[GRASS-user] Raster file from ascii file and flattening Africa .... : )

[Corrado]

Dear Moritz,

thanks for pointing out r.in.xyz .... I will get a look at it, to see if it is 
what I was looking for.

Concerning projection, of course I was not looking into flattening Africa with 
a digger .... but in "flattening" Africa's projection (I though it was clear 
from the context). 

My data are in WGS84 / lat-long, but I need to use an area preserving 
projection. I need to set the resolution to some sort of 100 km x 100 km 
squares, or equivalent area.

My data are 1 x measurement on several specimen of the same species in 
different sites, and I need to do some geographical analysis at that type of 
resolution in an area preserving projection / datum.

I am asking the gurus what is the best area preserving projection / datum I 
could use for Africa and the hows.

Thanks again.


On Wednesday 03 December 2008 17:05:15 Moritz Lennert wrote:
> On 03/12/08 10:26, Corrado wrote:
> > Dear friends,
> >
> > I am a kind of advanced newbie, if that makes sense.
> >
> > I have a text file of the form
> >
> > coordinate x,coordinate y,cat={real number between 250 and 450}
> >
> > where coordinate are expressed in latitude and longitude. The files
> > represents measurements of the size of a skulls on sites all over Africa.
> >
> > From it, I would like to build a raster file, 100 km by 100km.  There are
> > 2 problems:
> >
> > 1) Unfortunately,  in some 100km x 100km squares, there is one of the
> > points whilst in others there are maybe 20. How do I average, so that in
> > each square I only have 1 value representing the average?
>
> r.in.xyz does this for you directly during the import.
>
> Or you have r.resampl.stats, r.statistics, r.average, r.mode, r.median.
>
> > 2) How do we "flatten" Africa so that we may use 100km x 100km squares
> > instead of 1 degree x 1 degree, without committing a geographical crime?
> > What we need is to respect the areas ....
>
> I don't know what you mean by "flatten". IIUC, you are simply speaking
> about using a projection system. You have to create a location in the
> projection of your choice (I'll leave it to others to advise you on the
> best choice for the whole of Africa, but according to your criteria, you
> would need an equal area - see [1,2] for an introduction). Then use
> r.proj to reproject your map from the lat-long location to the projected
> location where you can then resample.
>
> Moritz
>
> [1] http://en.wikipedia.org/wiki/Map_projection#Equal-area
> [2] http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html



-- 
Corrado Topi

Global Climate Change & Biodiversity Indicators
Area 18,Department of Biology
University of York, York, YO10 5YW, UK
Phone: + 44 (0) 1904 328645, E-mail: ct529 at york.ac.uk