Z-Value

[JerryBiedenbender]

Hello,

   I have been struggling for a while with trying to collect an accurate Z-Value.

1. Currently I am checking out data from ArcMap to be used in ArcPad. I am collecting Accurate GPS data for a point layer. We are taking a tablet PC along with a Trimble ProXH into the field and collecting accurate coordinates. On the geography page of the attribute table while the GPS is collecting data, you can see XY and Z values are being generated...(See Attachment for a visual). When I hit OK the X&Y values stay saved in the attribute table but the Z-Value goes back to Zero. Is there something I need to change in the settings of ArcPad or the layer structure that will make the Z-Value save?

2. I have taken a notepad with me and wrote down a few Z-Values before hitting OK. These values are not very accurate, sometimes over 100ft off. Are there preferences or settings that I need to setup for Z-Value collection? The GPS and Antenna that I am using is collecting sub-foot accuracy for XY coordinates after post processing, so i figured it should be able to collect at least sub-Meter accuracy for Z-Value. Am i making false assumptions here?

Any help that you can provide to me would be appreciated,
Thanks,
Jerry

[BrettBlack]

1. Is your shapefile set up in ArcView with a Z value turned on? When creating a shapefile in ArcCatalog there is a box to check if you want the Z turned on, this must be done for the Z to be populated.

2. The simple explanation is the reason your Z is approximately 100 ft off is that GPS collects data in WGS84 latitude, longitude and ellipsoidal height.

Ground elevations are orthometric heights (Ellipsoid - (-Geoid) = Orthometric Heights/Ground Elevations.

In the GPS Preferences there is a place to enter a constant Geoid value. As of right now there is not a way to enter a geoid model to auto populate with true orhto heights or ground elevations as seen in survey type data collection software. I am sure you could develop a script to process this for you, but it would take some programming.

I hope this helps.


Brett

[JerryBiedenbender]

How do I figure out what my constant Geoid value is?

Jerry

[PeterAugello]

If you need to calculate the geoid separation at a single point, there are a number of geoid separation calculators available on the web.  For example:
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm96/intpt.html

You can also download raster data of the geoid separation
http://earth-info.nga.mil/GandG/wgs84/gravitymod/egm2008/egm08_gis.html

[JerryBiedenbender]

that should do it, thanks.

[JerryBiedenbender]

Once I check my data back into ArcMap I am able to use the (Add XY Coordinates) data management tool to grab the XY, but how do I grab the Z values? I cant figure out how to see them if i am not in ArcPad.


Thanks,
Jerry

[BrettBlack1]

If you are creating a new point shapefile in ArcCatalog, you need to make sure the Z value enabled. The picture attached shows where it is enabled (towards the bottom).

[JerryBiedenbender]

Okay, I can figure that part out. But how do I view the actual Z-Value values (Numbers) in ArcMap??

[BrettBlack]

If you have the Z value enabled on a shapefile and you are adding X,Y values in ArcToolbox, the Z value should be imported as well. If your originally created shapefile did not have the Z value, then a Z value will not be imported when adding X,Y data and you will have to create a new shapefile with the Z enabled.

The Help documentation in ArcMap outlines the procedure for obtain elevation (z) data.

[NenadPetrovic]

Hi,

Are you using a data entry form (apl, for example?). If so, you can have  Map.PointerZ in the "default value" field of a control, this will get populated for you automatically. Alternatively, you can use field calculator tools in ArcMap, after the field capture, to explicitly calculate Z values to a field that you designate for this data. Let me know if you'd like me to explain more.