why is eccentricity of zero an error?

Gerald I. Evenden gie at charon.er.usgs.gov
Sun Jul 4 14:33:03 EDT 1993

>Date: 04 Jul 1993 13:09:36 -0400 (EDT)
>From: "Jim Aanstoos (919) 541-6890" <AANSTOOS%LOUIE at rcc.rti.org>
>Subject: why is eccentricity of zero an error?
>Does anybody know why an eccentricity or flattening coefficient
>of zero is considered an error?  It makes it impossible to define a
>sphere of constant radius as one choice in the ellipsoid table.  I
>notice that such a choice appears in the default ellipsoid table
>($GISBASE/etc/ellipse.table) but is commented out.  If you un-
>comment it and then run something that uses that table (like
>m.datum.shift), it gives an error referring to that line.  The error
>comes from function get_a_e2 in get_ellipse.c of libes/gis, which
>complains if the eccentricity (squared) or flattening value is
>(less than or) equal to zero.  Should that test be changed to
>less than zero only, or would that cause problems elsewhere?
>            /\
>      jim  /--\
>          /    \ 
>Jim Aanstoos                      Internet: aanstoos%louie at rcc.rti.org
>                                         or aanstoos%louie at rti.rti.org
>Research Triangle Institute                                           
>PO Box 12194                                       Phone: 919-541-6890
>Research Triangle Park, NC 27709-2194                Fax: 919-541-6515

With repect to what program is a zero eccentricity or 1/f in error??

Certainly not with any software I am aware of---proj, GCTP, dma, ... .
Setting eccentricity or eccentricity squared to zero is the
normal way to specify a sphere.

The only place that proj will complain about a sphere being spec'ed
with 0 eccentricity is for UTM or UPS where it is ILLEGAL to use a sphere.
The DMA MPs will cart you off in this case ;-).

You have a legitimate complaint and bug report if you cannot have
a zero eccentricity [squared].  Are sure you aren't doing something
else wrong?

Gerald (Jerry) I. Evenden   Internet: gie at charon.er.usgs.gov
voice: (508)563-6766          Postal: P.O. Box 1027
  fax: (508)457-2310                  N.Falmouth, MA 02556-1027

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