Knowledge Base

Category: SurvCE
Topic ID: 624
Title: NAD83 Base GPS converted to Ground
Created: 0000-00-00Last modified: 0000-00-00
This is in regards to using GPS Equipment where you would like to setup your GPS Base over a known published NAD83 

***PLEASE NOTE: To correct your elevations to Orthometric values please scroll down to the bottom of this article and open the attached file "Carlson Geoid Files.pdf"

I would recommend the customer setup the base over one of the Monuments and use “From New” / Enter Grid Coordinates and pick the Point number they are setup on with Nad83 Grid Coordinates from the Point List which would give him “Perfect Raw GPS” shots at his Rover.

From there if they want to scale to ground they can:
1) go into Equip / Localization / Points tab

2) tap the “Add” button

3) Localize to one Point:
a. At this point if they have the raw Latitude and Longitude for the Base position they can just pick the point number from the File that the Base is setup on then use “Enter Lat_Long” for the raw data and hand enter the Latitude Longitude and Ellipse values  

b. If they can easily get to a 2nd State Plane monument with known coordinates in the CRD file they are using they can simply tap “Add” pick the point number from the file then plumb up over the 2nd monument with the Rover GPS and do a “Read GPS” with 20 shots or so

4) Once they have added in one localization point from either the Base or Rover tap the Green check in the Top Right and save this localization file

5) Go back into Equip / Localization = make sure the System Tab is set to the correct Nad83 state plane zone

6) Tap the GPS tab then tap the “Read GPS” button to the right of “Scale Factor” this will take a Rover GPS shot and apply a Grid to Ground Scale Factor

7) At this point you usually need to set a Geoid File using the Attached PDF document at the bottom of this article "Carlson Geoid Files.pdf"

8) After setting the Scale factor and loading a Geoid File you can begin taking Topo Shots or doing Stakeout work.

This is sort of the best of both worlds between a Grid Nad83 coordinate and a Ground Total Station coordinate.   It basically puts them in a Fake Nad83 Ground system where the 1 point in the Localization file is a perfect Nad83 State Plane coordinate but all the other points are scaled to Ground so they will be slightly different then the State Plane Monuments. 

The good part about setting up the Base over a known Monument and using “Enter Grid” will be they can always re-process the Raw GPS data at a Scale Factor of 1.000 if they ever need NAD83 coordinate data for their Topo shots but still be storing all the Field work in a Ground System that can be used to setup a Total Station on and do total station work from.

Please Note:  If they don’t use “Enter Grid” or “Enter Lat_Long” to start the base then the raw data stored to the *.RW5 file by the Rover will all be based on a sloppy 8 or 10 feet off “read GPS” result and they will never be able to re-process the data to a 1.000 Grid Nad83 coordinate in the office.


Scale Factor Info.pdf