Re: [gdal-dev] Reading SRTM30 grids 'at distance'

[Joaquim Luis]

Evan, Frank

Thanks very much. I keep insisting on the esri .hdr because I have a 
function to do all the work but it's definitively time to write one to 
do the same for .vrt (and learn more about it a same time).

Joaquim

Le lundi 14 novembre 2011 18:23:23, Joaquim Luis a écrit :
Hi,

I made a nice little tool for Mirone where one can very easily mosaic
either several flavors of SRTM grids or satellite images from Bing servers.
My question is about the SRTM30 grids (30 arc minutes) that are
available here (SRTM plus)
ftp://topex.ucsd.edu/pub/srtm30_plus/srtm30/data/
which I would like to be able to access directly with the vsicurl
mechanism. The trouble in this case is that those grids are raw files
without any header file. When we have them on disk the simple solution
is to create a esri .hdr header from the information extracted from the
file name. However, this logic does not work for files seating in the
Web because the file and the header won't be located next to each other.
The same occurs if one has those files compressed. In this case the
header file cannot be assumed to be stored inside the .zip file and the
vsizip mechanism will fail for exactly the same reason.
My question is than, if there is a mechanism to send the header info to
the GDAL library and thus pretend that file and header leave side by
side? Note that I want to do this programmatically so need for an
already existing ready to use solution.
One way to do this is to use the capability of VRT files to describe raw files (
http://www.gdal.org/gdal_vrttut.html ), that offer similar capabilities than
the EHdr header.

For e020n40.Bathymetry.srtm, a quick test shows that the following should do :

<VRTDataset rasterXSize="4800" rasterYSize="6000">
   <SRS>GEOGCS[&quot;WGS 84&quot;,DATUM[&quot;WGS_1984&quot;,SPHEROID[&quot;WGS
84&quot;,6378137,298.257223563,AUTHORITY[&quot;EPSG&quot;,&quot;7030&quot;]],AUTHORITY[&quot;EPSG&quot;,&quot;6326&quot;]],PRIMEM[&quot;Greenwich&quot;,0,AUTHORITY[&quot;EPSG&quot;,&quot;8901&quot;]],UNIT[&quot;degree&quot;,0.0174532925199433,AUTHORITY[&quot;EPSG&quot;,&quot;9122&quot;]],AUTHORITY[&quot;EPSG&quot;,&quot;4326&quot;]]</SRS>
   <GeoTransform>20, 0.008333333333333,  0.0,  40,  0.0,
-0.008333333333333</GeoTransform>
   <VRTRasterBand dataType="Int16" band="1" subClass="VRTRawRasterBand">
     <SourceFilename
relativetoVRT="0">/vsicurl/ftp://topex.ucsd.edu/pub/srtm30_plus/srtm30/data/e020n40.Bathymetry.srtm</SourceFilename>
     <ImageOffset>0</ImageOffset>
     <PixelOffset>2</PixelOffset>
     <LineOffset>9600</LineOffset>
     <ByteOrder>MSB</ByteOrder>
   </VRTRasterBand>
</VRTDataset>

(I've attached it to avoid line wrapping problems)



Thanks

Joaquim
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