[Proj] International 1924 Authalic Sphere needed for EASE-GridSnow data

[Gerald I. Evenden]

On Thursday 06 November 2008 11:22:34 am OvV_HN wrote:
> ----- Original Message -----
> From: "Hamish" <hamish_b at yahoo.com>
> To: "PROJ.4 and general Projections Discussions" <proj at lists.maptools.org>
> Sent: Thursday, November 06, 2008 2:45 PM
> Subject: Re: [Proj] International 1924 Authalic Sphere needed for
> EASE-GridSnow data
>
> > Markus Neteler wrote:
> >> However, the International 1924 Authalic Sphere seems to be
> >> missing from PROJ4.
> >>
> >> Could this be officially added? Is it as simple as this
> >> code change (maybe there is a better name to find)?
> >
> > definitely a better name would be needed, as International 1924
> > Ellipsoid != International 1924 Authalic Sphere!
> >
> I don't get it - I know, it's in the EPSG database.
> But: starting from the International 1924 ellipsoid, I get an authalic
> radius of 6371227.7113... m.
> Why don't they use that value?
> Hm, could be the same reason as why they use the well known integer value
> of 6370997 m for the authalic radius of the Clarke 1966 ellipsoid instead
> of the calculated value of 6370997.2406....
>
> Oscar van Vlijmen

To get the "Authalic" or radius of sphere with equivalent surface area in 
libproj use R_A option as in this lproj example:

gie at charon:~/CARTO/libproj4/src$ lproj +proj=tmerc +ellps=clrk66 +R_A -V
#Transverse Mercator
#       Cyl, Sph&Ell
# +proj=tmerc +ellps=clrk66 +R_A
#Final Earth figure: sphere
#  Radius: 6370997.241 

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