[UMN_MAPSERVER-DEV] Projections
Bradley Mclain
bradleym06 at GMAIL.COM
Mon Jul 9 21:11:47 EDT 2007
When I have the custom projection defined in my mapfile and the native
projection defined for layers the map does not show up when doing a WMS
getmap request, it just appears white.
When I change it so that all layers have the custom projection, the map
displays correctly from a WMS request but fails to display correctly in
OpenLayers giving an SRS error.
On 7/10/07, Ed McNierney <ed at topozone.com> wrote:
>
> 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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