[OpenLayers-Users] distances in spherical mercator

[Bill Thoen]

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
>