Detailed Terrain Map By Walking Around With a GPS?

My house is built on a bit of a ridge between two gullies (well, two collapsed lava tubes -- I do live on the side of an active volcano, after all). Grand plans include decks and terraces, but I can't envision them without a plan. Of course, I could hire surveyors to come in and map the place, but I have a GPS (actually, and I wonder if this is significant, I have two GPSes). What I want / wondering if it exists / wondering if I could code it is an application that takes a GPS track and creates a surface. Anyone know?

Why I wonder if having two GPSes is significant is because I would be very happy to leave one GPS sitting, perfectly motionless, while walking around with the other. While the GPS signal is salted with random imprecision, if that imprecision is the same for two different GPS devices, I would think that I could subtract the "jitter" of the stationary GPS from the track of the mobile GPS, perhaps providing me with the vaunted precision of "differential GPS." ... 5 minutesĀ later ... Hmm, notĀ an encouraging experiment. Perhaps the GPS have to lock on to the same satellites?