[Mapserver-users] classifying raster maps with more than 50 classes

[Jan Hartmann]

Hello list,

I'm trying to display some world gridded data (elevation, population, 
satellite) with user defined color ramps. All data are resampled to 
GeoTiff rasters with 256 values and a colormap. The colors are adequate 
for initial display, but I would like to give the user the option to 
choose between different predefined color maps (greyscale, equalized, 
different color panels). In Grass this is very simple: just adapt the 
color table. With MapServer I am able to create classes for the colors I 
need, but this is limited to a total of 50 and I have the impression 
that evaluating so many class-expressions is not very fast. Is there a 
possibility to adapt the colormap in a GDAL file directly before 
rendering it? It doesn't have to be within MapServer; a command line 
utility would be usable too.  I just don't want to duplicate the raster 
file for every color scheme I need.

Jan

Frank Warmerdam wrote:
> Jan Hartmann wrote:
> 
>> Frank Warmerdam wrote:
>>
>>>
>>> Jan,
>>>
>>> The SCALE=AUTO is done independently for each tile, and so it is 
>>> likely that
>>> different scaling ranges would be selected.
>>
>>
>>
>> Is this desirable? I can't see any use for tiled maps with differently 
>> scaled tiles. Would it be very difficult to scale from the minimum and 
>> maximum values of all displayed tiles combined?
> 
> 
> Jan,
> 
> The desirable thing would be for AUTO scaling to always be based on the
> whole image set for the input layer.  But it is not practical to recompute
> that each time a map is rendered, so the code currently just computes the
> auto-scaling on the basis of the window of imagery actually loaded to
> render the map.
> 
> This means that the scaling changes tile by tile in a tiled layer, and even
> that it it varies from render to render as you zoom in on a single 
> raster file.
> 
> The solution isn't to do more work each time the map is rendered, but 
> instead
> to have map generation tools that can collect appropriate scaling 
> information
> once in advance.  In the meantime I would strongly encourage you to just
> use "gdalinfo -mm <filename>" to get reasonable scaling ranges and enter
> them into the map file.
> 
> The short answer is, it would be very compute intensive (and a quite a bit
> of work to implement for that matter) to have auto raster scaling done on
> the basis of all displayed tiles combined.
> 
> Best regards,
>