Thank you Frank. I hadn't thought about file formats supporting
projections or not. Typically, you think about the software or
algorithm not supporting a projection or coordinate system, not the
format itself.
- John
Frank Warmerdam wrote:
John Callahan wrote:
Thanks Frank/Hermann for your responses. However, something odd
seems to be going on here. I feel like I'm missing something on how
GDAL handles coordinate system information.
>
Using the same input data set, and converting to different formats,
the description of the output coordinate systems are different. Is
this expected? Here are the gdal_translate commands and their
respective output files gdalinfo descriptions below.
John,
I think the key thing is that coordinate system encoding mechanisms
tend to vary from file format to file format. A few points of note:
o GeoTIFF has two general ways of encoding - one is to just embed
the EPSG PCS code for a whole coordinate system. The other is
to encode the projection method along with the parameters. You
will tend to get the first when specifying the coordinate system
with EPSG:n and the second when using PROJ.4 strings.
o Many geospatial formats have a particular way of representing
projections, and may not support all projection methods, preserving
datums, preserving names, etc.
[1] gdal_translate -a_srs "+proj=stere +lat_0=-90 +lat_ts=-70
+lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m
+no_defs" -of GTiff nt_200712_f13_v01_s.bsq nsidc_test.tif
[2] gdal_translate -a_srs "+proj=stere +lat_0=-90 +lat_ts=-70
+lon_0=0 +k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449 +units=m
+no_defs" -of HFA nt_200712_f13_v01_s.bsq nsidc_test.img
...
Fortunately, I can display these images fine. They overlay nicely in
QGIS with some vector data I have. However, the problem occurs when
projecting to EPSG:4326 (or similar) coordinate system; the output
files are incorrect/missing data. (It displays data from roughly lat
-39.25 only down to roughly -54.75 when data exists all the way down
to -90.)
One possible cause could be that gdalinfo reports (for all of these
datasets) latitude_of_origin as -70. From
http://nsidc.org/data/atlas/epsg_3412.html, the latitude of origin
should be -90 and the latitude of standard parallel should be -70.
Or the problem could be somewhere in gdalwarp, or specifically with
the Hughes 1980 ellipsoid. I have no idea. Thanks.
It sounds like there may be another issues with regard to the latitude
of origin in addition to the issue with GeoTIFF not working properly with
3412. If you can isolate and file a ticket that might be helpful.
I'm sorry I haven't been following some of the recent srs threads very
closely.
Best regards,
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