[GRASS-user] Importing DEM data with different projections

Roberts, David droberts at montana.edu
Tue Sep 17 21:01:17 PDT 2019

Hi Rich

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

    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.

    It's a pain I agree, but forcing it would lead to bad results.


On 9/17/19 11:41 PM, Rich Shepard wrote:
> The Oregon LiDAR data were acquired over several years under different
> contracts with the same provider company. These are available by 7.5"
> topographic quad names using the Ohio system. In the directory, 
> LDQ-45123G1/
> there are four input files.
> Using the GUI location manager I created a new location (G1) and imported
> the map 2009_OLC_Hood_to_Coast/.../hdr.adf. When I tried to import
> 2009_OLC_North_Coast/.../hdr.adf r.in.gdal complained that the 
> projection of
> the location did not match that of the new data to be imported:
> WARNING: Datum <Not_specified_based_on_GRS_1980_ellipsoid> not 
> recognised by GRASS and no parameters found
> WARNING: Projection of dataset does not appear to match current location.
> Location PROJ_INFO is:
> name: unnamed
> ellps: grs80
> proj: lcc
> lat_1: 44.33333333333334
> lat_2: 46
> lat_0: 43.66666666666666
> lon_0: -120.5
> x_0: 2500000
> y_0: 0
> no_defs: defined
> Dataset PROJ_INFO is:
> name: unnamed
> ellps: grs80
> proj: lcc
> lat_1: 43
> lat_2: 45.5
> lat_0: 41.75
> lon_0: -120.5
> x_0: 399999.9999999999
> y_0: 0
> no_defs: defined
> ERROR: x_0
> Why the source agency has different projections for flights over the same
> topographic quad area is not anything I want to pursue. My question to you
> gurus is how do I handle these situations? Do I just override the dataset's
> projection and force it to use the existing location's?
> I had one quad with 5 maps, two had the projection of the first one above,
> three had the projection shown second above. I created a separate location
> for each map then reprojected each to my statewide location. I'm sure
> there's a more efficient way to handle these discrepancies.
> TIA,
> Rich
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David W. Roberts
Department of Ecology, Montana State University
On sabbatical at WSL, Birmensdorf, Switzerland

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