Just curious how y'all handle a typical control survey such as this:
I typically put two GPS RTK points outside my project limits on a typical 4000 foot highway survey for sidewalks, say.
(Two on either end of the job). I then run a traverse through the project (four D & R)coming off the first two GPS points and then closing on the last two GPS points. I am aware of the grid to ground situation and I typically miss the last two GPS coordinates by a few tenths. Depending on the length of the project, I can usually guess how far I will miss the GPS coordinates.
Since my GPS RTK coordinates are SPCS, I can reduce my ground distances to the grid via a spreadsheet to check my closure. I understand that I can basically use a scale factor derived from the middle of the project area and put this into my DC, but I do it a little differently.
I leave everything on ground in my DC to avoid confusion later on.
Curious to hear from everyone.
I put everything on ground to start with and keep it there. When I'm shooting between the GPS points with a total station I know immediately if something is amiss.
Get on whatever system is required as deliverable and stay there.
Just about any LS adjustment software will handle this grid to ground issue.
No Reason To Ever Put GPS SPC Coords In A Ground Traverse
Once one has GPS coordinates in SPC one has sufficient information to start with the ground coordinate values of the GPS SPC. In reality GPS SPC coordinates do not exist on the face of the earth. They are a figment of imagination. All those GPS vectors you have are between actual ground points.
In other words, go to ground as quickly as possible.
Paul in PA
No Reason To Ever Put GPS SPC Coords In A Ground Traverse
Thanks. I just grab an azimuth from my first two GPS points, then traverse through to the last two. Since it's an open traverse, I like to check-in on the first GPS point to make sure I didn't mess-up somewhere.
Did you go to PSU in Wilkes Barre, with Dr. Ghilani?
> Just about any LS adjustment software will handle this grid to ground issue.
Yes, and should also be able to adjust the RTK-derived positions rather than treating them as error-free, which of course they aren't.
No Reason To Ever Put GPS SPC Coords In A Ground Traverse
I like a few intermediate points.
N
No Reason To Ever Put GPS SPC Coords In A Ground Traverse
And, I like static GPS. And, then I like real SPC coords, and average scale ground points. And, I want some OPUS, and some Sandwiches nice and ready when I am done!
I guess I'm just hongary!
N
You should use static for control.
Not RTK!
Also add some intermediate points.
I cook my GPS points (sometimes static, sometimes RTK, but your question isn't about preferred GPS methods), but I hold one of the GPS points fixed and scale the rest of them to ground before I ever traverse. I just average all four points scale factors and come up with one scale factor I apply to all points. I hold their existing azimuths and scale to ground with the average factor. Everything usually checks out great for the kind of work I do.
Just remember,
Friends don't let friends scale to the ground without truncating.
> Friends don't let friends scale to the ground without truncating.
The real answer to this problem is Oregon style low distortion projections which don't invite grid to ground conversions at all. Working in the Portland Zone I typically got CSF's on the order of 1 to 100,000. It's the future.