I am trying to use resolution passed into the StyleFunction to work out the size of my image Icons. Using tests, at a zoom where the scale line is 100m the resolution reported to the styling function is 2.3886.
I've take screenshots of the scale line and measured its length in pixels. A 100m scaleline is 68 pixels, or 1.4705 metres per pixel.
1.4705 !== 2.3886, so what is the resolution unit? The API documentation does not explain it and says it is just a number, but without an idea of units it is difficult to work out.
This is to accurately scale the icon to real world length BTW.
Using this jsfiddle.net/dz9gL0g0/ I find that 200m scaleline reports the 2.38, but 100m returns less (1.19). Is the resolution I'm getting from the previous zoomlevel? If I use the resolution passed in OR call the getResolution function directly, 100m scaleline always returns 2.83 for me, not the 1.19 I think it should, although 1.19 * 84 is mostly correct (scale line is bigger in example than my app, which gives me a 68 pixel scale line for 100m).
Moving the window alters the resolution - resize the jsfiddle and the resolution unit changes. My 100m scaleline still fits the geographical feature I use to test, but now reports 2.38.
After posting a bug request at github (https://github.com/openlayers/ol3/issues/3770) it was suggested that ol.proj.Projection.getPointResolution() be used to gather the correct resolution for the projection (which I assumed was being passed into the styling function at run-time - assumptions, eh?).
var view = map.getView();
var coords = view.getCenter();
var resolution = view.getResolution();
var projection = view.getProjection();
var resolutionAtCoords = projection.getPointResolution(resolution, coords);
This results in a much closer 1.50XXXX resolution being returned, which is dependant on the latitude. It remains to be seen if my tiled map source is scaled appropriately to the projection, but this means that my pixels per metre are now within a margin of error, correct.