I've taken many PMs on how to convert .RSO clutter files and have them import into Atoll correctly.
There are other ways to do this but this suits my work.

Problem is mainly when Global Mapper exports to Vertical Mapper Classified Grid format, it loses code 0 which is "Open" clutter code. I assume this is because 0 is treated as nodata.

To get around this, in Global Mapper, double click on the .SOL layer and on the Alter Elevation Values tab, set Offset to 1 meter. Now there is no "0" code.

Create a text file with the following content and save it as Clutter_palette.pal
192,192,192,"Open"
255,127,127,"Village"
0,124,255,"Suburban"
0,249,255,"Urban"
0,255,136,"Dense urban"
79,174,0,"Forest"
0,0,255,"Hydro"
255,255,0,"High urban"
11,220,11,"Park/wood"
255,0,0,"Building"
124,77,33,"Rail"
220,150,80,"Road"
99,150,50,"Airport"
255,0,255,"Tunnel"
144,162,181,"Rural"
255,0,0,"Building plaster"
255,0,0,"Building brick"
255,0,0,"Building glass"
255,0,0,"Building wood"
255,0,255,"Border"

Now export the .SOL layer to Vertical Mapper Classified/Clutter Grid File format and on the GRC Options tab, set Palette to "Custom Palette from file" and select the file name Clutter_palette.pal. This will embed all class codes and names, including open into the clutter file.

It should import into Atoll with the correct class codes and colors that match HTZ.


Another common error is using different projections. Make a note of the projection used in HTZ and ensure Global Mapper has the same projection for the imported maps. The Global Mapper configuration projection also needs to be the same as that in HTZ because exports use the configuration projection.
Lastly, ensure Atoll is set to use the same projection before importing files.