Transforms LiDAR from ellipsoidal to orthometric elevations using a grid in format that is usually provided by the national survey and expresses the vertical differences between some geoidal model and some elliptical model of the earth.
By default the conversion is from ellipsoidal elevations to geoidal (or orthometric) elevations. If you want to go the
other way, that is your input has geoidal (or orthometric) elevations and you want to compute ellipsoidal elevations, you need to add the switch ‘-backward’ to the command line.
Unless your point cloud input is already in longitude/latitude, the input LAS/LAZ file needs to come with projection information in the file. Alternatively it can be provided in the command-line. The GTX format defines the vertical difference grids using a longitude/latitude representation so that all LiDAR points need to be converted (temporarily) to longitude/latitude in order to look up and interpolate the difference value.
The default uses a bilinear interpolation (which we recommend), but you could request a bicubic interpolation with ‘-bicubic’.
The output projection will be identical to the input projection as only the z coordinate of the points will be adjusted using the difference grid.
See README file for more information.