[GRASS-user] Importing DEM data with different projections

Rich Shepard rshepard at appl-ecosys.com
Wed Sep 18 05:41:42 PDT 2019

On Wed, 18 Sep 2019, Roberts, David wrote:

>    I would guess that the contractor is trying desperately to make the
> Lidar data as directly vertical as possible and avoid oblique paralax.


Interesting. I thought that the flights-acquired data were processed by the
contractor to remove paralax prior to submitting results to the agency.

>    I think you have no choice but to re-project one set. Unless you're
> embedding them in a much larger study I don't think I would reproject them
> both to a state-level projection, but choose one of the two they use and
> re-project the other half. One is centered at 46 degrees and one at 45.5,
> so I would choose based on which latitude is more central to your study.

Now I know the value of r.import my work flow is to use r.in.gdal on the
first map for that quad (there are 2-5 maps per quad) and r.import on the
others. This results in all imports having the same projection as the first

The next step is to reproject these data to the state location (which
accumulates new data by project and makes all available to other
site-specific projects). Now all DEM maps have the same projection.

Finally, I reproject to the project location which seems to have slightly
different cell counts for columns and rows although the projection is the
same statewide one. Shrug. Now I can work with these in the appropriate
computational area.

Thanks for your insights,


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