[mapserver-users] Mathematical Scaling of Floating Point Data in Mapserver Map File

[Peter Willis]

Thanks Frank,

That helps somewhat.

It's going to be a bit of a bear making a 256 color lookup
that way. I guess I can script the CLASS lookup sections of
the map file(s).

Peter


Frank Warmerdam wrote:
> Peter Willis wrote:
>>
>> Hello,
>>
>> Is it possible to apply nonlinear scaling to classify
>> floating point tiff rasters.
>>
>> All examples that I have seen appear to assume a linear
>> dataset.
>>
>> If I wish to scale the data by the base 10 log
>> of the data and then scale the Red, Green and Blue values
>> of the classification, can I do it in the map file?
>>
>> What are the available functions that may be used
>> in the mapfile 'PROCESSING' and 'EXPRESSION' items?
> 
> Peter,
> 
> The short answer is that you cannot, in general, do non-linear
> scaling.  Depending on the dynamic range of your data, you could
> use classes to do something approximating non-linear scaling but you
> need to be aware that in the background there is still a linear scaling
> being done before the classification is applied to the "scaled buckets".
> But you can request as many as 64K buckets which would still give you
> pretty good control over something like four orders of magnitude.
> 
> You might use something like:
> 
>   PROCESSING "SCALE=0,100000"
>   PROCESSING "SCALE_BUCKETS=64000"
> 
>   CLASS
>     EXPRESSION ([pixel] < 2.5)
>     COLOR 255 0 0
>   END
>   CLASS
>     EXPRESSION ([pixel] < 10)
>     COLOR 235 20 20
>   END
>   CLASS
>     EXPRESSION ([pixel] < 25)
>     COLOR 215 40 40
>   END
>   CLASS
>     EXPRESSION ([pixel] < 100)
>     COLOR 195 60 60
>   END
>   CLASS
>     EXPRESSION ([pixel] < 250)
>     COLOR 175 80 80
>   END
>   CLASS
>     EXPRESSION ([pixel] < 1000)
>     COLOR 165 100 100
>   END
>   CLASS
>     EXPRESSION ([pixel] < 2500)
>     COLOR 145 120 120
>   END
>   CLASS
>     EXPRESSION ([pixel] < 10000)
>     COLOR 125 140 140
>   END
>   CLASS
>     EXPRESSION ([pixel] < 25000)
>     COLOR 105 160 160
>   END
>   CLASS
>     EXPRESSION ([pixel] <= 100000)
>     COLOR 85 180 180
>   END
> 
> Note that because the values between 0 and 100000 are classified
> into only 64000 buckets you don't really have very good precision
> at the bottom end of the range.  So even though we specify
> [pixel] < 2.5, each bucket is about 1.5625 wide so the first two
> buckets will get put into the first class, including values
> between 2.5 and 3.125.  This imprecision will become insignificant
> to a log scale as you move up to larger values.
> 
> Good luck,