Change VERTICAL datum using pre-calculated Raster?

[guy.mcwethy]

Hi Martin,

I have been trying to change the vertical datum of some older LAS files.

I have successfully changed the datum on the DEM surfaces derived from the LAS files using VDATUM and a raster array of correction factors.

(well enough that the elevations now match within the accuracy of the LiDAR data, anyway)

Would it be possible to apply these same corrections factors, as a raster grid, to all the LAS points as well?

This would seem to be MUCH faster than trying to calculate the actual corrections for each LAS point, and just as accurate.

Whatcha think?

Guy McWethy

WA Geological Survey

[Martin Isenburg]

Hello Guy,

yes. I'm not sure how closely spaced you need to have samples that express the different in elevation between two vertical datums (e.g. from elevations in respect to an ellipsoid to elevations in respect to a geoid) but having a sufficiently dense set of samples that are then linearly interpolated is certainly the preferred option - and I think Kirk Waters will agree - over putting each and every LiDAR point through the VDATUM software from NOAA.

https://vdatum.noaa.gov/

Once you have your set of samples (every 20, 50, 100, 500, 1000 meters, ... ?) you can store them to a LAZ file that you then run lasoptimize on (to bring it into a spatially coherent, compressed, and spatially indexed form). This set of external "ground points" whose z coordinate expresses the elevation difference between the two datums in the sampled x/y location you can then use as input to lasheight via the '-ground_points vdatum_samples.laz' option and perform a '-replace_z' operation to do the vertical datum change.

http://rapidlasso.com/lasheight

http://lastools.org/download/lasheight_README.txt

Regards,

Martin @rapidlasso

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[Kirk Waters - NOAA Federal]

Martin,

Your description of having a set of samples that are then bi-linearly interpolated to get the offset at a point is how VDatum operates. At least for the gridded vertical shifts such as the geoid models. Reference frame changes like NAD83 to ITRFXX are done with a Helmert transformation.

Kirk Waters, PhD                     | NOAA Office for Coastal Management

Applied Sciences Program      | 2234 South Hobson Ave
843-740-1227                          | Charleston, SC 29405    

coast.noaa.gov/digitalcoast