[Gdal-dev] Find elevation data at specific point

[Frank Warmerdam]

Gerald Brandt wrote:
> Hi,
> 
> I would like to know how to get the elevation from a specific coordinate
> (in latitude/longitude or UTM).  I can convert between lat/long and UTM
> as needed for the specific map data loaded, but I don't know how to get
> the elevation of that coordinate.
> 
> I'm assuming that I need to get the xpixel and yline values, and
> reference off of that somehow.  But, being new to GDAL, I'm not quite
> sure.


Gerald,

I haven't got time to prepare a proper demonstration.  But the brief answer
is you need to:

  o Transform your coordinate into the coordinate system of the image.
    For instance if you start with lat/long but the image is in UTM, convert
    it to UTM.
  o Convert that coordinate to image pixel/line coordinates.  Assuming the
    image is north-up (no rotational coefficients) then this would be:

    pixel_x = (geo_x - gt[0]) / gt[1]
    pixel_y = (geo_y - gt[3]) / gt[5]

    Where gt[] is the Geotransform array fetched with the GetGeotransform()
    method on the GDALDataset.

  o Read just that pixel with RasterIO().

    eg.

    double dfElev;

    poDS->GetRasterBand(1)->RasterIO( GF_Read, pixel_x, pixel_y, 1, 1,
                                      &dfElev, 1, 1, GDT_Float64 );

or something like that.

Good luck,

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