Pelvic area is measured within the vagina. A special set of calipers measures between the biggest distance, height X width, in the birth canal, giving you the squared area of the opening.
The height would be the distance between the vertabrae and the bottom floor of the pelvic bone.
That makes sense. I have one of those little tapes you put around the calves hoof for weight, I can also find the difference in circumference and pelivic area to predict calving difficulties early.