Arduino GPS guidance

I've done a little bit of digging on GPS guidance for the Arduino, my main concern being accuracy of the unit. I've got several projects backed up in my head that involve GPS to some extent, the main project being the Lawnmowbot. But it seems that these projects are in jeopardy as I've found out that the accuracy of the GPS unit ( http://www.robotshop.us/adafruit-gps-logger-shield-kit-arduino-1.html ) is 5 to 10 meters - that's almost 33 feet! I can't mow a lawn leaving a 10 meter buffer ring around obstacles, infact for my yard that may only be one stripe down the center. So unless we can figure out some fancy workaround, the Lawnmowbot is going to have to be shelved. One idea I had is to mount two or three GPS units onto the mowbot. They'll be fixed (and known) relative to eachother, so perhaps we can come up with a method to trim down the error based on their readings and their known distances from eachother. I dunno, I'll probably have to buy one and tinker with it for a while. Worst case, I move on to the next GPS project - I'm thinking a UAV based on a R/C helicopter; 10 meters error shouldn't be detrimental for something like that :)