Re: [Geotools-gt2-users] projection image rendering

[Adrian C. <ac...@gm...>]

On Tue, 2008-06-24 at 16:56 +0200, Wouter Schaubroeck wrote:
> Hey List,
> 
> I have the following question: what kind of projection method does
> geotools use, to create images of data? Especially if the data is in
> wgs84, and the resulting image is also asked in wgs84. Or am i missing
> something here?

Hey, 

Not knowing your level of knowledge, I'll answer at the risk of stating
the obvious. WGS84 uses a geographic coordinate system, something
necessarily defined in  a 'globular' mathematical space (a closed,
finite, unbounded manifold), therefore which cannot possibly be drawn on
a flat screen without some kind of alteration, called a projection. So
if you see WGS84 data on a flat surface, they have been projected and
are no longer in 'WGS84'---the real question is what projection  is
used.

If no specific projection has been defined, most modern tools
unfortunately fall back on the mindless projection, often formally
called "platte carré", where the long, lat values of WGS84 simply become
X,Y on the surface. You can recognize the shape of the world in this
projection since it is everywhere. Greenland, as is often the case, can
serve as a visual check on the distortion of this projection.

Google says:
  http://www.mapresources.com/results.asp?txtkeys=platte
+carre&name=Platte%20Carre%20World


Projections can be done with more care so as to preserve certain
characteristics from the original data (while sacrificing others).
Platte carré does not preserve any characteristics of interest, whereas
other projections preserve shape locally, direction from a particular
position of origin, OR relative areas, but not all at once. Hence, those
who are projecting data rather than images generally use formal
projections, area preserving, line preserving, or shape preserving
according to their needs.

Hope that helps,
--adrian



> I know that it's possible to ask the images in
> cartographic projections (ex lambert 72), but what happens if this
> isn't the case?
> 
> thx!!
> Wouter Schaubroeck
> 
> Geo Solutions