Measuring the the Cabin Footprint

This weekend, I stopped by my parents property and took a look at the cabin which we will soon move to our land.  Part of the reason for the trip was to measure the footprint of the cabin.  

The cabin floor is mounted to four 4" x 4" skids.  I'll need to create a level area which supports the cabin using these skids.  I've not yet determined the approach I'll use but I wanted to measure the distance between the skids so I have the information for when I start the ground work.

My measurements are all from the rear left skid of the cabin.  

The distance from the edge of the left skid to the far right skid is 15' 5".  This is roughly 15' from the center of one skid to the center of the other skid.  If you measure 5' in from the outside skids, you are at the innermost edge of the center skids.  This should make it fairly easy to measure out the footprint.  Now I just need to decide how many footings to create and what to make them out of.