Question

I’m drawing points on a map with OpenLayers like in this example: http://dev.openlayers.org/examples/draw-feature.html

Now I want to know, which area (in meters) is covered by such a drawn point. I know, this depends on the zoom level. And this is also my plan: I want to draw my points with a different size – depending on the zoom level. If the zoom level is maximum, the point should be big. If the zoom level is low, the point should be drawn very small.

Has anyone an idea, how to calculate the point size from pixel to meter?

Answer 1

You could use the map’s resolution, which is defined as map units per pixel.
So, assuming your map units are meters, the required pixel size would be:
size_in_meters / map_resolution.

You can use the above calculation in a style map to have points’ sizes change dynamically as map resolution changes:

styleMap = new OpenLayers.StyleMap({
    'default': new OpenLayers.Style({
        pointRadius: "${getSize}"
    },
    { context: {
        getSize: function(feature) {
            return size_in_meters / feature.layer.map.getResolution();
        }}  
    })
});