defining projections

[Brent Fraser]

Ben,

  If your shapefile data is in a different coordinate
system than your web map, you will have to specify
the projection information for both the data and the
rendered web map.

  For the web map, projection information is in the
"header" part of the map file (I put it after LINESET).
======================================
EXTENT 518510.066 5555675.320 1004018.796 5911026.391
UNITS meters
LINESET   symbols/line.sym

# View Projection definition
PROJECTION
  "proj=tmerc"
  "ellps=clrk66"
  "k=0.9996"
  "x_0=500000"
  "lon_0=117w"
END
======================================

The projection information for your shapefiles is in
the LAYER definition (for each layer):

LAYER
  NAME a.shp
  PROJECTION
    "geographic"
  END
  TYPE PolyLine
     :

The definition of the projection parameters in covered
in the PROJ4 doc, but they will look something like
(not that I've tested it!):

PROJECTION
  "proj=poly"
  "ellps=GRS80"
  "lon_0=77.75w"
  "lat_0=40.925n"
END

You may want to include a false easting ("x_0=0.0"),
a false northing ("y_0=0.0") and possibly a scale
factor ("k=1.0").

I expect that MapServer/Proj4 will handle geographic
values for the extent but the coordinates of the extent
MUST be in the coordinate system of the web map.
You cannot specify a web map projection of polyconic
then give the extents in degrees.

Hope  this helps...

Brent Fraser
bfraser at geoanalytic.com
GeoAnalytic Inc.
#300 , 700 - 4th Avenue SW Calgary, AB
Canada T2P 3J4
Tel:  (403)213-2700 Fax:  (403)213-2707
www.geoanalytic.com


----- Original Message -----
From: "Ben Lewis" <benlewis1 at hotmail.com>
To: <mapserver-users at lists.gis.umn.edu>
Sent: Tuesday, October 31, 2000 1:14 PM
Subject: defining projections


> Mapserver users,
> First I want to say I am really impressed with MapServer. It seems to be
> very fast and robust. I got the demo version of mapserver running on NT,
and
> I've been able to set up a site pointing at other data in the UTM
> projection, but am having trouble getting data in Polyconic space to
> display.  There is no error message, just does not show up. What is the
> syntax for referring to data in:
>
> Polyconic projection
> elipsoid GRS80
> Long of origin -77:45:00
> Lat of origin  40:55:30
> Units meters
> Extent is minx, miny, maxx, maxy?
>
> Also if I go with geographic how to I specify extent?  Does PROJ4 handle
> degrees?
>
> Am I in effect asking about PROJ4 commands here?  Would this kind of
> information be in the PROJ4 documentation?
>
> - Ben Lewis
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