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When you export heightmap from L3DT in ASC format it comes out with header like this:

ncols         4096
nrows         4096
xllcorner     0.000000
yllcorner     0.000000
cellsize      10.000000
NODATA_value  -9999

This format is acceptable to terrain builder even though it uses the easting 200000 and northing 0, these 0.0 values are acceptable.

First line ncols is columns in the heightmap grid, err actually I cant remember which way they go, but this is one part of the heightmap grid ;)

Second line nrows is the rows, otherwise same as above ;)

Third line xllcorner and fourth line yllcorner are the real world data coordinates, this is how Global Mapper, QGIS etc sofware will precisely position this heightmap. Only difference in Terrain Builder is that it uses make belief easting 200,000 and northing 0 values.

Fifth line cellsize is same as Terrain Builder Cell Size, easy.

Sixth line NODATA_value is for ocean surface elevation, this… umm err as I'm typing this text in 2024-08-04T09:51:00Z I honestly cant remember which way it was, that -9999 is the default value for ASCII Grid heightmap format, it means… something I cant remember right now, yes default value but “WHAT DOES IT MEAN!?” I cant remember heh. There was some trick I used to remove ocean in global mapper using this value but honestly its buried somewhere in PMC ArmA 3 Terrain Dev Diaries and I have to look it up, I'll try to get that info soon, remind me on PMC Tactical Forum if you're reading this text. PMCTODO find the info and add to this paragraph

ASCII Grid heightmap file extension is .ASC, for example my_cool_heightmap_file.asc or so.

If you import heightmap that is in wrong coordinates for example 0/0 which is default L3DT ASC export coordinates, then you cannot fix the .ASC header and refresh from source because Terrain Builder does NOT refresh the coordinates, just the elevation data. In such case you have to adjust the coordinates from the properties dialog.