# [Proj] Projecting a rectangle to another coord. system?

Mikael Rittri Mikael.Rittri at carmenta.se
Mon Sep 13 08:39:35 EDT 2004

```Glynn Clements wrote:

G>
G> Mikael Rittri wrote:
G>
G> M> Although I agree that data stored in a polar projection should not be
G> M> viewed in a cylindrical one, I think that one may have geodata stored
G> M> in longitude/latitude, which should be viewed in a polar projection.
G>
G> That case works OK. Projecting a lat/lon rectangle to a polar
G> projection produces a ring segment (like a letter "C") around
G> the pole, and the bounding box of the projected border will
G> bound the projected interior points.

True enough, for rectangles and polygons defined in the geo-database.

G> It's the reverse case which doesn't work. If you project a
G> rectangle which encloses the pole, the north and south edges
G> of the bounding box will be determined by the min/max
G> latitudes of the border points; the projection of the pole
G> will lie above or below the bounding box of the projected border.

But we would need the reverse case as well, because of "demand-driven
geodata access" (or whatever it should be called).  When the user looks
at a polar projection, the zoom level etc. of his window determines a
bounding rectangle which is defined in the polar projection.  The
geodata, on the other hand, are defined in lat/lon in a database,
which can be queried with a bounding lat/lon-rectangle.  That is,
the database returns only the polygons that intersect the lat/lon-
rectangle of interest.  So we need to find a lat/lon-rectangle which
covers at least the polar rectangle --- that's the reverse case.

Since I thought all GIS systems behave like this, I hoped the
fully general problem (including discontinuities) had been solved
(and published).
--
Mikael Rittri <Mikael.Rittri at carmenta.se>

```