CC3 Free now huh ? hee hee

Seriously tho, a CAD tool or dedicated mapping app - esp a vector based one should be able to do it but its not too much of a pain to do this for any raster image and although its approximate, you might be able to get the area without any tracing which might in some cases be quite tedious - thinking of the UK coastline.

Since not everyone has CC3 and unless somebody says that there is a nice free app to do just the thing, ill do a quick tut to show how you might be able to do it. Not saying its the best way but its a way.