Great, thanks
Joaquim
Le jeudi 24 mars 2011 19:03:57, Joaquim Luis a écrit :
Even, Chaitanya
Thanks for hint.
I did read the http://trac.osgeo.org/gdal/wiki/UserDocs/ReadInZip but it
hasn't any mention to the need of using "vsicurl"
Yes I am aware that it is lightly documented. I've added recently a few
pointers though :
- in the GDALOpen() doc :
http://gdal.org/gdal_8h.html#e97be045eb4701183ad332ffce29745b
which points to :
http://gdal.org/cpl__vsi_8h.html#4f791960f2d86713d16e99e9c0c36258
Which mentions :
"This special file handler can be combined with other virtual filesystems
handlers, such as /vsizip. For example,
/vsizip//vsicurl/path/to/remote/file.zip/path/inside/zip"
Perhaps a more didactic page would be needed in the wiki. Any taker :-) ? I
remind that the wiki is open in writing for everyone with an osgeo id...
I was about to do the tests you did, so thank you also for that.
The point here is not to read that file in particular. I just wanted to
use one as example and one that is not a *.tar.gz that the docs say it
won't work.
Ah... well actually reading in a .tar or a .tar.gz is now supported (
http://gdal.org/cpl__vsi_8h.html#d6dd983338849e7da4eaa88f6458ab64 ). Seems
that I have changed my mind since. But seeking will still be very slow of
course (especially if you combine with /vsicurl !). Ok, I'm going to rectify
the page about that point.
Ok, I have enough information to make it possible to read some
compressed files sitting somewhere in the web and have the data land
directly in the internals of GMT.
Thanks
Joaquim
Joaquim,
The correct syntax would be :
gdalinfo
/vsizip/vsicurl/http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w02
0n90.dem.zip/W020N90.DEM
or just :
gdalinfo
/vsizip/vsicurl/http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w02
0n90.dem.zip
because the zip file only contains one single file (W020N90.DEM)
... But the W020N90.DEM inside the zip file isn't directly recognized by
GDAL (even if you download it and unzip the file). It's just a RAW file,
that neads an header to tell the dimension, georeferencing, datatype
etc, so the above won't directly work.
You can for example create a VRT that refers to the raw file :
<VRTDataset rasterXSize="4800" rasterYSize="6000">
<GeoTransform>-20, 8.3333333333300008e-03, 0, 90, 0,
-8.3333333333300008e-03</GeoTransform>
<VRTRasterBand dataType="Int16" band="1" subClass="VRTRawRasterBand">
<NoDataValue>-9999</NoDataValue>
<SourceFilename
relativetoVRT="0">/vsizip/vsicurl/http://dds.cr.usgs.gov/srtm/version2_1/
SRTM30/w020n90/w020n90.dem.zip</SourceFilename>
<ByteOrder>MSB</ByteOrder>
</VRTRasterBand>
</VRTDataset>
I've deduced this VRT from the
http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w020n90.hdr.zip
file that sits next to the .dem.zip file.
It's a shame that they didn't put the .hdr and the .dem file inside the
same zip. It would have they been possible to open it directly...
With the SRTM3 in HGT format, you can directly do :
gdalinfo
/vsizip/vsicurl/http://dds.cr.usgs.gov/srtm/version2_1/SRTM3/Africa/N00E0
06.hgt.zip
Best regards,
Even
Hi,
How do we access to a compressed files by URL? We can do that, can't we?
As an example I try this (on Windows)
gdalinfo
/vsizip/C:\http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w020n90
.de m.zip ERROR 4:
`/vsizip/C:\http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w020n9
0.d em.zip' does not exist in the file system,
and is not recognised as a supported dataset name.
and this
gdalinfo
/vsizip//http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w020n90.d
em. zip
but the error message is similar.
The idea is to fetch the data directly into GMT, but first I need to
understand well how it works.
Thanks
Joaquim Luis
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