GPS Vertical Datum

[JasonTipton]

We are collecting 3D points in ArcPad. When I set the 3D points up, I was worried about 500 other things and didn't set up a Vertical Datum. What is the default vertical datum that ArcPad uses or does the default vary on the map units or the XY coordinate system?

Any other information about Z datums would also be appreciated. Basically, I want the same datum that you would find on a topo map.


FYI: ArcPad 10 SP1

[BrettBlack1]

It may depend on what GPS unit you are using and the correction source, but GPS uses ellipsoidal heights for the vertical datum. To get to orthometric heights you would load a geoid model or have a vertical datum defined. To get from ellipsoidal height to orthometric height you could use a tool like Corpscon or VDatum from the NGS.

[AlexMahrou]

Brett is correct.  Also, you will be needing MSL Heights if you are looking to reference apples to apples between USGS topos (post 1975 please) and your GPS Data.

Vertcon will be your best bet.  Someone would be rich if they wrote an extension that would perform vertcon tasks completely within ArcGIS.:D

[BrettBlack1]

If I remember right, VERTCON is not capable of converting a height from ellipsoidal to orthometric. It is to be used for taking orthometric heights in NGVD29 to NAVD88.

[AlexMahrou]

Brett, you are right again!

Jason, you can use Corpscon to perform the conversion.  Here's how I've done it in the past if I remember correctly.

1.  Populate your columns of the feature class (or shp) with x,y,z values.  (Data Management tools)
2.  Export the table out to a comma delimited format. 
3.  Download Corpscon (get the one that includes CONUS): corpscon_conus_geoxx.exe around 84Mb
4.  Start Corpscon and set it up correctly.(see image)  If you are converting from WGS84, you will be using the setup from below...

5.  You can convert one point at a time, using the manual conversion (which is no fun), or you can actually feed that comma delimitted table into the "convert user defined data file" dialog.  (see below).  You might have to play around with the parameters to get it to accept your data file.  The easiest way is to make sure that your values are in DD. 

[JasonTipton]

Thanks guys, that will help a lot.

[GrahamW]

I don't know how well this Australian based method will work in the US.
I haven't had to do this for lines and polygons....

I created a TIN of my local geoid. It was fairly easy as its just an ascii XYZ file.

in arcmap(arcinfo) I then subtract that height from the point data to give accurite local heights

[ShawnBenteau1]

I agree that ESRI needs to step on board with a truely 3D coordinate transformation. We always survey infrastructure to cm level utilizing RTK but have the problem of trnasforming the collected ellipsoidal heights to orthometric. Not a problem for points, but for lines and polygons is a major pain. We have a workaround but this fragments our workflows immensely. All we need is the option to input a geoid model into Arcpad to do this computation on the fly. Seems like every other mobile survey software can do this, just not in the context of GIS collection utilizing attributes as well as geometry.