[GRASS5] Re: [Proj] Continuing confusion of datums vs. projections

[Gerald Evenden]

Sorry, but I have been through this issue several times but I see it is 
still
necessary to reiterate my position: cartographic projections are a 
subject
area unto themselves and have *no* relationship to datums nor other
aspects of common cadastral and geodetic activities.  One can spend
their entire life studying and developing cartographic projections and
not have one whit of knowledge of datums nor how to manipulate them.

I take John Snyder as an excellent example: he wrote many articles and
books related to cartographic projections.  He created new projections.
He was recognized as a world class expert on cartographic projections.
However, you do not need to take your shoes off to count the number
of paragraphs he wrote on datum operations.  In fact, I cannot think
of one case other than his listing of the projection parameters for
both NAD27 and NAD83.  Surely, Snyder may have been aware of
datum shifting and how to do it but he did not seem to include it in
his research activities.

Surely, projections are required in the practical world of cadastral and
geodetic activities, but so are sine and cosine and a bunch of other
transcendental functions.  The software development business
normally compartmentalizes solutions by individuals specializing
in solving different parts of a larger problem with the result of a
collection of libraries that are linked together to produce a more
general application program that utilizes a wide area of disciplines.

Lastly, the argument that because the major axis and eccentricity are
major details of a datum and that they are required by a projection, 
then the
projection should be included as part of a datum library just obfuscates
the issue.  This whole subject area uses the sine function in many
places---does that mean that we have to include the sine function in a
cadastral library?  I think not.

I am sorry to bore y'all but I think this is an important issue and 
needs
to be understood.
_____________________________________
Jerry and the low riders: Daisy Mae and Joshua