[Proj] datums and libproj4
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
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
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
hidden away for security reasons. All very hush-hush. It was
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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