[pdal] Setting SRS on las file

[Howard Butler]

> On Feb 6, 2015, at 2:13 PM, Matthew Hanson <mhanson at appliedgeosolutions.com> wrote:
> 
> 
> We have las files that have no spatial reference embedded in the file.   For one, I'm not sure it's necessary to actually add the spatial reference to the las file.  We want to create DEM's, so perhaps it's better to just leave them as is then set the SRS on the final GeoTiffs.
> 
> However, for the moment assuming that this makes sense, when I run pdal translate
> $ pdal translate -i input.las -o output.las -a_srs EPSG:XXXXX
> 
> I get a file that is bigger than the original because it's seemed to add 4 new dimensions: Time, Red, Green, Blue.   The original file did not have them, although I notice that lasinfo reports min and max for all those fields in both files, even though they are missing from the original.
> 
> Are these standard dimensions in las files, and is there any way to control if they are added or not?  

Control it with the "format" option, and set it to 0 if you don't want Time or RGB. It defaults to 3, which includes those dimensions. 

> $ pdal translate -i input.las -o output.las -a_srs EPSG:XXXXX --writers.las.format=0


>  If we want to add the SRS to files, I'd rather not increase the size of them all.  Although maybe this is moot since, as per above, it just doesn't make sense to add the SRS anyway.

It is probably easiest to leave the SRS alone until the final GeoTIFF write (and then set it with a GDAL VRT). The only advantage of setting it on the file before processing is so you don't have to do it in a later step.

Another problem here is setting the SRS is going to cause a rewrite of the entire file due to the VLRs needing to be modified. This is one entire extra i/o step that's not needed.

Hope this helps,

Howard