iGage
X9x User Manual
page 39 of 53
Building, Loading and Using a GSF (GEOID Separation File)
GPS receivers measure Ellipsoid Height. Users typically need Orthometric Heights.
The GEOID Separation File (GSF) is a file organized as a grid of height differences. Field data collection tools like SurvCE use the GSF
file to derive an orthometric height at a Latitude - Longitude - Ellipsoid height from the ellipsoid.
It is very important that you survey with a GSF loaded for moderate sized jobs. If you don’t, water will not necessarily flow downhill
and fields will not be leveled for irrigation purposes.
Every three to five years, the National Geodetic Survey (NGS) produces a new GEOID realization for the United States. The most
recent GEOID is Geoid12A.
The GEOID file (actually it is not a single file, but a group of files) for the United States is too large to load into data collector
memory (when using SurvCE and FAST Survey) at once, so it is necessary to extract a portion of the GEOID for field use. A typical
maximum file size is 400 Kbytes, while the full GEOID model is typically 40 megabytes for the United States.
GEOID Tools
Carlson SurvCE includes a Carlson XPort license number. You can download the latest XPort tool here:
http://www.carlsonsw.com/support/software-downloads/?product=SurvCE
Typically the XPortSetup tool is button number 1:
The Carlson XPort tool automatically downloads needed grid files from the internet as needed. Thus it is important to have internet
access when building GEOID files in an area for the first time.
If you are using Spectra Precision FAST Survey, or your computer is not online, you can use the Ashtech Geoid Extraction tool. In
addition, the Ashtech tool can be used to visualize the GEOID subsets that you create:
The Ashtech Geoid Extraction tool can be downloaded with this link:
http://ashgps.com/mirror/master/GEOIDS/Install_Geoids.exe
The full Geoid12A for use with the Ashtech tool can be downloaded with this link:
http://ashgps.com/mirror/master/GEOIDS/Install_GEOID12A_RevC.exe
Building a GSF file for a Project:
1.
Start the Carlson XPort tool using the desktop icon:
2.
From the main menu of the X-Port tool, select
“Tools: Geoid Manager”:
3.
Choose the correct Geoid. If you are in the United
States, the current correct GEOID is ‘USA (Geoid
12A)’, however other geoids are available for use if
you need to match an older job.
4.
Enter the center of job in Dd.MmSsss; select an
appropriate size (I prefer 125 miles):
5.
Click OK, the tool may need to go online to
download the base geoid files, then choose an
appropriate location for the output file. I prefer to
use the Geoid name, followed by the State,