[gdal-dev] accessing zip files by URL

[Joaquim Luis]

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"

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.
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/w020n90.dem.zip/W020N90.DEM
>
> or just :
>
> gdalinfo
> /vsizip/vsicurl/http://dds.cr.usgs.gov/srtm/version2_1/SRTM30/w020n90/w020n90.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/N00E006.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/w020n90.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.dem.
>> 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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>> gdal-dev at lists.osgeo.org
>> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>