MonoTouch MKMapView with Zoom Level Property

MKMapViewZoomLevel.cs

using System;
using MapKit;
using CoreLocation;
using CoreGraphics;
using System.Drawing;
using Foundation;

namespace BinaryQuest
{
  [Register("MKMapViewZoomLevel")]
	public class MKMapViewZoomLevel : MKMapView
	{
		const double MERCATOR_OFFSET = 268435456;
		const double MERCATOR_RADIUS = 85445659.44705395d;

		public MKMapViewZoomLevel ():base()
		{

		}

		public MKMapViewZoomLevel(IntPtr handle):base(handle)
		{
		}


		private double longitudeToPixelSpaceX(double longitude)
		{
			return Math.Round(MERCATOR_OFFSET + MERCATOR_RADIUS * longitude * Math.PI / 180.0);
		}
		
		private double latitudeToPixelSpaceY(double latitude)
		{
			return Math.Round(MERCATOR_OFFSET - MERCATOR_RADIUS * Math.Log((1 + Math.Sin(latitude * Math.PI / 180.0)) / (1 - Math.Sin(latitude * Math.PI / 180.0))) / 2.0);
		}
		
		private double pixelSpaceXToLongitude (double pixelX)
		{
			return ((Math.Round(pixelX) - MERCATOR_OFFSET) / MERCATOR_RADIUS) * 180.0 / Math.PI;
		}
		
		private double pixelSpaceYToLatitude (double pixelY)
		{
			return (Math.PI / 2.0 - 2.0 * Math.Atan(Math.Exp((Math.Round(pixelY) - MERCATOR_OFFSET) / MERCATOR_RADIUS))) * 180.0 / Math.PI;
		}

		private MKCoordinateSpan coordinateSpanWithMapView (MKMapView mapView, CLLocationCoordinate2D centerCoordinate,	int zoomLevel)
		{
			// convert center coordiate to pixel space
			double centerPixelX = longitudeToPixelSpaceX(centerCoordinate.Longitude);
			double centerPixelY = latitudeToPixelSpaceY(centerCoordinate.Latitude);
			
			// determine the scale value from the zoom level
			int zoomExponent = 20 - zoomLevel;
			double zoomScale = Math.Pow(2, zoomExponent);
			
			// scale the map’s size in pixel space
			CGSize mapSizeInPixels = mapView.Bounds.Size;
			double scaledMapWidth = mapSizeInPixels.Width * zoomScale;
			double scaledMapHeight = mapSizeInPixels.Height * zoomScale;
			
			// figure out the position of the top-left pixel
			double topLeftPixelX = centerPixelX - (scaledMapWidth / 2);
			double topLeftPixelY = centerPixelY - (scaledMapHeight / 2);
			
			// find delta between left and right longitudes
			var minLng = pixelSpaceXToLongitude(topLeftPixelX);
			var maxLng = pixelSpaceXToLongitude(topLeftPixelX + scaledMapWidth);
			var longitudeDelta = maxLng - minLng;
			
			// find delta between top and bottom latitudes
			var minLat =  pixelSpaceYToLatitude(topLeftPixelY);
			var maxLat =  pixelSpaceYToLatitude(topLeftPixelY + scaledMapHeight);
			var latitudeDelta = -1 * (maxLat - minLat);
			
			// create and return the lat/lng span
			MKCoordinateSpan span = new MKCoordinateSpan(latitudeDelta, longitudeDelta);
			return span;
		}

		public void SetCenterCoordinate (CLLocationCoordinate2D centerCoordinate, int zoomLevel, bool animated)
		{
			// clamp large numbers to 28
			zoomLevel = Math.Min(zoomLevel, 28);
			
			// use the zoom level to compute the region
			MKCoordinateSpan span = coordinateSpanWithMapView(this, centerCoordinate, zoomLevel);
			MKCoordinateRegion region = new MKCoordinateRegion(centerCoordinate, span);
			
			// set the region like normal
			this.SetRegion (region, animated);
		}

		public int ZoomLevel
		{
			get{
				double zoom = 21 - Math.Round(Math.Log(this.Region.Span.LongitudeDelta * MERCATOR_RADIUS * Math.PI / (180.0 * this.Bounds.Size.Width)));
				Console.WriteLine("Return Zoom:" + zoom);
				return Convert.ToInt32(zoom);
			}
		}
	}
}

Made some changes to the original to take into account some changes with the Xamarin framework.

Originally taken from: http://troybrant.net/blog/2010/01/set-the-zoom-level-of-an-mkmapview/