Solutions Geoid Model
Posted By Terry Strickland on 1/12/2002 at 9:36 AM

Which geoid model should I be using for locus in ashtech solutions? I found the different models give a different elevation for non-control points, but do these affect the x/y coords as well? I missed a benchmark by 0.28' using the default worldwide model. My elevations are not critical, but I hope I don't have to re-adjust all the work that I've done for horizontal control as well. Any help would be appreciated.
terry strickland


Geoid99 in the USA
Posted By Phil athome on 1/12/2002 at 4:02 PM

The best Geoid model for use in the USA is Geoid99.

Vertical errors do affect horizontal accuracy.

It does not always make you wrong - just different.

If you want the long version let me know. I have some nice graphics I can send you.

Or come to one of my seminars. The Geoid model is one of the topics.

http://www.landsurveys.com/


Re: Solutions Geoid Model
Posted By Bob LeMoine on 1/13/2002 at 6:08 PM

Solutions supports most of the common Geoid Models. Solutions selects the appropriate Geoid Model depending upon the combination of Horizontal Coordinate System and the Vertical system (ellipsoid or Orthometric) selected by the User.

To access the available Geoid Models...
Project | Settings | Coordinate System...

If the User selects "Geodetic" has the Horizontal System Type, and Orthometric for the Height System - only the EGM96 Worlwide Geoid Model is available for selection.

If the User Selects "GRID" as the Horizontal System Type, then further selects: "State Plane Coordinate 1983"
and Orthometric for the Height System ,many more Geoid Models become available for selection by the User.

Examples will include:
EGM96, Geoid96, Geoid99 and the GSD Geoid Model for Canada.

Geoid96 and Geoid99 are Models for North America, Mexico and the Caribean. Thes models only have Gravity data for North America.

EGM96 is a World Wide Geoid Model, works all over the world.


Modified By Bob LeMoine on 1/13/2002 at 6:10 PM

Makes a difference
Posted By Terry Strickland on 1/13/2002 at 9:09 PM

Using the Geoid99 I'm hitting benchmark posted elevations within 0.02' and better as apposed to tenths with the EGM96. Very little difference in horizontal coords in the state plane regardless of geoid model. Thanks, everybody, for the input.
terry


Re: Solutions Geoid Model
Posted By Paul Michael on 1/13/2002 at 9:17 PM

Using the Geoid99 model myself. Elevations have been real good. My first GPS survey with the locus recievers I set an elevation on a control point for the base map survey. I checked the elevations by running a 1 mile level loop from the control monument used in the GPS survey to check the elevation for my own benefit. I nailed it to the hundreth.... I was impressed.

Subsequant surveys have revealed a couple of hundreths either way when checked with existing control.

Paul Michael Va LS


Re: Solutions Geoid Model
Posted By Dave Doyle on 1/14/2002 at 8:35 PM

Don't use EGM96 in the States. Differences between EGM96 and GEOID99 can easily approach 1 meter in the lower 48. EGM96 does not include the thousands (6000+) GPS on bench marks used to fix GEOID99 to NAVD 88. Also, watch for GEOID02