I have been attempting to use M9's merge function to combine irregular lidar mosaics but have run into a problem with no data bounding box tiles having a pixel height value of zero rather than null. In the M9 screen grab below the irregular lidar layers appear fine with no data bounding box pixels being invisible as expected:
Note that the brown layer is on top of the green/blue layer.
If I then merge the two images in M9, the problem becomes apparent in the resulting image:
The stepping around the image (seen in the south) are M9 image tiles that comprise a mixture of some data and some true null pixels, it seems that only full no data tiles making up the image bounding box get set to a value of zero.
Next I tried exporting from M9 to a number of offered surface export formats to see if exporting one of the irregular lidar tiles would be transparent across the no data portion of the bounding box. I tried ESRI flt/hdr, Geotiff, Surfer Grid and ESRI ASCII grid and all exported the M9 image tiles with partial data (the stepping) correctly with some data and null pixels, and the remainder of the bounding box tiles with zero.
M9 ESRI flt/hdr Export in Global Mapper. The red bounding box pixels have a value of zero.