Area Calculation

Area Calculation

I need to measure the area(squarefoot of some farm fields) they don't have any know shapes, I mean the field are neither triangle,nor circle,nor rectangle, nor...,nor....,nor......

I have been told that there is a device or equipement or instarument or whatever you call it named GPS, that I can walk with around the boundaries of the field and will pick up the data and this data can be feeded to a software on the computer and can be converted to a map and there is a different software that can calculate the area on the map. Don't take my statement word by word, but I hope you got the point of what I am looking for: Calculate an area of a field that doesn't have any know shape to which you can apply a formula that will come out with the squarefoot of the area.

If GPS or other equipment that can do that, please provide a link

Thanks