Gentlemen, I am going to cut my teeth on my parents walkway. My father has removed the failing stamped concrete with the skid steerer. He was able to remove the some soil to various depths. My question is: How do you determine the proper depth for a 4" base when the ground level changes going along the 90' distance. What I mean is that if I set up a level string and take measurements from the string to the grass level, along the 90' that level may change by 3" to 4". Do I measure 7" (4" base, etc.)from the lowest point? We plan on using the Grinnell Naples 4 peice system. The walkway will be 90' by 4' with a semi circle and a 1/4 circle. Thank you.