Born of this thread...

http://www.cartographersguild.com/showthread.php?t=2279

about a free way to get the area of a part of a raster image - such as an island. here is a (and not necessarily the best) way of doing it.

If you have a map of a known area or you can chop out a rectangle of known area that covers an irregular shape in the middle then you can use this method.

What you will need is a black and white image of the bit of map you want to measure. You can usually flood fill or use some contrast / brightness foo to get you that image but if not then you will have to trace it - maybe in a new layer.

So, to start with I have my Modron island chain. Naturally I chose this because we can empirically calculate the area to check the result. The image is 1024x1024 and the circles have radii of 200, 100, & 50 pixels.

If the image was not square then it does not matter, you can resample it to 1024x1024. It makes no difference - its the black to white ratio thats required.