[OpenLayers-Users] EPSG:4326 WMS map aspect ratio
Richard Greenwood
richard.greenwood at gmail.com
Thu May 30 13:59:45 PDT 2013
On Thu, May 30, 2013 at 9:52 AM, Smart, Gary <Gary.Smart at utas.utc.com>wrote:
> Thanks for the response. However, I am not in control of the WMS maps
> supplied.
>
Have you issued a getcapabilities request against the server and verified
that only epsg:4326 is available? A request would look something like:
http://www.youserver.com/wms?service=WMS&version=1.3.0&request=GetCapabilities<http://maps.greenwoodmap.com/cgi-bin/mapserv?map=MAP_22_MAIN6&service=WMS&version=1.3.0&request=GetCapabilities>
and would return an xml document. Available coordinate systems would appear
between <CRS> </CRS> tags.
> I certainly cannot insist that the customer re-projects his entire WMS
> contents.
>
The customer is always right, but we can try to educate them.
> ****
>
> I am looking for a way to make the client (openLayers) adjust the
> projection (to its native spherical Mercator) prior to display?
>
You are asking OpenLayers to reproject raster data and that's not something
that OpenLayers can do. Maybe someone else knows of a kluge that would make
it look better.
> ****
>
> Incidentally, WorldWind has no problem displaying maps from the very same
> WMS and displaying a square OS grid.
>
I am not familiar with WorldWind but I have seen other desktop GIS software
that will do some undefined projection of lat/long data to make it look
nicer on screen. And that sounds like what you're looking for. Still, in my
mind, the "right" way to do it is to use an appropriate local coordinate
system. That's what they are there for.
Good luck,
Rich
****
>
> ** *From:* Richard Greenwood [mailto:richard.greenwood at gmail.com]
>
> *Sent:* 25 May 2013 16:03
> *To:* Smart, Gary
> *Cc:* OpenLayers
> *Subject:* [External] Re: [OpenLayers-Users] EPSG:4326 WMS map aspect
> ratio****
>
> ** **
>
> I would suggest that you use an appropriate map projection for your area
> of interest. That's what projections are for. Ideally you project all your
> data and save it in that projection, although it is possible to reproject
> on the server, or on the client.****
>
> Rich****
>
> ** **
>
> On Wed, May 22, 2013 at 7:59 AM, Smart, Gary <Gary.Smart at utas.utc.com>
> wrote:****
>
> I really would like to display the contents of my local WMS in a form
> which the users are used to (i.e. square OS grids, rather than the
> ‘squashed’ appearance which comes from mapping the EPSG:4326 onto a
> flat-Earth projection in high latitudes). I have found numerous examples
> of’ how to resolve this problem’, but none seem to have any effect on the
> distortion OR they result in no map being displayed at all. I cannot
> connect directly to map servers on the web (spherical Mercator), because my
> system is an isolated network and the maps are supplied by the customer.**
> **
>
> ****
>
> Salient extracts from the js code follow:****
>
> var map, options = { ****
>
> projection: new OpenLayers.Projection('EPSG:4326'), ****
>
> //displayProjection: new OpenLayers.Projection(‘EPSG:3574’),****
>
> //units: ‘m’,****
>
> //maxResolution: 156543.0339,****
>
> //maxExtent: new OpenLayers.Bounds(-20037508, -20037508, 20037508, 20037508)****
>
> zoom: 14****
>
> };****
>
> map = new OpenLayers.Map('map', options);****
>
> ****
>
> var my50kLayer = new OpenLayers.Layer.WMS( "OpenLayers WMS", ****
>
> "
> http://expdev2/cgi-bin/wms?", ****
>
> {****
>
> layers:
> 'UK_ASRP_1to50000',****
>
> format: 'image/png',*
> ***
>
> //transparent:
> 'true',****
>
> continuousWorld: true,
> ****
>
> attribution: "OS
> 1:50K",****
>
> }); ****
>
> map.addLayers([my50kLayer]);****
>
> ****
>
> As the code exists above, the displayed map works fine (apart from the
> squished grid rectangles). Having adapted the ‘measure example’ from the
> OpenLayers examples page, I can measure from the map, and if the geodesic
> selection is made, the measurements are clearly correct (1Km across both
> dimensions of the OS grid). With geodesic selection not clicked, the
> measurement across the map is approximately 1.5Km. So the transforms and
> projections are clearly in sync. But how do I stretch the maps in the ‘y’
> direction so they appear equant (not squished vertically)?****
>
> ****
>
> Adding the displayProjection seems to do nothing. Using any of the other
> options with my WMS, results in a blank map.****
>
> ****
>
> I have also plagiarised the polar-projections example from the OpenLayers
> example site, and replace the buttons with the following projections:****
>
> var projectionOptions = ****
>
> {****
>
> 'EPSG:4326':****
>
> {****
>
> projection: new OpenLayers.Projection('EPSG:4326')****
>
> },****
>
> 'EPSG:3857':****
>
> {****
>
> projection: new OpenLayers.Projection('EPSG:3857'),****
>
> maxExtent: new OpenLayers.Bounds(-5505054, -5505054, 5505054,
> 5505054),****
>
> maxResolution: 5505054 / 128,****
>
> numZoomLevels: 18****
>
> }****
>
> }; // end projectionOptions array****
>
> ****
>
> And I have applied the projections as per the example code (i.e.
> setProjection function was copied and is being called). However, pressing
> EPSG:3857 button results in a blank map, and pressing EPSG:4326 after that,
> also results in a blank map (albeit with a white pair of tiles in the
> middle)!****
>
> ****
>
> Can anyone tell me whether it is possible to display a square grid for OS
> maps coming from an EPSG:4326 WMS?****
>
> Thank you.****
>
> ****
>
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>
>
--
Richard Greenwood
richard.greenwood at gmail.com
www.greenwoodmap.com
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