[UMN_MAPSERVER-DEV] Projections

Mon Jul 9 19:11:26 PDT 2007

Being defined by server and clients at the moment shouldn't be an issue
because I am testing off the server at the moment which I have updated the
proj epsg file. Is this correct have I correctly understood this concept?

How do I get the correct extent for this new projection I have absolutely
now idea how to do it? I'm not using bbox so this shouldn't be a problem.

On 7/10/07, Ed McNierney <ed at topozone.com> wrote:
>
>  Bradley –
>
>
>
> Don't just experiment with combinatorial testing – think about what you're
> trying to do, make sure you understand it, then be sure your map file and
> request do it.  Since we've explained that whatever EPSG code you're using
> is not standard, you need to be sure that it is defined in both the server
> and all clients.  Have you done that?  Failure to do so would be an obvious
> cause of an "SRS error" from the client.
>
>
>
> Remember that when you change your map file output or your WMS request to
> a different SRS, you need to change the extent or BBOX parameters, too.  In
> a WMS request the BBOX parameter always uses the units of the SRS used, so
> you can't just keep the same GDA94 coordinates and expect them to work.  If
> you don't change them, you're requesting a Mercator output image at some
> random location that bears no relation to your input data, so it is quite
> likely to give you a white image (since no input data overlaps the requested
> map area).
>
>
>
> -          Ed
>
>
>
> *From:* UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] *On
> Behalf Of *Bradley Mclain
> *Sent:* Monday, July 09, 2007 9:12 PM
> *To:* MAPSERVER-USERS at LISTS.UMN.EDU
> *Subject:* Re: [UMN_MAPSERVER-USERS] [UMN_MAPSERVER-DEV] Projections
>
>
>
> 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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