title: How to transform a distance from degrees to metres?
body: <p>I'm using OpenLayers with an ordinary mercator map and I'm trying to sample a bounding box by finding a grid of points in latlong.
  The bbox is expressed in latlon, e.g.</p>
  
  <pre><code>48.1388,-15.3616,55.2057,-3.9359
  </code></pre>
  
  <p>I can define a distance in <strong>degrees</strong> (e.g. x: 2.5, y: 2.4) and work out the points from there.
  But I'd like to express this distance in <strong>metres</strong> (e.g. 50000) in order to relate it to the user mindset (people understand metres, not degrees). 
  How can I convert this distance? I know how to reproject a point, but not a distance.</p>
  
  <p>Thanks for any hints!
  Mulone</p>