USGS DEM and OGC WMS -- Bug

[Frank Warmerdam]

On 7/27/05, Julien, Heryk <hjulien at nrcan.gc.ca> wrote:
>  
>  
> 
> Hi, 
> 
> This is the first time I get a request to add a USGS DEM to Mapserver. 
> 
> I would like to use the original USGS format to be able to query the DEM
> using the WMS getFeatureInfo. 
> 
> If I convert the USGSDEM to a simple raster (ex:*.tiff) don't I loose the
> elevation data? 

Héryk,

You can convert USGS DEMs to GeoTIFF without losing any precision
of the original elevation data.  The USGS DEM ASCII format is particularly
inefficient to access, so conversion to GeoTIFF should give you some
performance benefits. 

To serve the data via WMS you pretty much need to scale it to 8bit on 
the fly using the pixel data scaling options discussed in the Raster 
HOWTO (or classification).   I would suggest you review these items 
in the raster howto. 

I would also encourage you to review the map file I recently provided
to the fellow asking about SRTM WCS servers.  That map file offers the
elevation data as scaled WMS layers as well as for WCS access and
can provide you some ideas. 

For raster query of the original data values I think you will need to 
have an extra "pure" layer to do the queries against.  This query 
layer would not be scaled or classified.  As opposed to the layer
used for display purposes. 

Best regards,
-- 
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