[GRASSLIST:2206] Re: How to display satellite data correctly?

[Kris Nackaerts]

Hi,

I had the same problem when trying to visualize hyperspectral images. I 
created a script in R (www.r-project.org) to analyze the histogram 
output files and create a GRASS script automatically to correctly 
rescale the values for visualisation. I could then say like remove the 
5% tails.

If you are interested in this script, let me know. Otherwise, I also 
used just a PCA transform to get a first impression of what was in the 
images.

Regards,

Kris

Hamish wrote:

>> currently I am doing some first steps with GRASS to see how it all
>> works. I've got some sample data (Landsat TM) that I would like to
>> analyze with some of the available functions. However it seems that
>> GRASS is not very good at displaying color images -- at least not if
>> the histograms of the individual channels aren't properly equalized.
>>
>> There is r.colors that should do the trick (I've tried grey.eq) but
>> the results weren't too good. To my opinion the histogram is
>> transformed to be a little like black-and-white but the dynamics
>> aren't well. The display of d.rgb is extremely bright colored but it
>> is difficult to see the real structure -- there are quite a few
>> colored noise pixels that also degrade the quality.
>>
>> Are there better ways to display and visually analyze satellite data?
>> What I am especially missing is a simple linear stretch of the
>> histogram with a 1% cutoff at each end.
>>   
>
>
> is it really bad data or just each channel using a different color scale?
>
> see
> http://article.gmane.org/gmane.comp.gis.grass.user/2836
>
>
> then use d.rgb, not r.composite.
>
> A custom color map is a less destructive way if you aren't sure..
>
> If it is bad data, you can use r.mapcalc or r.null to get rid of it.
>
>
> ?
> Hamish
>
>
>  
>