Another thought on Survey-grade GPS vs. hand-held:
GPS satellites broadcast data on 2 frequencies, roughly 1200Ghz. The civilian L1 band is broadcsat in the clear, and is only accurate to a few meters. Period. The signal comes through a sine-wave similar to FM radios, and is easy to pick up.
Surveyors use the military, or L2 band, but CANNOT decrypt the signal. The encryption involves using Pseudo-Random-Noise or PRN added to the carrier-wave to further confuse the receiver. This noise is generated using a published computer algorithm, and varies with each satellite, repeating every month or so. Surveyors use the noise to get cm accuracy. HUH?
Imagine a rock band playing a long song, say Iron-Butterfly's Innagoddadavita. If you didn't speak the language, all you would get would be the wild beat. No great loss, but if you listened to it long enough, over and over, you'd know when, exactly, the drum-solo starts, and ends. If it were to be played very loud, on seakers a mile apart, you'd be able to pinpoint your position by listening for a specific cymbal crash, or high-hat coming from each speaker, timing the difference yields your position. No need to listen to the words, even if they told you exactly where you were. You'd need to know where the speakers were, but the L1 band sends this data already.
The Military combines both channels, and an ordinance mounted GPS gets the data on-the fly to cm accuracy like a Garmin handheld. Surveyors need hours of data-collection to get the same accuracy from a single receiver, or multiple receivers with one on a fixed, known point.