[OpenLayers-Users] Conversion to Spherical Mercator - 100m error

[Geo-rge]

Hi Chris,
I don't know why I just got a copy of this message several months late.
However it reminds me that I solved the problem and never reported it.
It is not a problem of Spherical Mercator or of OpenLayers.

The problem is with the definition of the "Gauss Boaga" projection  
(that ESRI call Monte Mario ITaly). I did specify that the ellipsoid  
is the Hayford 1909 (+ellps=intl) but I did not specify that the datum  
is Rome 1940, i.e. that the ellipsoid is rotated towards Monte Mario  
(a mountain near Rome).

In fact I never worked out how to do it, so I used a workaround in my project.

Regards

-- 
George Mu'ammar


Quoting "Christopher Schmidt-2 (via Nabble)"  
<ml-user+64170-846031244 at n2.nabble.com>:

>
>
>
> On Fri, Aug 08, 2008 at 01:02:31PM -0700, George.Muammar wrote:
>> The road layer (Teleatlas / Navteq) comes in UTM - WGS84 and I also combine
>> it with my cadastral data. I play with these projections every day and are
>> very confident with them  - but not so with the so called "spherical
>> mercator"
>
> Using Google Maps in EPSG:4326 *must* lead to offsets within OpenLayers.
> There is no possible way to avoid this, as Google Maps data is
> *projected* data, and EPSG:4326 is a 'flat' projection, using lat/lon as
> planar units. The end result is that there is no way that you can
> correctly overlay data on top of Google Maps, Virtual Earth, or any
> other commercial baselayer.
>
> However, reading the following, I realize you are using spherical
> mercator? At least, I Think? In which case, the answer is "yes, I have
> used this with 'very accurate maps', and had no problems getting pixel
> perfect overlays on top of Google Maps. However, I have also seen
> significant offsets, *especially* in VirtualEarth (in the 100M range)
> even in areas where I would typicaly expect better results.
> (Specifically, New Orleans in VE at some zoom levels is *bad*.)
>
> I don't see anything wrong with your configuration here; Have you tried
> laying over Google instead?
>
> Another comment is that it is possible that your proj.4 version may
> affect things: I've heard people reporting problems with older proj.4
> libraries not respecting the +nadgrids=@null hack.
>
> Just a few thoughts.
>
>>
>> Does anybody have any opinion in the matter ? has anyone tried to overlay
>> Google / VE / Yahoo with very accurate maps ?
>>
>> For the record I am doing the following: (maybe something wrong)
>>
>> projections in Proj.4 epsg file:
>>
>> # Spherical Mercator for Google etc.
>> <900913> +proj=merc +a=6378137 +b=6378137 +lat_ts=0.0 +lon_0=0.0 +x_0=0.0
>> +y_0=0 +k=1.0 +units=m +nadgrids=@null +no_defs
>> #	GAUSS BOAGA EST
>> <3004> +proj=tmerc +lat_0=0 +lon_0=15.0 +k=0.999600 +x_0=2520000 +y_0=0
>> +ellps=intl +pm=0 +units=m  no_defs <>
>>
>>
>> I define my map thus:
>> 	var smproj = new OpenLayers.Projection("EPSG:900913");
>>
>> 	mapOptions = {
>> 	    projection: smproj,
>> 	    units: "m",
>> 	    displayProjection: geoproj
>> 	};
>>
>>
>> my layers thus:
>>            var hyblayer = new OpenLayers.Layer.VirtualEarth( ....
>> 'sphericalMercator': true});
>>
>> 		var pmwms = new OpenLayers.Layer.WMS( .....
>>                           {'projection': smproj});
>>
>>
>>
>> --
>> View this message in context:   
>> http://www.nabble.com/Conversion-to-Spherical-Mercator---100m-error-tp18898188p18898188.html
>> Sent from the OpenLayers Users mailing list archive at Nabble.com.
>>
>> _______________________________________________
>> Users mailing list
>> Users at openlayers.org
>> http://openlayers.org/mailman/listinfo/users
>
> --
> Christopher Schmidt
> MetaCarta
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> ______________________________________
>
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> http://n2.nabble.com/Conversion-to-Spherical-Mercator---100m-error-tp1829789p1829790.html
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>
>



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View this message in context: http://n2.nabble.com/Conversion-to-Spherical-Mercator---100m-error-tp1829789p1877997.html
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