GPS units have a resolution about about ten feet, which means that you can move it around a bit and it will keep showing the same latitude/longitude until you get about ten feet away from your "start" location and only then will it show its new position. This is fine for applications like driving but not so useful for a small robot trying to cut te grass in the back yard without also cutting the flowers in the garden.
So I'm wondering about the sensitivity and repeatability of GPS units.
Suppose I have a GPS unit displaying lat/long, at some position in my yard. I mark the "initial position" on the ground. I then slowly move the unit, inch by inch, in one direction. When the unit "transitions" to a different lat/long position I make a mark on the ground. I bring it back to the initial position and again move it, inch by inch, in the same direction. Will it transition at the same point that it did the previous time? If I do this a hundred times and each time marking where the transition occurred will all the marks be in precisely the same place, or scattered about? If they're scattered how large a scatter "area" will they occupy? If I do this in many directions away from my initial point will I get something comparable to a circle (or a square?) drawn by all the dots I've drawn?