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Re: map_set stereographic projection [message #48501 is a reply to message #48497] Fri, 28 April 2006 01:33 Go to previous messageGo to previous message
peter.albert@gmx.de is currently offline  peter.albert@gmx.de
Messages: 108
Registered: July 2005
Senior Member
Hi Daniel,

I found it often useful to use the 8-element limit keyword with
map_set. You are not resticted to the corners of the mapped region then
and have more freedom to choose the "anchor points". Moreover,
sometimes the corners aren't actually on earth, in those cases you
_must_ use the 8-element vector. It's a bit tricky to get used to, but
it's worth trying. Basically, you are specifying 4 points (as lat/lon
pairs) _anywhere_ on the left, upper, right and lower boundary of the
area. "Anywhere, how's that supposed to work?" Well, it just does.
Funny enough.

But then this is probably not the right way for your problem, as you'd
like to get the regular lat-lon projection. What you actually need are
the lon/lat values for each pixel. With those, you can either project
your data in any projection (using e.g. Liam Gumleys IMAGEMAP() routine
or our MAP_IMAGE__DEFINE object found at
http://wew.met.fu-berlin.de/idl).

If you actually want to transform the data itself into regular lon /
lat grid, e.g. for pixelwise comparison with other datasets, you might
want to use LONLAT2REG(), found on the same website. This routine
averages irregularly gridded lon / lat data into any regular lon / lat
array.

But now, how do you get the lon/lat values for each pixel?

First try: Ask the data provider. They should have the data. Somewhere.
Second try: Download the "proj" software and calculate them yourself.
(http://proj.maptools.org/). proj is _not_ easy to get started with,
but if you have to do map transformations more than once (including,
possibly, different ellipsoids), it's well woth every minute spent
reading the manual.

I would recommend calculating all map coordinates in map space first
(i.e. coordinates being given in x = meters east of greenwich meridian;
y = meters from the equator) and use proj to transform those into
lat/lon. I have to admit that I only hat to deal with sinusoidal
projection so far, which was pretty easy, and do not know how to
actually approach polar stereographic. But in case you don't get the
lat/lon data from the data providers, I would recommend to give proj a
good try. And the proj people have a quite helpful mailing list, too...


Best regards,

Peter

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