Accurate GPS receivers have revolutionized Mobile GIS, allowing field crews to capture horizontal positions with sub-meter, sub-foot, and even centimeter accuracy in real time. However, capturing accurate elevations, in a way that can be used practically, is still lacking in most Mobile GIS software.
Incoming real-time corrected GPS elevations are based on an ellipsoid (typically GRS 80).
The problem is, we need orthometric heights, or heights above mean sea level, in order to work consistently with other high accuracy elevation data.
High-precision datum transformations available in most Mobile GIS software applications handle the horizontal component of the position (or real-time correction services handle the horizontal transformation before transmitting corrections).
But what are we to do about the elevation component?
Fortunately, high resolution geoid height models, which allow us to convert elevations from ellipsoidal to orthometic (mean sea level), are available from the NGS in the U.S. and various other agencies in their respective countries worldwide.
Mobile GIS software applications that can take advantage of centimeter-level GPS and handle the geoid model transformation on the fly for you result in useful elevations stored in the Z value of your captured feature coordinates.
So, which Mobile GIS applications support both centimeter-level GPS and handle geoid height transformation on the fly, and which fall short?
Here is a short list of the most popular Mobile GIS applications. If you found any other solutions that I missed, please comment! :
|Software||Provides real time orthometric height for cm-level GPS?|
|Esri ArcGIS Mobile||No|
|Esri ArcPad||Yes (via GeoBullseye extension)|
|Trimble TerraSync||Yes (requires TerraSync Centimeter Edition and Trimble GPS receiver)|
Note: There are many published articles that explain ellipsoid heights, orthometric heights, and geoid height models in great detail. Therefore, I focused this blog post on which Mobile GIS software options are available for you to capture accurate, useful GPS heights in the field. Please see the following articles for an in-depth explanation of the underlying concepts: