Hello all,
I have a question in regards to a large geodetic network, but first let me give a little backstory to the project.
We are?ÿusing the trimble system here. TBC, R10 and TSC3 data collectors W/ Trimble access.
We are processing baselines and tying our network to known NGS control in the area.
Our r10's collect in WGS84 and the NGS points we want to use are in NAD83 2011 epoch.
My question and concern comes from our project settings/coordinate system set up. Our project was initially set up with NAD83 Conus, but now we are second guessing ourselves. With the raw data coming in being WGS84 but the NGS sheets being in NAD83, does TBC take into account the different datums and?ÿadjust for that? Will the network adjustment make this issue moot and bring everything to the NAD83? Is there a different datum I should be looking for?
We have ran two different iterations of this project, one with the initial datum of nad83 conus and another with ITRF to nad83 2011 and all the coordinates from nad83 conus match much closer to the NGS data sheet.
Thanks for any help
Your R10??s do collect in WGS84, however if you have a defined coordinate system set in your project settings (i.e. NAD83 state plane, Colorado, central zone 0502) those WGS84 values will automatically convert to a Northing/Easting coordinate value. ?ÿAre you setting up on a base station on one of the known NGS points, or using a VRS / RTN network for your survey work? ?ÿ
Also, what are your project seatings in the data collector?
Unless your project is VERY large (in area), you will not see any noticeable difference using either as your processing datum.
That said, I like to always "seed" my processing with an ITRF current epoch coordinate. I submit an occupation to RTX, and then enter the resulting ITRF current epoch coordinate as a global coordinate (add coordinate...). I first process with autonomous coordinates (i.e. don't enter anything), then I enter in one station as ITRF (from RTX), fix it, do a free adjustment, then reprocess. I always have the datum set to WGS84. Not sure about TBC, but a lot of software treats WGS84 as equivalent to NAD83 (i.e. shift=0,0,0). In reality there is justa very very small rotation, but it only makes a difference in spatially large network, and even then very small.?ÿ
I do some networks that cover tens of thousands of square miles, and smaller ones as well that might just cover a few sq miles.?ÿ I just make it a habit to always do the above two step process to be consistent.?ÿ?ÿ
?ÿ
Given that the original poster expresses confusion about the difference between WGS and NAD83, I wonder how well he understands the various iterations of both WGS84 and NAD83?
There is a nice non-official tabulation of the versions of WGS84 on this site here:?ÿ https://surveyorconnect.com/community/gnss-geodesy/itrf20052008-0-grs80/#post-486426 ?ÿLikewise there are a number of versions of NAD83. The poster mentions he wants coordinates in ??NAD83 epoch 2011.? This is the datum tag reflecting the most recent nationwide adjustment. As the adjustment did NOT include non-GPS observations, there are points with data sheets that have other datum tags.
The reason for the many versions of WGS is to insure consistency between WGS and the International GNSS Service orbits. Their goal, if I remember correctly, is to maintain consistency at the better than 5 cm level.
?ÿLike Mr Hamilton, I process data in the current version of the ITRF/IGS (in my case using CORS or IGS sites), in order to consistent with precise IGS orbit data. While it is possible to access NGA precise orbits from their site, I do not use them. I have yet to perform the comparisons I intended.?ÿ
As you use a commercial software package that I no longer have access to, I cannot tell you how it deals with data in different reference frames. As I recollect it was possible to use IGS precise ephemeris when processing data in TBC, I wonder how it treats the differences between ITRF and NAD83. You risk initiating a wrong transformation if you lie to the software (if it knows what it is doing).?ÿ
As far as ??collecting data in WGS84?, what does that mean? ?ÿAre you using point positioning? If you are using differential GPS with respect to a reference site, your baselines are determined with respect to the reference site. I do not know of a source for WGS84 reference sites. While I belabor the point, it is important to keep the details and nomenclature straight.
In closing, using good quality NAD83(2011) control points as reference sites in both data reduction (GPS observations to vectors) and adjustment will yield NAD83(2011) coordinates. I do not see any reference to using NGS or other continuously operating GPS sites in your project.
As Mr Hamilton also states, the adjustment process should be iterative starting with a ??free? or ??minimally constrained? adjustment which validates your work. Performing work along the lines of the NGS ??Constrained Adjustment Guidelines? would be my recommendation.?ÿ
I have added a map showing NGS CORS sites in the Las Vegas, NV area. I also include one of my favorite cartoons.
Good luck with your project,
?ÿ
DMM
Thank you everyone for your time. Although my question was poorly written, I got my questions answered through all of your responses.
We are collecting?ÿbaselines via fast static observations and simply bringing in the t01 and t02 files into TBC for baseline processing
The project is 100 x 90 miles and contains over 500 baselines and 160 points with 16 of those being NGS points with the NAD83(2011) epoch.
I have already performed a loop closure analysis getting the horizontal down to 1 ppm and the 3d to below 2 ppm. I have finished with the minimally constrained adjustment and was working on the fully horizontally constrained adjustment when the question came up.