Do you really need GPS?
Jan 07 '00 (Updated Jul 07 '00)
Before you buy a handheld GPS system, you should have some understanding of their capabilities and their limitations.
How does GPS work: Satellites in orbit transmit a signal that the GPS receives. These signals are used to determine latitude and longitude position, altitude, and time. Many receivers are also programmed to calculate other information based on the satellite transmission. The signal can be blocked in heavy tree cover, indoors if not near a window, in mountainous terrain, and when surrounded by buildings. Most of the time, however, you'll be able to get adequate reception.
How accurate is GPS: GPS receivers are capable of determining your latitude/longitude position to within 15 meters (roughly 50 feet). With Clinton's recent approval, Selective Availability (which introduced a unpredictable error of up to 100 m) is gone. This means all units are capable of 15m accuracy! (Switch your navigation setting to UTM grid: the last two digits are 10's of meters.) More expensive GPS units are not more accurate, and being within 15 m of a spot on the earth is pretty good.
How will positional error affect how the GPS is used: Positional error is the inherent inaccuracy of the GPS unit. It is random error, and can be averaged away. Without selective availability, you won't notice the effects of position error under most applications. If you do, it will be when you're moving slowly, approaching a specified point (a waypoint, and you'll need to be very close), when comparing your known altitude with the GPS's reported altitude, or retracing your steps. Really, the altitude issue will probably be the only thing you notice. Using the averaging function on the GPS, you should be able to essentially eliminate positional error within a few minutes (with SA, this would take hours).
How can I use GPS: GPS only tells you where you are and where other points are in relation to you. With some planning, you can use GPS to get to a point you've identified on a map or a place you've been to before (your car…). If you're navigating to a point using the compass tool on the GPS, once you stop, the direction of travel arrow won't line up with magnetic north: having a compass and reading off the heading from the GPS is the best and most accurate method for navigating around.
What GPS features do I need: A short list of features distinguishes cheap, moderate, and expensive GPS's. Cheap ones (<$125) have a 2-channel receiver (this is the number of satellites the receiver can track at once). These units will have trouble following a signal in many areas, and will probably leave you frustrated (save yourself the heartache of a 2 channel receiver unless you get lost often in cornfields and other wide-open spaces). Moderate units ($150 range) will have 12-channel receivers, and the expensive units (>$300) have 12-channel receivers and installed mapping software.
I'm a bit of a minimalist here, and I don't see much offered in the more expensive GPS receivers that I consider a necessity. The most important feature, which almost all have, is a 12-channel receiver. A tough, weatherproof case is also key if the GPS is going with you out into the woods. If you want to use the GPS in the car, a 12-volt adapter is a good investment, and getting a combination PC/power cable really expands the utility of the GPS. Considering these factors led me to get the Garmin GPS 12. Paying more than $145 for a GPS does not improve the accuracy, but may add some degree of convenience or luxury.
So, do you need to buy a GPS? Well, where will you use it?
In the city:
No Selective Availability expands the utility of GPS for this application (after all, SA was introduced to keep hostiles from using GPS to target missiles with enough accuracy to hit a specific building). You'll still want to get a GPS with maps installed or a PC cable and mapping software (a much better investment for about the same price). No SA should put you +/- one or two houses of an address. You'll still get mixed up at complicated intersections, but when you do, you'll have an expensive electronic gadget to let you know that you're going the wrong way. If you just need directions, GPS won't add anything that PC mapping software-- or the mapping utilities found on the internet (or a good map)-- doesn't already add.
On the highway and backroads:
This is maybe the one area where GPS can totally supplant a map. You'll still need the mapping software or one of the more expensive GPS's, but once you've programmed the GPS, sit back and let the GPS tell you where to go. You'll find exits easily, and as you approach intersections, you'll know which way to go. You'll also always have a way to backtrack to a point you're familiar with if you're lost. An extended application of this is when you're traveling, especially outside of the US (GPS works everywhere). If you're cruising around and exploring, you can always get back to main roads without having local maps.
On trails:
You'll be able to answer the question "are we almost there yet" with annoying precision. But, if you stay on trails, your GPS is best used as an emergency backup. Track your course, and if you lose the trail or make a wrong turn, you'll still be able to find your way back to a known point. In this role, the GPS makes up for the complacency many have when they take off on trails. You could use a map and compass just as well, and you'll still need to have M/C with you, but the GPS will add a level of comfort.
Off trails:
Yes, get a moderately priced GPS. In this role, the GPS makes navigation much easier and adds a level of safety; but this is also where the unit is most likely to fail. Barring an incident that makes the GPS inoperable (see below) though, you'll really have to work hard to get lost (which is one reason why you'll still bring your map and compass with you). You'll always get back to your car, scenic vista, secret campsite, or whatever. Learning to read a topo map and use a compass is not only a necessary backup, but also adds another level of accuracy to GPS.
When using a GPS in the backcountry: extra batteries are an absolute necessity- bring several sets. Also, when you're in heavy cover or mountainous terrain, the receiver will be 'out of view' of some of the satellites during some times: during these times, you either won't get a location from the GPS, or it's error will be much greater. Stopping or repositioning the GPS can sometimes remedy this situation. The GPS screen is the most sensitive part of the receiver. Besides cracking, it may not be readable in cold (<20 F) or very hot temperatures.
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