<div> <font face="Arial, Helvetica, sans-serif">Corrado:<br>
<br>
</font></div>
<div> <font face="Arial, Helvetica, sans-serif">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. <br>
<br>
</font><font face="Arial, Helvetica, sans-serif">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. </font><font face="Arial, Helvetica, sans-serif">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.<br>
<br>
</font><font face="Arial, Helvetica, sans-serif">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 </font><font face="Arial, Helvetica, sans-serif">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 si
milar to the cylindric equal-area (because the two standard parallels would be nearly latitudes of opposite sign), </font><font face="Arial, Helvetica, sans-serif">and in that case it cannot approach Lambert for low distortion across your region of interest. </font><br>
<font face="Arial, Helvetica, sans-serif"><br>
Regards,<br>
— daan Strebe<br>
<br>
</font></div>
<div> <br>
</div>
-----Original Message-----<br>
From: Corrado Topi <ct529@york.ac.uk><br>
To: proj@lists.maptools.org<br>
Sent: Fri, 5 Dec 2008 1:18 am<br>
Subject: [Proj] Mapping Africa in an equal area projection<br>
<br>
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<pre style="font-size: 9pt;"><tt>Dear friends,<br>
<br>
I would like to map distribution of some specimen over Africa at a<br>
resolution of 100 km x 100 km (or even down to 25 km x 25 km): I believe I<br>
need an Equal area projection. My original files, with the data, are .csv,<br>
with lat/long + measurement (real number). I need to carry out some file<br>
(.csv) transformation between projections. <br>
<br>
I gathered some information, and ended up with the following list of<br>
candidate projections:<br>
<br>
1) Sinusoidal<br>
2) Equatorial Lambert Azimuthal Equal Area<br>
3) Albers Equal Area (Conic)<br>
4) Mollweide<br>
5) Cylindrical Equal Area<br>
<br>
I do not really know which one to choose. What do you recommend?<br>
<br>
For each of them, which ellips
oid and datum would you choose? <br>
<br>
I already read the usual stuff (like UGSPP 1395 document - Map Projections -<br>
A Working Manual and co.).<br>
<br>
Regards<br>
-- <br>
Corrado Topi<br>
<br>
Global Climate Change & Biodiversity Indicators<br>
Department of Biology, University of York, UK<br>
<br>
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</tt></pre>
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