Spherical->Ellipsoidal Earth models in OpenMap

On Tuesday 2002-01-22 at 13:00:40(-0500) Eliot Lebsack wrote:
> Good afternoon.
>
> I've been examining the source code of the 4.4.2 release, and I have
> noticed
> that the Mercator projection assumes a spherical earth, instead of an
> ellipsoid. I've
> dug up the Snyder book "Map Projections - A Working Manual", and have
> found the
> corresponding ellipsoid-based formulas for performing the forward
> projection
> transformations.
>
> Before I get bogged down in the details of the Java class organization,
> has
> anyone given any thought as to how such a change would be implemented in
> a
> systematic manner (working within the current class framework)? Since
> the sphere is a degenerate form of the general ellipsoid, it should be
> rather straightforward to modify the arithmetic to support it.
>
> Any thoughts on this idea would be welcome.

This is a philosphy response to a technical question. Don't mind me,
I've just always wondered about this...

I'm curious about why people want ellipsoid-based projections in a
display-oriented component. I can understand why you would want to use
them in a computation component - i.e. something that deals with
distances, areas, buffering, etc.

Is there any understanding in other geospatial software of the
difference between quick-and-dirty display where you want everything
to go as fast as possible and the accuracy needs of the compute side
of things? Does everyone always use the same projection math for both
parts?

In a Model-View-Controller paradigm, it would seem to me that the view
could generally be considered to be an imprecise and incomplete window
into the model anyway. Displays have fat pixels, not infinitesimally
small dots.

Furthermore (at least this used to be true), the OpenMap projections
are geared towards display and have additional methods such as
"plotable()" which used to be a way to see if a given point was
visible (i.e. was not clipped off the edge of the screen, nor "behind"
other points in the orthographic projections. These methods are not
needed for computing distance, etc.

So what does this all mean? Maybe it means that there should be two
kinds of projection classes - those optimized for drawing, and those
optimized for computing. There could be others, or at least variations
on these as well.

End of philosophy...

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Allan Doyle                         http://www.intl-interfaces.com
adoyle@intl-interfaces.com
                                                   
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