as NAD83 lat long? That shift would be about 3 feet in my area.
Log a 2 hour file on one of your recent points and send it in to opus for a tiebreaker.
James
Hey, if you are along the West Coast of California, that difference could be achieved in like 10 years. Make sure of the coordinate Epoch used and adjust.
Here (SoCal) the movement is 0.15 ft NW'ly per year.
Other places may be even worse.
How Was The Project Data Saved ?
If all they did was save the SPC values it is a wild goose chase.
Even looking at the original project GNSS Solutions reports should give you solid clues.
If it is 1.5' consistent in your work area from various control points I suspect the the foot used. Simple check, get the Lat Lon of the office and calculate the SPC for the office from the SPC reference point in both US and International feet. A big clue is that elevations are close, elevations are always in US feet but it makes little difference since at most it would be only the hundreds of feet from the control station. Actually you do not even need Lat Lon, all the work can be in SPC.
BTW, do not just assume that it is the old settings that are wrong.
Assuming you are in Vermont, the 1.5' is reasonable. I just opened a project in eastern PA in GNSS Solutions. Switching from US to International feet in my "Settings", I have a 5.4' shift East.
Paul in PA
