[UMN_MAPSERVER-DEV] Projections

Ed McNierney ed at TOPOZONE.COM
Mon Jul 9 15:20:05 EDT 2007 

Paul -

Those parameters look good.  There is obviously not a "standard" EPSG code
for this projection.

Bradley, you need to decide whether you need to serve these WMS images out
to the wide world (i.e. clients over which you have no control) or only use
them "internally" where you can control both your WMS server and the client.

If it's the former, you have a problem, as there is no EPSG code describing
the projection you need that seems to be in general usage so your clients
can be expected to support it.  If it's the latter, you're fine.  You can
simply pick any number you like that's not already taken and add that line
to your EPSG file.  It's just a text file.  Let's assume you'll pick 41002
(since 41001 seems to be in popular usage, but is wrong).  Then you'd add
the single text line:

<41002>+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

To the EPSG files on your server and on each client.  Then you can refer to
it as SRS=EPSG:41002 in your WMS requests and you're all set.

    - Ed

> From: Paul Spencer <pspencer at DMSOLUTIONS.CA>
> Date: Mon, 9 Jul 2007 14:55:22 -0400
> To: Ed McNierney <ed at topozone.com>
> Cc: "MAPSERVER-USERS at LISTS.UMN.EDU" <MAPSERVER-USERS at LISTS.UMN.EDU>
> Subject: Re: [UMN_MAPSERVER-DEV] Projections
> 
> 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 <pspencer at DMSOLUTIONS.CA>
>>> Reply-To: Paul Spencer <pspencer at DMSOLUTIONS.CA>
>>> Date: Sun, 8 Jul 2007 22:17:03 -0400
>>> To: <MAPSERVER-DEV at LISTS.UMN.EDU>
>>> 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
>>> 
>>> +-----------------------------------------------------------------+
>>> |Paul Spencer                          pspencer at dmsolutions.ca    |
>>> +-----------------------------------------------------------------+
>>> |Chief Technology Officer                                         |
>>> |DM Solutions Group Inc                http://www.dmsolutions.ca/ |
>>> +-----------------------------------------------------------------+
>> 
> 
> +-----------------------------------------------------------------+
> |Paul Spencer                          pspencer at dmsolutions.ca    |
> +-----------------------------------------------------------------+
> |Chief Technology Officer                                         |
> |DM Solutions Group Inc                http://www.dmsolutions.ca/ |
> +-----------------------------------------------------------------+
> 
> 
> 
> 
>

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