GNSS Offset?
I’m adding property line info for 80 parcels to existing mapping from a few years ago for a project to bury all the electric lines in town (Cruz Bay, St. John). I have a tight closed loop with a robot aligned to 4 GPS OPUS SPC points, all within 0.10′. But I’m missing the old mapping (1 control point from the company that did the mapping with GNSS) and over a dozen manholes, catch basins, curbs etc. by a consistent 3.7’N, 1.4’E. I had a problem with US Feet / Int. Feet on another project, but that was more E than N as the numbers are greater, so that’s not it. I can just move all my work to match the old and it will be all good, but me and the guy that did the mapping would like to know what’s up. Any ideas?
- 15 Replies
That type of little shift is usually international feet vs US feet or SPC adjusted to surface around a point. Without Metadata I would either hold the design control or move their data over universally to SPC.
My first guess would be that the older GNSS mapping data was mistakenly processed against an autonomous position rather than a published or adjusted position.
Or, if they processed it against an OPUS solution, someone could have screwed up and entered one or both of the ITRF geodetic values rather than NAD83 values. Seen that before too.
Or someone managed to grab all the points in CAD and move ’em somehow…
“…people will come to love their oppression, to adore the technologies that undo their capacities to think.” -Neil Postman
I’ll just move mine to match. It’s kind of backward – construction has started but no one knows where the property lines are, so easement acquisition can’t start until I finish drawing all the property lines. Our power co. would be right at home in Texas.
Thanks, guys.
That looks like the offset between WGS84 and NAD83. In Topcon Magnet many people will incorrectly use NAD83/2011 settings when should use NAD83_NO_TRANS . In my area, the difference is 3.78 ft.
Friends don’t let friends scale SPCs without truncating.
I would suspect that MightMoe’s thought about surface level values is the most likely. A very quick computation for St. John shows that the differences in US Feet vs. International or ITRF14/WGS 84 vs. NAD 83 (2011) do not match the values you indicated.
How old is the underlying work?
ITRF v. NAD83, HARN?
Whenever I have a 3?? +/- difference it always comes down to US vs International Foot. Civil 3D requires the drawing settings be set to USF as well as the Survey module. Ridiculous you need to check both places on every project every time you import as well as export. Always check your northings and eastings on both importing and exporting. The cvs and drawing coordinates should match. How many countless projects have fallen to this poor programming? It should be set it once and done.
To further complicate this, the construction co. surveyors miss my work by N 0.8′, E 0.0′.
But I found that there are 2 OPUS Share points right next to the project that I hit 0.05′ and 0.04′. So I’m confident I’m good.
@larry-best
Your agreement with additional contemporary opus derived positions is good. But that still leaves the question of how old the underlying work is. Using CORS data goes pretty far back. And the published positions of CORS stations have been revised over the years. So coordinates for a specific point or project, that I generated from CORS data and the published CORS positions from 1998 would be different than an opus position determined today for those same points.
There are a some relatively new stations on St Croix established by GPS. And a bunch of older stations established by triangulation. These older stations have NAD83 (86) coordinates but were not adjusted to the NAD83 (2011). So the prior survey may have been adjusted to older non-GPS stations.
When I surveyed on St Thomas in the early 90’s, there were no GPS stations at all, we had to tie to triangulation stations. And there was only one second order station at the time (T 41), at a location where the GPS signals were getting wiped out by a nearby microwave tower. So we had to set an eccentric and sight an offshore lighthouse to tie to the existing station. And of course only NAD83 (86) coordinates were available. When I was there we were using Trimble 4000SSE receivers, I see that NGS did tie it in with GPS using newer receivers (the next gen 4000SSI was more immune to radio interference).
Here is part of a map of St Croix, open triangles are triangulation stations, black triangles are GPS.
@larry-scott Shared Solutions for DL3595 and DL 3598 say observed 2020-12-02
@larry-best
good to know.
I agree with Lee’s deduction. That is about the shift I have seen between NAD83(2011) and the previous datum correlation.
Is this just a subset of the data shifted? I once found a string of street furniture all shifted that the crew collected, about 100% sure it was a bad fix, relative to themselves they all fit. Bad RTK fix for a bit, looked good, ya know good to a few hundredths +/- a meter 🙂
SHG
Log in to reply.