Polar Stereographic

[Chris Helm]

Dominic,

I dont know if this will help but I use polar
stereographic to display data in high latitudes. Most
of the layers in my mapfile are are defined in lat
long coords. and I've used some javascript that
changes the entire mapfile projection block to
streographic based on where the user is in the
application.

I use different projection definitions to display data
in different regions, using lon_ts to center the map
in a particular region.

s_polar =
"proj=stere,ellp=WGS84,lat_ts=-71,lat_0=-90,lon_0=0,k_0=1.0"
n_polar =
"proj=stere,ellp=WGS84,lat_ts=71,lat_0=90,lon_0=0,k_0=1.0"

And for a map centered on alaska:
n_polar_ak =
"proj=stere,ellp=WGS84,lat_ts=71,lat_0=90,lon_0=-140,k_0=1.0"


I originally thought I could use lat_ts=90 but in
calculating the inverse equations (for coord display)
I found if i used something like -71 it worked fine.

I also dont use x_0 and y_0.

This is kind of a confusing answer but I hope it
helps,

Chris

--- Dominic Lowe <d.lowe at RL.AC.UK> wrote:

> Hi All,
>
> This query is probably more to do with map
> projections than with
> Mapserver, depending on where I'm going wrong...
>
> I'm trying to display Polar Stereographic maps (of
> Europe) in Mapserver.
> I can successfully display the maps, but something
> is not right as they
> don't reproject properly to other coordinate systems
> (eg epsg:4326) - by
> this I mean they reproject but the map will appear
> distorted in the
> wrong part of the lat lon world.
>
> So anyway I think that I've just created an arbitary
> projection and got
> them to display somehow. What I'd like to do is
> display them in an epsg
> compliant Polar Stereographic...
>
> I've added a line to the epsg file:
> <9810> +proj=stere +lat_ts=90 +lat_0=90 +lon_0=0
> +k_0=1.0 +x_0=0 +y_0=0
>
> this is based on the information here:
>
http://www.remotesensing.org/geotiff/proj_list/polar_stereographic.html
> North Pole (NatOriginLat > 0): *
>
>   +proj=stere +lat_ts=/Latitude at natural origin/
>               +lat_0=90
>               +lon_0=/Longitude at natural origin/
>               +k_0=/Scale factor at natural origin/
> (normally 1.0)
>               +x_0=/False Easting/
>               +y_0=/False Northing/
>
> *So I think by setting lat_ts to 90 I've assumed
> that the "projected
> image" is tangential to the north pole.
>
> So then I've tried different methods for converting
> the lat lon
> coordinates into Eastings Northings to be used in a
> world file to
> accompany the image. If the 9810 definition is ok
> then I think this is
> where I'm going wrong - there seem to be several
> possible coordinate
> systems that lay claim to the name 'Polar
> stereographic' - (eg UPS - is
> that the same as vanilla Polar?) and I'm not sure
> which one relates to
> epsg:9810 that I 'think' I've defined.
>
> Anyway, I'd very much appreciated it if anyone has
> any experience of
> using Polar Stereographic in Mapserver and could
> help clarify these issues.
>
> Regards
>
> Dominic
>




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