It's a digit in the latitude of gps numbers. Example and I'm jus making these numbers up. When you leave the Oak your location is N43.14.75, W 78.32.45,. So in this example, the "14" will increase as you drive north. So when you get to N43. 29.75 your at the "29" line. Also known as 500 foot of water. Hope this helps Sent from my E6810 using Lake Ontario United mobile app