Thanks for the clarification Ed (and for fixing the cross-list-
posting-bug :)). I just looked into this more and you are correct (I
was completely wrong). From the site that Bradley quoted, they seem
to think that the spherical projection is what is used by Google
etc. The (unverified) proj.4 parameters for this seem to be:
+proj=merc +lat_ts=0 +lon_0=0 +k=1.0 +x_0=0 +y_0=0 +a=6378137.0
+b=6378137.0 +units=m
Cheers
Paul
On 9-Jul-07, at 2:06 PM, Ed McNierney wrote:
Paul -
This doesn't seem correct to me. You're suggesting a Transverse
Mercator
projection on an ellipsoid when the request was for a Mercator on a
sphere.
I did a little Googling around and while it appears that the 41001
projection is often referred to as "Transverse Mercator", and there
are
several references to the PROJ definition you mention (+proj=tmerc,
etc.),
there is a more sensible reference to 41001 being defined in a
GeoServer
install as:
PROJCS["WGS84 / Simple Mercator",
GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS_1984", 6378137.0, 298.257223563]],
PRIMEM["Greenwich", 0.0],
UNIT["degree", 0.017453292519943295],
AXIS["Lon", EAST],
AXIS["Lat", NORTH]],
PROJECTION["Mercator_1SP"],
PARAMETER["central_meridian", 0.0],
PARAMETER["scale_factor", 1.0],
PARAMETER["false_easting", 0.0],
PARAMETER["false_northing", 0.0],
UNIT["m", 1.0],
AXIS["x", EAST],
AXIS["y", NORTH],
AUTHORITY["EPSG","41001"]]
This is more sensible (at least it's a Mercator projection!) but it
is still
a ellipsoidal rather than spherical projection, so it's not quite
right.
However, the entire universe of Web-based "geographic" projection
fans,
stimulated by Google and Microsoft, obviously don't really get
bothered when
their maps are "not quite right", so I should learn not to be so
fussy.
- Ed
P.S. I modified the reply to go only to the MAPSERVER-USERS list,
since the
original cross-posting to both lists only complicates matters for
people
trying to help.
From: Paul Spencer <[EMAIL PROTECTED]>
Reply-To: Paul Spencer <[EMAIL PROTECTED]>
Date: Sun, 8 Jul 2007 22:17:03 -0400
To: <[EMAIL PROTECTED]>
Subject: Re: [UMN_MAPSERVER-DEV] Projections
You need what is commonly referred to as EPSG:41001, defined as:
<41001> +proj=tmerc +ellps=WGS84 +units=m +no_defs <>
To use this, add the above line to your 'epsg' file and also add a
projection object to your map object:
PROJECTION
"init=epsg:41001"
END
You will also need to make sure that every layer in your map file has
a proper projection so mapserver can reproject it correctly.
Cheers
Paul
On 8-Jul-07, at 9:44 PM, Bradley Mclain wrote:
This is kind of hard to explain but what I want to do is use
Mapserver to project my data which is in the GDA94 projection as
shown below:
"# GDA94 / MGA zone 55
<28355> +proj=utm +zone=55 +south +ellps=GRS80
+towgs84=0,0,0,0,0,0,0 +units=m +no_defs <>"
to a projection that will be able to be properly merged with Google
maps (using OpenLayers) which uses a projection something like the
following:
"Update 4: Agreement frrom Melita Kennedy and David Burrows that
Google Maps and Virtual Earth use spherical equations for the
Mercator projection. The correct proj4 settings are:
+proj=merc +latts=0 +lon0=0 +k=1.0 +x0=0 +y0=0
+a=6378137.0 +b=6378137.0 +units=m
Note this is different than using the ellipsoidal equations which
would be:
+proj=merc +latts=0 +lon0=0 +k=1.0 +x0=0 +y0=0
+ellps=WGS84 +datum=WGS84 +units=m no_defs"
I believe that I need to use Mapserver to do this but I have no
idea how to go about it. Currently my Mapserver is running fine
displaying the data correctly using the GDA94 projection except i
had to use a .vrt file to correctly align the raster image which
was slightly out of alignment. I have also set up and can call my
Mapserver as a WMS (thought having issues with the WMS Layer
function in OpenLayers, just displays a white screen but I believe
that this is to do with projections)
My Mapserver has been compiled with GD, GDAL, GEOS, PostGIS, PHP
and proj4.
In my mapfile I have defined each layer using "init=epsg:28355" as
well as globally defining the map using the same projection.
This reason I am trying to do this is because currently I don't
think that my projection can be properly imported into OpenLayers
and merged with Google Maps.
Thanks
Brad
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|Paul Spencer [EMAIL PROTECTED] |
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|Chief Technology Officer |
|DM Solutions Group Inc http://www.dmsolutions.ca/ |
+-----------------------------------------------------------------+
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|Paul Spencer [EMAIL PROTECTED] |
+-----------------------------------------------------------------+
|Chief Technology Officer |
|DM Solutions Group Inc http://www.dmsolutions.ca/ |
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