Measure the four sides of the area with the measuring tape and one of the diagonals and record the measurements. The choice of diagonal does not matter.
Add up the lengths of the two sides and the diagonal that form a triangle to find the perimeter of the triangle. Do the same for the other two sides that also form a triangle.
Divide the perimeter of each triangle by two and record the value.
Calculate the area of each of the two triangles with this formula:
area = square root (s * (s-a) * (s-b) * (s-c))
Where s is the value of one half the perimeter found in Step 3 and a, b, and c are the lengths of each side of the triangle.
Add the area of the two triangles created by the diagonal together to find the total area of the land.