[Proj] Re: Mapping Africa in an equal area projection

[strebe at aol.com]

 Corrado:



 Of the candidates you list, I would call Mollweide inappropriate because it stretches Africa north-south to in order to reduce distortion elsewhere. It really is best left for world maps, where that compromise results in some benefit. Any of the others you list could be reasonable, depending on how they are constructed. 

Because Africa spans roughly
the same latitudinal range north as south, an equatorial Lambert
azimuthal equal-area is a fine choice, and will give you the lowest
distortion of the group. Nor are there any better projections to recommend without going exotic. Sinusoidal is not a common modern choice, and I do not recommend it in this case because you will end up with higher distortion around the periphery than the other projections, and with no compensation.

Unless your application would benefit from straight parallels and meridians, or unless your two standard parallels happen to be exactly latitudes of opposite sign, then Albers will always perform better than cylindric equal-area. But the advantages of Albers are largely lost on Africa unless you were willing to do something exotic like a heavily oblique aspect. Left with its standard parallels set to the graticule parallels, it would end up very similar to the cylindric equal-area (because the two standard parallels would be nearly latitudes of opposite sign), and in that case it cannot approach Lambert for low distortion across your region of interest. 

Regards,
— daan Strebe



 

-----Original Message-----
From:
 Corrado Topi <ct529 at york.ac.uk>
To: proj at lists.maptools.org
Sent: Fri, 5 Dec 2008 1:18 am
Subject: [Proj] Mapping Africa in an equal area projection










Dear friends,

I would like to map distribution of some specimen over Africa at a
resolution of 100 km x 100 km (or even down to 25 km x 25 km): I believe I
need an Equal area projection. My original files, with the data, are .csv,
with lat/long + measurement (real number). I need to carry out some file
(.csv) transformation between projections. 

I gathered some information, and ended up with the following list of
candidate projections:

1) Sinusoidal
2) Equatorial Lambert Azimuthal Equal Area
3) Albers Equal Area (Conic)
4) Mollweide
5) Cylindrical Equal Area

I do not really know which one to choose. What do you recommend?

For each of them, which ellipsoid  and datum would you choose? 

I already read the usual stuff (like UGSPP 1395 document - Map Projections -
A Working Manual and co.).

Regards
-- 
Corrado Topi

Global Climate Change & Biodiversity Indicators
Department of Biology, University of York, UK

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