[Proj] datums and libproj4

Gerald Evenden gerald.evenden at verizon.net
Fri Feb 25 11:28:20 EST 2005

Deja vu all over again.

As far as libproj4 is concerned, the structure returned by pj_init
only has information on the ellipsoid: major axis and 
flattening/eccentricity, etc.
It contains absolutely NO information on the datum.  The ellipsoid does 
define a datum.  The ellipsoid is part of the definition of a datum and 
is of
interest to libproj4 only in the context of performing a transformation
between coordinate types: geographic to and from planar.

As a creator/maintainer of libproj4 I only have to maintain a knowledge 
how to perform the translation between coordinate types.  As for all the
technical machinations of datum transformations; "I know nothing."  And
I don't need to know anything other than the ellipsoid used by a 

An example of the separation of datum and projections is in using the
math library.  The math library has a sin(x) function but the library 
no interest in the use or meaning of "x"; it only computes the sine of
the value of x.  It is the same with libproj4.

I took this stance of ignorance some years ago while the Cold War was
still in full swing.  Information on datums was very difficult to come 
by; often
hidden away for security reasons.  All very hush-hush.  It was 
difficult enough
to get information of projections.  So I only took on what I could 
get information about and where some mentors where available.
Jerry and the low riders: Daisy Mae and Joshua.
"The whole religious complexion of the modern world is due to the
absence from Jerusalem of a lunatic asylum." Havelock Ellis, 1914

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