Hi Chris,
<br>I don't know why I just got a copy of this message several months late.
<br>However it reminds me that I solved the problem and never reported it.
<br>It is not a problem of Spherical Mercator or of OpenLayers.
<br><br>The problem is with the definition of the "Gauss Boaga" projection
<br>(that ESRI call Monte Mario ITaly). I did specify that the ellipsoid
<br>is the Hayford 1909 (+ellps=intl) but I did not specify that the datum
<br>is Rome 1940, i.e. that the ellipsoid is rotated towards Monte Mario
<br>(a mountain near Rome).
<br><br>In fact I never worked out how to do it, so I used a workaround in my project.
<br><br>Regards
<br><br>--
<br>George Mu'ammar
<br><br><br>Quoting "Christopher Schmidt-2 (via Nabble)"
<br><<a href="http://n2.nabble.com/user/SendEmail.jtp?type=node&node=1877997&i=0" target="_top" rel="nofollow">ml-user+64170-846031244@...</a>>:
<br><div class='shrinkable-quote'><br>>
<br>>
<br>>
<br>> On Fri, Aug 08, 2008 at 01:02:31PM -0700, George.Muammar wrote:
<br>>> The road layer (Teleatlas / Navteq) comes in UTM - WGS84 and I also combine
<br>>> it with my cadastral data. I play with these projections every day and are
<br>>> very confident with them - but not so with the so called "spherical
<br>>> mercator"
<br>>
<br>> Using Google Maps in EPSG:4326 *must* lead to offsets within OpenLayers.
<br>> There is no possible way to avoid this, as Google Maps data is
<br>> *projected* data, and EPSG:4326 is a 'flat' projection, using lat/lon as
<br>> planar units. The end result is that there is no way that you can
<br>> correctly overlay data on top of Google Maps, Virtual Earth, or any
<br>> other commercial baselayer.
<br>>
<br>> However, reading the following, I realize you are using spherical
<br>> mercator? At least, I Think? In which case, the answer is "yes, I have
<br>> used this with 'very accurate maps', and had no problems getting pixel
<br>> perfect overlays on top of Google Maps. However, I have also seen
<br>> significant offsets, *especially* in VirtualEarth (in the 100M range)
<br>> even in areas where I would typicaly expect better results.
<br>> (Specifically, New Orleans in VE at some zoom levels is *bad*.)
<br>>
<br>> I don't see anything wrong with your configuration here; Have you tried
<br>> laying over Google instead?
<br>>
<br>> Another comment is that it is possible that your proj.4 version may
<br>> affect things: I've heard people reporting problems with older proj.4
<br>> libraries not respecting the +nadgrids=@null hack.
<br>>
<br>> Just a few thoughts.
<br>>
<br>>>
<br>>> Does anybody have any opinion in the matter ? has anyone tried to overlay
<br>>> Google / VE / Yahoo with very accurate maps ?
<br>>>
<br>>> For the record I am doing the following: (maybe something wrong)
<br>>>
<br>>> projections in Proj.4 epsg file:
<br>>>
<br>>> # Spherical Mercator for Google etc.
<br>>> <900913> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
<br>>> +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs
<br>>> # GAUSS BOAGA EST
<br>>> <3004> +proj=tmerc +lat_0=0 +lon_0=15.0 +k=0.999600 +x_0=2520000 +y_0=0
<br>>> +ellps=intl +pm=0 +units=m no_defs <>
<br>>>
<br>>>
<br>>> I define my map thus:
<br>>> var smproj = new OpenLayers.Projection("EPSG:900913");
<br>>>
<br>>> mapOptions = {
<br>>> projection: smproj,
<br>>> units: "m",
<br>>> displayProjection: geoproj
<br>>> };
<br>>>
<br>>>
<br>>> my layers thus:
<br>>> var hyblayer = new OpenLayers.Layer.VirtualEarth( ....
<br>>> 'sphericalMercator': true});
<br>>>
<br>>> var pmwms = new OpenLayers.Layer.WMS( .....
<br>>> {'projection': smproj});
<br>>>
<br>>>
<br>>>
<br>>> --
<br>>> View this message in context:
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<br>>>
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<br>> --
<br>> Christopher Schmidt
<br>> MetaCarta
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<br>>
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