title: Map scale factor when converting from Mercator to Equirectangular
body: <p>Lets say I have a map in Mercator projection, and I know top and bottom latitudes:</p>
  
  <p>topLatitude = 80; bottomLatitude = -55;</p>
  
  <p>I also know width and height of a map:</p>
  
  <p>width = 800; height = 500;</p>
  
  <p>I want to rescale the map to Equirectangular projection, keeping the same width.</p>
  
  <p>How can I calculate new height of a map?</p>