[GRASSLIST:2703] Re: stitching geotiffs

[Richard Greenwood]

Martin du Saire wrote:

> Outstanding!  Thank you Richard and  Glynn.  I think I may eventually 
> have to r.patch the individual maps.  I was playing around with two maps 
> within a single zone, and the r.stats -a -n map1,map2 output did not 
> reflect what I expected to be the composite of the two (Actually, the 
> numbers made no sense at all).  One last question:
> 
> Without having specific datum in my adjacent zones (14, 16), is there 
> some way to make upper-bound estimates on the amount of distortion 
> resulting from the r.proj method=nearest (the maps are land-use/cover)?
> 
> Thanks again.
> 
> Martin

That's a hard question that I will not try to answer directly. Map 
distortion can involve linear, directional, and/or area distortion. So 
first you have to define which of those are relevant to your 
application. I think in your case the most significant distortion will 
be angular. And I would guess that areas would be most important for 
land use applications. Within a UTM zone I am thinking that area 
distortion is limited to about 0.04% because the central meridian has a 
scale factor of 0.9996 (anyone is welcome to correct me on that). But as 
you move outside the zone, the increase in distortion would not be linear.

I am kind of a trial-and-error guy, so if it was me, I would simply 
reproject the extreme cases into the target location and see if they 
meet your requirements. Extending UTM zones is not generally considered 
a good practice (although I am guilty of having done it a couple times). 
A better solution might be to create a projection for your project.


-- 
Richard Greenwood
www.greenwoodmap.com