Change projection for LAS file

I found a minute to dig into this. We don't quite know what your specific file contains, but here's a quick and dirty way to update the GeoKeyDirectoryTag VLR (page 11) using values as described in the geotiff spec.

This does not currently account for the storage of individual projection parameters in a separate GeoDoubleParamsTag or GeoAsciiParamsTag record. I don't have any example data on hand that contains those params, so if you need help extending this I might need you to post something for me.

This updates the file in place, so be warned.

import laspy

src_path = r"C:exampletest.las"

# see here for possible values:
# http://geotiff.maptools.org/spec/geotiff2.7.html
replace_keys = {
    3072: 26912,  # set ProjectedCSTypeGeoKey to NAD83 / UTM 12N
    3076: 9001,   # set ProjLinearUnitsGeoKey to meters
    4099: 9001    # set VerticalUnitsGeoKey to meters
}


def update_geo_keys(path, lookup):
    with laspy.file.File(path, mode='rw') as f:
        for i, vlr in enumerate(f.header.vlrs):
            # search for the GeoKeyDirectoryTag record
            if vlr.record_id != 34735:
                continue

            modified_body = []
            j = 0

            # loop over geo key entries, stored in groups of 4 values
            while j < len(vlr.parsed_body):
                key_id, tag_loc, count, value = vlr.parsed_body[j:j+4]

                if j > 0 and tag_loc != 0:
                    raise NotImplementedError('modification of projection '
                                              'parameters not implemented')

                # update the value for this key ID, if necessary
                try:
                    value = lookup[key_id]
                except KeyError:
                    pass

                # maintain a new list of values to be written
                modified_body.extend((key_id, tag_loc, count, value))
                j += 4

            # update the vlr with our new values, quit searching
            vlr.parsed_body = modified_body
            break
        else:
            raise ValueError('no GeoKeyDirectoryTag found in file')

        f.header.save_vlrs()


update_geo_keys(src_path, replace_keys)