When setting the base up at unknown point and setting a point for a localization with the rover using n5000,e10000,elev=100 as explained in a TDS manual for property survey measurements, I am wondering how far away from the base can you go while keeping the elevation component accurate. I know how to perform and understand other types of GPS surveys, but just to do a rough topo I am wondering if my process mentioned above would be accurate and if the elevations would be relative to the first point I localized on assuming the above mentioned coordinates.
Modified By gary kuroski on 7/23/2001 at 8:54 AM

Gary,

It's difficult to provide an exact distance from the RTK Base.
Several factors can influence this distance.

In my opinion, the biggest factor would be the topography (change in elevation) of the job site. Due to discrepancies in the Geoid Models and Tilt of the Vertical.

The flatter the project site, the further I would trust the assumed elevation at the RTK Base. (1-3 Miles) Generally, if there's not too much change in elevation over the project site, an assumed RTK Base elevation will work OK.

On the other hand, if the project site has a lot of elevational change, I would limit the distances I travel away from the RTK Base station.

I try to encourage Customers to work with a somewhat realistic elevation for the RTK Base station. This can be obtained from a topgraphic Quad sheet if you have nothing else to use.

If the project site has an MSL (ortho) elevaton of say 3000 ft, and you use a assumed elevation of 100 ft, this can adversely effect the resulting elevations.

A general rule of Thumb:
For every 20 meters of elevational error, you can add about
1 ppm of error. Gross elevational errors can corrupt the Horizontal coordinates. Same rule applies in Post-Processed static network adjustments.

Basic RTK Rules:

1 point localization, Scale Factor = 1, Rotation = 0

2 Point localization , Scale Factor = 0.9999xxxxx, Rotation = xx.xxxxxx

3 Point localization = more realistic Scale & Rotation, but N/A on Residuals (Control ties)

4 Point localization = Best possible fit
Solves for Scale, Rotation & generates Residuals on project Control.

Bob LeMoine
Ashtech Precision Products

Bob thanks for the input. I kind of thought that was the answer. Yeah the terrain was relatively flat and the ellipsoid ht. was like 20 or something,(the ellipsoid ht. is shown in meters on the Z surveyor) so I felt the z factors were acceptable. I was really impressed with the Pac Crest Radio on this job site. It was set up in an open cornfield but was surrounded by dense vegetation and I had no problems. Radio reception never dropped below 95%. It was the first time I did an arbitrary localization. I checked in to the base at the end and the coords were dead on. I will keep the advice about the aassumed topo elevation for the future. I noticed my Husky doesn't have a Geoid file loaded, There must be a default file that the receiver uses to calculate elevations or do I need to load one for South Carolina. I have never loaded a Geoid file to the Husky FS3 and would like to complete this process once just to see how to do it.Thanks again. Always something to learn with this type of Surveying.