[UMN_MAPSERVER-DEV] Projections
Ed McNierney
ed at TOPOZONE.COM
Mon Jul 9 22:32:45 EDT 2007
Bradley -
BBOX is a required WMS parameter, so you must be using it.
You have to be able to express the geographic extents of your area of
interest in the units used by your desired map projection. Otherwise
there's no way you can know what you're asking for. I don't use
OpenLayers and can't help you on its configuration, but you can use the
PROJ command-line tools to reproject individual coordinate pairs from
your input to your output projections so you can calculate a starting
bounding box for your map.
- Ed
From: Bradley Mclain [mailto:bradleym06 at gmail.com]
Sent: Monday, July 09, 2007 10:11 PM
To: Ed McNierney
Cc: MAPSERVER-USERS at lists.umn.edu
Subject: Re: [UMN_MAPSERVER-USERS] [UMN_MAPSERVER-DEV] Projections
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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