[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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