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

Tue Dec 9 01:55:01 PST 2008

Hello,

here is a list of some Africa projections-datums. Some of the equal area
projections, for example the "aea" is suitable.

regards: Janne.

List follows:
--------------------

# esri.extra.pin:102011
# Africa Sinusoidal
proj=sinu
lon_0=0
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.extra.pin:102022
# Africa Albers Equal Area Conic
proj=aea
lat_1=20
lat_2=-23
lat_0=0
lon_0=25
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.extra.pin:102023
# Africa Equidistant Conic
proj=eqdc
lat_0=0
lon_0=0
lat_1=20
lat_2=-23
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.extra.pin:102024
# Africa Lambert Conformal Conic
proj=lcc
lat_1=20
lat_2=-23
lat_0=0
lon_0=25
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.pin:102011
# Africa Sinusoidal
proj=sinu
lon_0=0
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.pin:102022
# Africa Albers Equal Area Conic
proj=aea
lat_1=20
lat_2=-23
lat_0=0
lon_0=25
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.pin:102023
# Africa Equidistant Conic
proj=eqdc
lat_0=0
lon_0=0
lat_1=20
lat_2=-23
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# esri.pin:102024
# Africa Lambert Conformal Conic
proj=lcc
lat_1=20
lat_2=-23
lat_0=0
lon_0=25
x_0=0
y_0=0
ellps=WGS84
datum=WGS84
units=m
no_defs

# world.pin:nw-africa
# Northwest Africa
proj=lcc
ellps=clrk80
lat_1=34dN
lon_0=0dE
x_0=1000000
y_0=500000
k_0=0.99908
no_defs

--------------------

-----------------------------------------------------------

Corrado Topi [ct529 at york.ac.uk] kirjoitti: 
> Dear Daan,
> 
> additional information .... the journal we would like to submit to requests
> the maps to be in equal area projection.
> 
> 
> They say:" Maps that display area data and organism distribution at a
> continental, hemispheric, or world scale must always use an equal-area map
> projection (e.g., Mollweide or Aitoff's). Note especially that Mercator's
> projection is not acceptable for such data. Please indicate the precise
> projection employed in the caption. On these maps, the equatorial scale
> should be indicated, while scale information should be provided, preferably
> as a scale bar within the figure, for all maps of whatever size and area;
> use 'km' or 'kilometres', not 'kilometers'. Maps should include adequate
> geo-referencing information."
> 
> Reference:
> 
> http://www.blackwellpublishing.com/submit.asp?ref=0305-0270&site=1
> 
> Best,
> 
> 
> strebe at aol.com wrote:
> 
> > 
> > 
> > 
> > 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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