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

[strebe at aol.com]

Corrado:

You did not indicate if you will be making a single map of all of Africa, or a series of much larger-scale maps. If it is a map of all of Africa, then your choice of ellipsoid is not going to matter much, in which case WGS84 is a good default because GPS data sources are already referred to the WGS84 datum and because datum conversion parameters normally are specified against WGS84.




Otherwise, use the ellipsoid and datum from which your source data are culled unless you have some specific reason not to. If your data come from many datums and ellipsoids, then again, it surely makes most sense to use WGS84 as a standard.




If you will be making a series of large-scale maps of (known) high accuracy, then it makes more sense to standardize on a datum that fits Africa better than WGS84. The Clarke 1880 ellipsoid dominates the extant African datums, but I am not aware of any named datum that has been formulated as a best fit for the entire African continent, though there are mean solutions for large regions, such as ARC 1950.




If your project is such a series of large-scale maps, by the way, I would not use an equal-area projection if I were you. A conformal projection will give more benefits on the ground and the areal inflation will be negligible until you reach medium-scale.





Regards,

— daan Strebe



-----Original Message-----
From: Corrado Topi <ct529 at york.ac.uk>
To: proj at lists.maptools.org
Sent: Mon, 8 Dec 2008 2:50 am
Subject:=2
0[Proj] Re: Mapping Africa in an equal area projection








Thanks Daan!

What ellipsoid / datum, would you choose? WGS84, ETRS89, or waht else?

Regards


strebe at aol.com wrote:

> 
>  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


-- 
Corrado Topi

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

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