Polar Stereographics with rotation

Hi,

I am working with data in polar-stereographic projection defined by
- latitude and longitude of origin
- orientation (angle relative to 0-meridian)
- number of cells in grid (sizex, sizey)
- cell-size in meters (dx)

I need to convert the grid-cells so I can work with each cell in a
simple latitude-longitude based system, and I have obtained a formula
(piece of FORTRAN code actually) that will do the trick. Only problem
is that this
formula doesn't require the latitude and longitude of origin - rather
it
requires the position of the pole in grid coordinates, and I simply
can
figure out a way to obtain the pole-grid-coordinates from the
lat-lon-of-
origin.

Hope somebody can help me out a little or at least point me in the
right .
direction.


Thank you in advance
Hans Olav Nymand
hon@pdc.dk

On 24 Feb 2005 12:03:19 -0800, hon@pdc.dk (Hans Olav Nymand) wrote:



Dear Hans,

If I have understood you correctly, you have the latitude and
longitude of the corner coordinate of your grid, the orientation (or
central meridian) and the cell sizes in metres and the dimensions of
the grid in cells. So, what you have is a simple translation problem.
You are lacking one pience of information , though, and this may cook
your goose!

Assuming that the latitude of true scale is at the Pole (not the usual
case, but all you can do with the information given), than compute the
projection coordinates of the corner point using the central meridian.
This should be straightforward given the code you have - if not, proj
(www.freegis.org) provides an excellent and reliable means of doing
these calculations. Then simply apply your cell sizes to determine
which cell has the coordinates 0,0. This cell is the position of the
pole within the grid.

I mentioned that you do not in general have sufficient information.
Polar Stereographic projections usually apply a scale factor that
corresponds to a latitude of true scale - if this were not the case,
the scale error would always increase away from the pole. There are
conventions for Antarctica (-71S) but I am not aware of any for the
Arctic.

Good luck

Paul