An A-frame is a tool used to determine landscape contour. It is simple and cheap to make, and appropriate to situations where other contour-mapping tools such as LiDAR or GPS are unavailable, impractical, or prohibitively expensive. It consists of three straight rods or planks, fastened together in the shape of a capital "A", with a plumb bob (weight) suspended from the apex. Markings on the cross-piece are compared to the plumb bob line to determine slope.