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question for survce users
I do not post much but do read every day. Enjoyed the posts and have learned some things. Been a PLS here in NC for over 30 years. My question is under Job Settings, the System Tab, Edit Projection List, under name I’m using USA/NAD83/NC. If view is selected the EDIT/VIEW SYSTEM screen is displayed. There is a tab to Edit/View Datum. When this tab is selected then the Define System screen displays. It of course it shows the parameters of the Nad83 etc. There is a checkbox at the bottom of the screen (GSF File is relative to WGS84). My question is using Nad83 , the North Carolina VRS should this checkbox be checked or not checked. Survce came with this checked. I have collected several GPS CONTROL monuments with it checked and unchecked. Results show very little differences in x,y,z. I understand about geoids and the Grid, just wondering about this checkbox. Called Carlson support on this and the help guy said he had never seen this screen.(Hum) Just wondering what you fellows that use survce have to say? My version is 5.07 10/18/17
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4 Replies
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I’ve always left it as “GSF file relative to WGS84” and not worried about it. My reasoning is that there is only a few metres difference between WGS84 and NAD83 and if the geoid separation is such and such “here”, it’s not going to be any different 2 metres away. So I don’t think it matters if the software thinks the lats and lons are WGS or NAD. I wouldn’t expect it to make any difference to be honest.
I’m assuming, when geospatial agencies create geoid files, they are using WGS84 as the default reference frame.
You could wonder why, for a North American geoid, they wouldn’t use NAD83? And I don’t know why.
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Every GPS receiver I know about collects basic system data in WGS84.
Being a flatlander, I only use WGS84 and convert into other systems when the client requests a certain system.
When the data is downloaded, it is converted into the system that you have chosen.
With RTK, the output is in whatever system you have chosen to work in based upon the control you have chosen.
0.02
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If you are only reporting distances between points, then WGS84 versus NAD83 is of little concern because that couple of meters difference will be essentially constant over any local project (not so for pipelines).
If you report coordinates, NAD83 on a point will stay pretty close to the same for a long time, whereas WGS84 will show a measurable difference in a very few years. The North American tectonic plate moves the two apart by about 2 cm per year for me.
There is also about a meter difference in vertical in the middle of the USA, so be careful about how you convert to NAVD88.
If I couldn’t figure out what was going on in the equipment, I’d go to a monument with NGS data sheet and see how well the numbers agreed.
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Thanks guys, To Bill, I am aware of the tectonic shift and the meter. I did exactly as you said a several different times on a NGS Monument which is less than a mile from my house. I found these results compared to the published:
Published NAD83 SPC NC Coords- NORTH 753,607.44 EAST 1,634,109.92 sFt, Ortho Height: 698.06 feet (adjusted). Geiod 12B vert order Second -Class II
Mine with the checkbox off : -NORTH 753,607.4294 EAST 1,634,109.9395 sFt, Ortho Height: 698.12 feet Geiod 12B
Mine with the checkbox on : -NORTH 753,607.4281 EAST 1,634,109.9476 sFt, Ortho Height: 698.14 feet Geiod 12B
I get this same difference in ortho height about 0.06′ to 0.08′ every time I observed it compared to the Published
Now this is the VRS- 180 epochs back to back with no time in between, a cors station is just across the road from this monument, in fact this is a reference station for the cors station. Seems to me the checkbox makes little difference in the horizontal, the vertical bugs me. This difference in vertical between mine and the published is still close to the 2cm range.
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