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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

GeographyPage.jpg

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MikeLiska

Eric B - Do you have an example of how to create a model that could use a Geoid Raster to calculate the Orthometric Elevation of existing points based on their X,Y? I am trying to come up with some type of automated way to calculate the Ortho Elevation instead of only having the Ellipsoid.

Thanks,

ericbock

If you have entered a geoid seperation value inside of ArcPad and are using Trimble's GPSAnalyst to checkin and process the data, then it reverts back to HAE. You can recalculate MSL using an adjust z-values tool in arctool box. Keep in mind that this Geoid separation value you input in ArcPad or ArcGIS is for one specific location on earth. THe software should really support the full geiod model to work properly. Go to the Esri ideas page and vote for this support. Alternatively, you could create a model that would use the geoid as a raster, grad the geoid seperation values based on an xy location, then adjust your elevation. Keep in mind that vertical elevations on several mapping grade GPS units is typically 2-5times worse than the horizontal accuracy, unless you are using GPS units that get better vetical accuracies.

Eric b

ISAAC_GUTIERREZ

Im using arcpad 10, and everytime i add XY fields i get weird z-values (using Add XY tool). I added the Geoid and called Esri but i think they didnt understand my questions these points should be in the 122 -133 range. If i go to arcpad and look in the Geography section on my trimble it has the correct values... is there wa way to IMport the Geography information from arcpad? Yes i have a Z values enabled.. attached is a pic of how my values look when they import!!

ZValues.jpg

PatrickMurphy

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.

Can you give details on how this is done? I have xy point data and no z values from a shape file I created in Arcpad.

Thanks,
Pat

ZoltanKovacs1

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?

Well, there is a constant used to be estimate your Z accuary, which is 1,5-2x. That means if you can get 50 cm horizontal accuracy for a point, its vertical accuracy is around 0,75-1 m.
However, to capture XYZ coordinates in a Shapefile, you need to enable the Z value.
Anyway, for postprocessing you should use GPS analyst for ArcGIS Desktop or Pathfinder Office (and GPScorrect for ArcPad to log data for PP). These output 3D data to geodatabase/shapefile...
In addition, be sure that you DO NOT change the GPS Datum (in GPS Preferences dialog) from WGS 84. Our users often do this, and they are wondering why they got all the coordinates 90-100 off site. ArcPad automatically does the on-the-fly conversion to your system - if the correct coord. sys is set to your Shapefiles.

Hope this helps.

RolfBroch

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

Start editing your point layer, click the sketch properties button (rightmost button in the editor toolbar) and then double-click the point using the edit tool.

Rolf

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.

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.

JerryBiedenbender

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

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).

NewShapefile.jpg

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

JerryBiedenbender

that should do it, thanks.

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

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

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

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