You have been captured by a mad king, he has placed you in a large field, he says you are free to go once you have calculated the area of the field.

He has left you with 1 pencil, 1 piece of paper, and 1 30cm ruler.

What is the quickest way to work out the area of the field, so you can go home to your family?

NOTE, from any position inside the field you can see the entire perimeter, assume that the field has four straight sides, though the angles may all be different.

Also, the area of the field is large enough to make it impractical to measure with the ruler