calculate scale from TMS/Google/OSM zoom level

ZoomScale

    double latitude = self.mapView.mapCenter.latitude;
    metersPerPixel *= cos(latitude * (M_PI / 180));
    double metersPerUnit = 1609.344;
    int numberOfUnits = 1;
    NSString *unitName = MILES_UNIT_STRING;
    if (metersPerPixel < (1000/maxScaleSize)) {
      unitName = METERS_UNIT_STRING;
      metersPerUnit = 1;
    } else {
      unitName = KILOMETERS_UNIT_STRING;
      metersPerUnit =1000;
    }
    
    double unitLength = metersPerUnit / metersPerPixel;
    if (unitLength < maxScaleSize) {
      numberOfUnits = pow(10, round(log10f((maxScaleSize/2.0)/unitLength)));
      unitLength *= numberOfUnits;
    }
    
    if (unitLength < (maxScaleSize / 5)) {
      numberOfUnits *=5;
    } else if (unitLength < (maxScaleSize / 2)) {
      numberOfUnits *= 2;
    } else if (unitLength > maxScaleSize && numberOfUnits > 1) {
      numberOfUnits/=2;
    }
    unitLength = metersPerUnit * numberOfUnits / metersPerPixel;
    
    unitName = [NSString stringWithFormat:@"%d %@", numberOfUnits, unitName];
    
    dispatch_async(dispatch_get_main_queue(), ^{
      [self setScaleWithLabel:unitName andSize:unitLength];
    });
  });