[Geotiff] USGS Geotiff parameters

[Frank Warmerdam]

Church, Brian J wrote:
> Hello, I am new to the Geotiff format and raster image processing in
> general.   My question concerns the Geotiff files from the NED downloaded
> via the seamless.usgs.gov website.  Is it possible to extract elevation
> data from these files?  From what I understand, the 3rd value in the
> PixelScale tag provides the scale for the elevation data.  Is that correct?

Brian,

For a 2D projected or geographic coordinate system, I have always seen
the 3rd (elevation scaling parameter) be zero.

> However, this value is zero for the few files I have downloaded.   Also, I
> understand the TiePoints tag would provide the base elevation.  But again,
> there is only the x,y values.

Right.

I don't have a NED GeoTIFF file handy, but generally speaking you will
need other means than the geotiff tags to interpret the vertical datum
and any scaling that might be applied to elevation data.  So you would
use GeoTIFF tags for horizontal positioning of the elevation samples,
and read the values from raster data, but any offset or scale would
need to be handled by other means.

As far as I know the NED data is in meters from sea-level but you would
have to check it's supporting metadata to be sure.

Best regards,
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