[OpenLayers-Users] distances in spherical mercator

[Alessio Di Lorenzo]

Hi Bill and thanks for the answer,

I solved with this Tim Shaub's patch 

http://trac.openlayers.org/attachment/ticket/1819/geodesic.patch

ciao, alessio








Bill Thoen wrote:
> 
> Alessio,
> 
> I think you can just use the planar distances because the google SRS is 
> a projected coordinate system and the coordinate units are meters.. 
> Whether it's very accurate over long distances I don't know. I doubt it, 
> but t might not be too far off depending on your needs. Google 
> coordinates certainly won't work with Vincenty because that expects 
> coordinates  in decimal degrees.
> 
> - Bill Thoen
> 
> Alessio Di Lorenzo wrote:
>> Hi all,
>>
>> I'm using the measurement tool in an application that uses google maps as
>> base.
>> My problem is that the measurement tool calculate the planar distance
>> (instead geodesic)...
>>
>> I tried to use the Util.distVincenty and it works (very good) only if my
>> map
>> projection is 4326.
>>
>> Snippet:
>>
>> function calcVincenty(geometry) {
>>     var dist = 0;
>>     for (var i = 1; i < geometry.components.length; i++) {
>>         var first = geometry.components[i-1];
>>         var second = geometry.components[i];
>>         dist += OpenLayers.Util.distVincenty(
>>             {lon: first.x, lat: first.y},
>>             {lon: second.x, lat: second.y}
>>         );
>>     }
>>     return dist;
>> }     
>>
>>
>> function handleMeasurements(event) {
>> 	
>>     var geometry = event.geometry;
>>     var units = event.units;
>>     var order = event.order;
>>     var measure = event.measure;
>>     var element = document.getElementById('output_misure');
>>     var out = "";
>>     if(order == 1) {
>>     	
>>     	if (map.getProjection()== "EPSG:4326") {
>>     		out += "Distanza: " + calcVincenty(geometry).toFixed(3)+ " " +
>> units;
>>     		
>>     	} else {
>>     		out += "Distanza: " + measure.toFixed(3)+ " " + units;
>>     	}
>>     } else {
>>     	if (map.getProjection() == "EPSG:4326") {
>>     		out += "Area: " +
>> (OpenLayers.Util.approxPolyAreaOnSphere(geometry)/1000000.0).toFixed(3)+
>> " "
>> + units + "q";
>>     	} else {
>>     		out += "Area: " + measure.toFixed(3) + " " + units + "q";
>>     	}
>>     }
>> 		element.innerHTML = out;
>> }
>>
>>
>> If I apply the calcVincenty function to the 900913 projection I get
>> enormous
>> distance errors...
>> Is there a way to measure geodesic distances (and areas) also for 900913
>> projected maps?
>>
>> Thx in advance, alessio
>>   
> 
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