[osg-users] [vpb] osgdem and HAE/MSL

[Nick Modly]

Hi,

When providing elevation data to osgdem. is it expected to be in HAE (height above ellipoid), or MSL (mean sea level), or can it tell from the input? For example, I am providing elevation data in the AIG/Arc/Info Binary Grid format (.adf). The data represents the elevation of the area in meters MSL. However, when I osgpick a point and convert it to Lat-Lon-Alt MSL, the altitude is off by about 30m (the difference between HAE and MSL at this location).

This are the options I ran osgdem with
$ osgdem --geocentric -o output.ive -l 8 -d ./path-to-dted/ -t image.tif

Is it possible to specify a vertical datum or coordinate system, such as EGM96?


Thank you!

Cheers,
Nick

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[Glenn Waldron]

Nick, Shayne,

I have not looked at VPB in a while, but I believe it expects height values to be referenced to the WGS84 ellipsoid. If that's the case you'll need to convert it first to HAE.

By the way: DTED heights are referenced to the EGM96 geoid (not to the WGS84 ellipsoid). So they need conversion in order to be properly displayed. GDAL (up until recently?) does not convey this information so there's no way for the application to know. You just have to know :)

Glenn Waldron / @glennwaldron

On Thu, Sep 12, 2013 at 1:45 PM, Tueller, Shayne R Civ USAF AFMC 519 SMXS/MXDEC <Shayne....@hill.af.mil> wrote:

Nick,

Osgdem will work with the input elevation. It will build the database based
on the input source. For example if you're using DTED and the geocentric
option, it will place the point at the specified elevation (in meters) above
the ellipsoid. The point is represented in the Cartesian ECEF geocentric
space (e.g. x,y,z).

The default ellipsoid used is WGS84. You can specify the polar and
equatorial radii  in osgdem to fit to another ellipsoid. Just make sure you
specify those radii in your EllipsoidModel class on the rendering side so
that things match up. The EllipsoidModel class also defaults to WGS84...

HTH

-Shayne

[Tueller, Shayne R Civ USAF AFMC 519 SMXS/MXDEC]

Glenn,

VPB does reference to the WGS84 for the --geocentric option.

So to do the conversion, do you have to remove the contribution of the geoid
(gravitational equipotential surface) from the raw DTED first before pumping
it into VPB? Is there an application that does this?

Thanks,

[Tueller, Shayne R Civ USAF AFMC 519 SMXS/MXDEC]

Nick,

Osgdem will work with the input elevation. It will build the database based
on the input source. For example if you're using DTED and the geocentric
option, it will place the point at the specified elevation (in meters) above
the ellipsoid. The point is represented in the Cartesian ECEF geocentric
space (e.g. x,y,z).

The default ellipsoid used is WGS84. You can specify the polar and
equatorial radii in osgdem to fit to another ellipsoid. Just make sure you
specify those radii in your EllipsoidModel class on the rendering side so
that things match up. The EllipsoidModel class also defaults to WGS84...

HTH

-Shayne

-----Original Message-----
From: osg-user...@lists.openscenegraph.org
[mailto:osg-user...@lists.openscenegraph.org] On Behalf Of Nick Modly
Sent: Sunday, August 25, 2013 11:46 PM
To: osg-...@lists.openscenegraph.org
Subject: [osg-users] [vpb] osgdem and HAE/MSL

[Glenn Waldron]

Shayne,

Correct, for each height value in your DTED grid, you have to sample the EGM96 geoid at that point and add the two together. That will give you HAE. I don't know of any tools off the cuff, sorry. osgEarth has vertical datum support so you could probably write a tool using that.

Glenn Waldron / @glennwaldron / osgEarth

[Tueller, Shayne R Civ USAF AFMC 519 SMXS/MXDEC]

Good stuff to know. Thank you.