Geoid adjustment in the wrong direction using lasheight

[Gregory Phillpotts]

Hello,

I am trying to adjust a laz file to a geoid I have as a .txt using lasheight. I'm finding it is decreasing the z values where they should be increasing.

This is the command I am using:

lasheight -i output.laz -ground_points *.txt -replace_z -o AdjustedOutput.laz -olaz

The horizontal co-ordinates for both files are in Alabama West state plane in survey feet.

My geoid file is formatted like this:

1679200 226100 93.689
1679200 226400 93.689
1679200 226700 93.689
1679200 227000 93.689
1679200 227300 93.689
1679200 227600 93.689
1679200 227900 93.689
1679200 228200 93.689

I've tried adding a plus to the z values but git the same outcome.

Is there something causing the shift to be in the wrong direction? Is there a way to force it to the correct direction?

[Michael Perdue]

Hello,

The relationship between the geoid and the ellipsoid is given by:

H = h - N

Where:

H = orthometric height

h = ellipsoidal height

N = undulation (geoid height)

This relationship indicates that negative undulation values occur when the geoid is above the ellipsoid.

lasheight was principally intended to calculate the height above a surface that is typically ground. So when a measurement is above the reference surface, it has a positive value. This is opposite of the convention used with the geoid. If you want to use lasheight for applying a geoid, you must invert the sign of the geoid.

The better tool for applying geoids to data is lasvdatum. lasheight requires preprocessing your geoid and uses Delaunay triangulation for interpolation. lasvdatum takes the raw geoid, uses bilinear interpolation and allows both forward and backward transformations. lasvdatum will generate the same answers as the proj library where lasheight may not depending on where you are and the localized nature of the geoid. That said, the official interpolation method in Canada (and probably the US) is biquadratic interpolation, so you may see small differences between these tools and the tools provided by national geospatial organizations.

Cheers,

Mike

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[Gregory Phillpotts]

Thanks. 

Inverting the sign has worked and has given results which are suitable for what I need in this case. As lasvdatum is the better option I'll keep it in mind for the future.

Thanks again,

Greg