[gdal-dev] Re: Poor Aggrement Between Layers
Kent Eschenberg
eschenbe at psc.edu
Mon Jan 14 15:07:11 EST 2008
This problem has been traced, I believe, to an error in the original relief map
in the Lambert projection provided by the National Atlas of the US (NAUS). The
same map in the Albers projection seems correct.
On 1/2/2008 Kent Eschenberg wrote:
> The two map layers described at
>
> nationalatlas.gov/mld/srld48i.html (relief)
> nationalatlas.gov/mld/roadtrl.html (roads)
>
> don't seem to agree. If I look at the Los Angeles area I must shift
> the roads about 10,596 feet to the right and 1,849 down.
Further testing has been done with these 3 datasets:
A: nationalatlas.gov/mld/roadtrl.html (roads, geographic)
B: nationalatlas.gov/mld/srld48i.html (relief, Lambert)
C: nationalatlas.gov/mld/srld48a.html (relief, Albers)
The comparisons were done by converting one map to the same projection as the
other map as follows:
Compare A to B->geographic: disagree
Compare A to C->geographic: agree
Compare A->Lambert to B: disagree
Compare B->geographic to C->geographic: disagree
Compare A to C->geographic->Lambert->geographic: agree
The transformations at the "->" were performed by GDAL. For example,
B->geographic was performed with
gdalwarp -t_srs WGS84 srld48i200l.tif srld48i200l_WGS84.tif
where the projection of the input file is read from the file and associated
files in the same directory.
Note that both the first and the last comparison exercised the GDAL
transformation from Lambert to geographic. It seems therefore that this
transformation is being done correctly by GDAL. The only conclusion I can make
is that B is in error.
The funny thing is that Jay at the NAUS sent an image from their online
MapMaker utility that uses A and B, and the results agree. Maybe MapMaker
doesn't use exactly the same version of B as is available to the public for
downloading.
Kent
Pittsburgh Supercomputing Center
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