[GRASS-user] r.patch, images vs coloured elevation data, aerial photo mosaic

[John Stevenson]

Nikos Alexandris wrote:
> On Mon, 2009-02-16 at 11:35 +0000, John Stevenson wrote:
>   
>> Hi,
>>
>> I have two questions, first a specific one, secondly a more general one.
>>
>> 1)  I have some aerial photos that I want to mosaic.  I am able to do 
>> this with r.patch.  However, the photos only cover about half of the 
>> region.  In the remainder of the region, I would like to show a shaded 
>> relief map as a background to give context to the photos.  When I 
>> include the shaded relief map in the list to patch, it comes out as 
>> varying shades of red.  My question is therefore, what do I have to do 
>> to the shaded relief map to turn it into an 'rgb image' that I can 
>> include in r.patch.
>>     
>
> Do you really want to patch the "shaed relief" map along with the
> orthophotos in one map? What about r.blend?
>
> If you insist on patching, then, I think, you would need to create 3 new
> versions of your shaded map which will correspond to R(ed), G(reen) and
> B(lue), rescale them to 0,255 (r.rescale). Then you patch them with the
> R, G and B mosaic's of the orthophotos and compose (r.composite) an RGB
> map.
>
> I would expect the shaded part to look grey-scaled since you will have
> the same pixel value in all of the R, G and B layers. But I am 100% sure
> that it will work.
>
>
>
>   
Thanks Nikos,

In the end, I made a composite from the original shaded relief:

     r.composite red="nesja_shade" green="nesja_shade" 
blue="nesja_shade" levels=32 output="elev_shade_comp"

then patched it with the photos:

     r.patch 
input="p43_trimmed,p44_trimmed,p33_trimmed,elev_shade_comp"  
output="aerial"  

This gave me coloured aerial photos where the data exist, and a grey 
shaded relief map where they don't.

>> 2)  More generally, what is the best way of converting a good-looking 
>> map into a georeferenced image e.g. by converting the elevation data 
>> into rgb pixel colour data.  Currently I would use d.out.file then 
>> gdal_translate, but that is dependent on the screen resolution.
>>     
>
> What about "d.out.file in=YourMap out=YourMap resolution=2 # or
> resolution=4" ?
>
>   

Good point.

>> Cheers
>>
>> John
>>     
>
> Kind regards, Nikos
>
>
>   


-- 


Dr John Stevenson
Postdoctoral Research Associate
School of Earth, Atmospheric and Environmental Sciences
Williamson Building (Room 2.42)
University of Manchester
Manchester M13 9PL, UK
tel. +44(0)161 306 6585; fax. +44(0)161 306 9361;
john.stevenson at manchester.ac.uk