Function msCalculateScale

mapscale.c:77–111

** Calculate the approximate scale based on a few parameters. Note that this assumes the scale is ** the same in the x direction as in the y direction, so run msAdjustExtent(...) first. */

** the same in the x direction as in the y direction, so run msAdjustExtent(...) first.
*/
int msCalculateScale(rectObj extent, int units, int width, int height, double resolution, double *scale)
{
  double md, gd, center_y;

  /* if((extent.maxx == extent.minx) || (extent.maxy == extent.miny))   */
  if(!MS_VALID_EXTENT(extent)) {
    msSetError(MS_MISCERR, "Invalid image extent, minx=%lf, miny=%lf, maxx=%lf, maxy=%lf.", "msCalculateScale()", extent.minx, extent.miny, extent.maxx, extent.maxy);
    return(MS_FAILURE);
  }
  
  if((width <= 0) || (height <= 0)) {
    msSetError(MS_MISCERR, "Invalid image width or height.", "msCalculateScale()");
    return(MS_FAILURE);
  }

  switch (units) {
  case(MS_DD):
  case(MS_METERS):    
  case(MS_KILOMETERS):
  case(MS_MILES):
  case(MS_NAUTICALMILES):
  case(MS_INCHES):  
  case(MS_FEET):
    center_y = (extent.miny+extent.maxy)/2.0;
    md = (width-1)/(resolution*msInchesPerUnit(units, center_y)); /* remember, we use a pixel-center to pixel-center extent, hence the width-1 */
    gd = extent.maxx - extent.minx;
    *scale = gd/md;
    break;
  default:
    *scale = -1; /* this is not an error */
    break;
  }

  return(MS_SUCCESS);
}

double msInchesPerUnit(int units, double center_lat)
{