[gdal-dev] Interpretation of DTED elevation data

[Alex Barféty]

I've used the ReadRaster method with a short array and it working perfectly
fine, the elevation data recovered are correct.

Thanks for your advice :)

Alex Barféty


-----Message d'origine-----
De : Even Rouault [mailto:even.rouault at mines-paris.org] 
Envoyé : mercredi 28 mars 2012 10:23
À : AlexRNL
Cc : gdal-dev at lists.osgeo.org
Objet : Re: [gdal-dev] Interpretation of DTED elevation data

Selon AlexRNL <alex.barfety at signalis.com>:

> Hi everyone!
>
>
> I have been trying to extract elevation data from a DTED file using 
> GDAL (in Java). The file being loaded is of level 0 precision and 
> seems to be correct as an import into Global Mapper is rendering the
expected result.
>
>
> I managed to retrieve the image data but I don't know to transform 
> those values into a altitude in meters.
>
>
> According to the DTED format specification, the elevation values are 
> supposed to be two-bytes integer in meters. But, when using the 
> http://www.gdal.org/java/org/gdal/gdal/Band.html#ReadRaster_Direct(int
> , int, int, int, int, int, int, java.nio.ByteBuffer) ReadRaster_Direct 
> method, the buffer retrieved is supposed to be composed of Int16 
> number but when i'm converting the raw data (bytes) into a short array 
> i'm getting all sorts of value which doesn't make a lot of sense. The 
> range of the elevation is spanning from 1 to 65286!
>
>
> Is there a conversion that i'm missing? How do I get the meter value 
> from the raster returned by GDAL?

You should make sure to request with buf_type = gdalconst.GDT_Int16. Then
iterating the buffer with getShort() should deliver the expected values.

Otherwise use the version of ReadRaster() that accepts a short[]. That will
be simpler to use.

The benefit of using ByteBuffer instead standard Java arrays only appears
with
*huge* arrays

>
>
> I've been searching for a while but it doesn't seem like anyone has 
> that kind of problem...
>
>
> Thanks in advance for any information that may help me :)
>
> Alex.
>
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