Calculating the diameter of a google map - google-maps

Question

I want to display a google map on my site along with a caption of how large of an area is covered in miles. I will use the area between the left and right borders for this.
For example under the map it will say "3 miles" and then when the user zooms in, it will change to "1 mile" and so on
Thanks

Answer 1

The API has a distance method which returns the distance in meters. So you can do it like this:
var bounds = myMap.getBounds();
google.maps.geometry.spherical.computeDistanceBetween(bounds.getNorthEast(), bounds.getSouthWest());

Answer 2

Start by calling the getBounds() method on the Map object. This will return a LatLngBounds object from which you'll know the latitude and longitude of the bounding box the map defines.
Now that you're armed with the latitude and longitudes you'll simply need to apply the Haversine formula to computer the distance. For a good overview of this equation, and an example in Javascript see this page. An excerpt of which is:
var R = 6371; // km
var dLat = (lat2-lat1).toRad();
var dLon = (lon2-lon1).toRad();
var lat1 = lat1.toRad();
var lat2 = lat2.toRad();
var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(lat1) * Math.cos(lat2);
var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
var d = R * c;
Since the above equation operates in kilometers, and you asked for miles, one additional modification you could perform would be to convert the output (d) to miles. Converting kilometers to miles is simply a matter of dividing by 1.609344. So, you could simply:
d /= 1.609344;
Expanding upon this, you could then even allow the user to choose between miles/kilometers, and wrap the whole thing up in an if statement:
if(miles)
d /= 1.609344;