I assume what youre talking about is a hand held consumer GPS unit.
I am a licensed land surveyor in California. I have been using GPS equipment since 1992. Ive owned and operated all sorts of systems from really cheap hand held units to high-end, survey-grade systems. Ive used equipment that could only receive American NAVSTAR signals and systems that could receive both NAVSTAR and Russian GLONASS.
So
heres a brief overview. (If you want more, PM me.)
Garmin and Magellan both make excellent models well worth looking at.
I would suggest you look through a few web sites to look at the functions available and determine what you absolutely need, what you really want, what would be nice, and what you can easily live without.
NAVSTAR, the American GPS system, is a constellation of 24 satellites; four satellites in each of six orbital planes around the globe. The SV (Space Vehicles) orbit at an altitude of approximately 20,200 kilometers above the earths surface.
The signals put out by the SVs are very weak, something on the order of looking at a 50 watt light bulb from about a mile away. For this reason, and this reason only, the signals cannot penetrate buildings, vehicles or very much water. However, signals CAN bounce around inside a vehicle or a building and make a cheap hand held appear to be working. This is called multipath error. It can be mitigated by using some complex algorithms. You will not find this feature on low end units. Of course, in about 15 years, when the constellation is filled with enough satellite broadcasting the L5 frequency signal, we should be able to use GPS under 30 feet of water or so!
Essentially, the system is really a distance ranging system. Your hand held figures out how far it is to a particular satellite. By knowing where the satellite is supposed to be from a predicted orbit, the unit can determine where on the ellipsoid the unit is. The ellipsoid is a mathematical model of the earths surface - the real earth is much to difficult to model perfectly and work with (OK, for all the other tech heads, Geod 03 is a damned fine approximation, particularly in my world in southern California).
So, what do you need?
Look for a system that can handle data input, latitudes and longitudes, in degrees/minutes/seconds and decimal degrees. Make sure that the input will handle a tenth of a second - anything less, a second, is too loose and anything more is fibbing - the system isnt that accurate. Make sure the unit can handle your local model, such as WGS84, or the local model; for your part of the world.
If youre going to use it a lot to store huge chunks of data, get a system that will store the data on a removable memory card - very few hand helds have really efficient data input systems.
You can get systems that will allow you to upload huge chunks of maps, such as navigation charts or street maps. Cool, but the more they will do, the more they will cost.
From personal experience, the "ruggedized" units are just plain old NOT. On the other hand, there are a few brands of excellent dry bags specifically for use with GPS hand helds. Get one and use it. Belive me, a $24 bag is a lot cheaper than buying another unit. (Been there. Done that. Got that dirty t-shirt.)
Color screen v. monochrome. I like color. I put my units to significant professional use and I like color. It can show me a heck of a lot more data than monochrome. You make the choice. Color is much more expensive.
Accuracy. Do not let anyone tell you their unit is more accurate. It isnt. Plain and simple. Unless you spend upwards of $50k on an RTK type system or you are in the military and have a military signal receiver, youre not going to do any better than 10 feet, and thats only if you have active WAAS (Wide Area Augmentation System). Yes, you can get a system for about $10K with a correction signal system that wil allow accuracies of about ½ a meter. And, yes, in certain areas with broad cast base station systems, for about $12K to $15K, you can get down to a few centimeters - sometimes if everything is perfect (if youve made this argument, call Roger Frank at Johnson and Frank).
With the signals being sent by the satellites, the perturbations in the orbits of the SVs, atmospheric distortions and a host of other variables, the best that can be expected from a non-WAAS enabled system is about 10 meters.
As a land surveyor, I use a survey grade RTK/Static system in my work. It cost well over $40K and can produce accuracies on the order of millimeters. (Topcon HiPer Lite +) I also have three Ashtech (now Thales) ProMarkII L1 receivers. They can give me accuracy to about +/- 10 cm on the fly and millimeters in post-process mode. To get to the site and for use on the weekends, holidays, diving, etc., I use a Magellan Color hand held. Ive owned two other Garmin hand helds and have used the eTrex.
All models do the same basic job. The extra dollars buy more features. The key is to figure out what you need and what you can live without.
