[Proj] Common term for geographic and projected coordinates?

Gerald I. Evenden geraldi.evenden at gmail.com
Wed Feb 4 15:16:20 EST 2009

```On Wednesday 04 February 2009 2:43:46 pm Eric Miller wrote:
> >>> On 2/4/2009 at 10:37 AM, "Karney, Charles" <ckarney at Sarnoff.com>
>
> wrote:
> >>  From: Mikael Rittri [Mikael.Rittri at carmenta.com]
> >> Date: Wednesday, February 04, 2009 05:19
> >> Subject: [Proj] Common term for geographic and projected
>
> coordinates?
>
> >>    I am trying to document a method that tranforms pixel
>
> coordinates
>
> >> to either projected or geographic coordinates, depending on whether
> >> the window is presented with a map projection or just as lat-lon
>
> (all
>
> >> right, Pseudo Plate Carrée).
> >
> > In the vision world, the conversion for "world" coordinates to pixels
>
> is
>
> > accomplished by a "camera matrix"
> >
> >     http://en.wikipedia.org/wiki/Camera_matrix
> >
> > In general this is a projective map from 3d to 2d.  The simple case
> > you're considering is a similarity transform.  The transformation
>
> from
>
> > pixel to projected coordinates is given by the inverse of the camera
> > matrix.
>
> I don't think that's what the op is interested in.  It's just 2D to 2D
> scaling and translation with some type conversion handling (int <->
> double).  I couldn't find a good explanation online but search for
> "converting screen coordinates to map coordinates" or something similar.
>  The map coordinates are always some projection as the op noticed.  It's
> basic algebra/geometry.

I certainly agree, but I was so confused by the response that I figured that
for once I'd better keep my mouth shut.  ;-)

--
The whole religious complexion of the modern world is due
to the absence from Jerusalem of a lunatic asylum.
-- Havelock Ellis (1859-1939) British psychologist
```