A couple of Saturdays ago, I was invited to sit in on an orientation course, an appetizer, if you will, on the Global Positioning System, and now I’m ready for the full-course meal.

Thanks to Peg Smart at The Map Store in Old Town, and instructor Dick Parker, I came away feeling as if I could actually master this wonderful tool (if I had a few more weeks to devote to it). Parker teaches such a course at United Technologies Center, and I think I’ll try to put aside the time to take his next seven-week class, which begins March 28.

Smart offered the use of a Magellan 315 model GPS so I could follow along as Parker led the class through the haze of terms and little quirks of these machines.

Until that Saturday, I’d never held one in my hands for more than a minute because I hadn’t the slightest idea of what happened after turning on the power. I’ve developed a reasonably good understanding of a map and compass, and have looked at the GPS as an expensive toy – something you might buy if you got a little windfall and you didn’t need to fill your oil tank.

After Parker’s orientation I have a different outlook. I’m still glad I have an understanding of topographic maps and a compass. But when you combine this working knowledge with a GPS, I think you’d have to work pretty hard to get lost.

As long as you remember to turn the machine on and punch a button at the place you begin your trip (and the batteries are good), all you have to do to get home again is punch another button and the machine does everything but carry you home. Even if you run into a geographic obstacle and have to walk around it, the GPS will show you the new heading you have to follow to get back.

In the simplest of terms, a GPS is a sophisticated calculator and radio signal receiver that takes its information from four or more of the 24 GPS satellites in orbit around Earth. Basically, the hand-held unit calculates the time it takes for the signal to reach it. It does this for four satellites to pinpoint its exact location on Earth.

You can get that information in the form of latitude and longitude readouts (which is what many users in this country request) or in Universal Transverse Mercation, or UTM, which is what most of the rest of the world uses because it’s based on the decimal system.

By combining knowledge of how a topographic map is displayed, what the numbers on it mean, and how the globe is divided up in topographic terms, you can take the readout from your GPS, apply it to a map and pinpoint your exact location (you’ll need a simple plotting tool to help with the longitude).

Imagine the globe as an orange divided into 24 segments, each 1,000 miles across at the equator (your time zones), each representing 15 degrees across.

(The UTM system, which Parker said makes more sense, breaks the orange into 60 zones of 6 degrees each, then specifies position in meters north of the equator and east from the center line of the zone.)

Back to your orange, at the widest point, start marking off 90 parallel lines upward to the top and you’ll get your latitude markings. What you have now is a bunch of squares drawn on your orange.

Turn on your GPS, and, if you’ve done the initial setup properly, you’ll find a readout that shows you exactly where you are in terms of latitude and longitude.

OK, so what do you do with this information?

Your topo map has this same information, if you know how to read it. The numbers on the four corners of the map tell you the latitude and longitude, and, with a little practice, you can follow latitude up the side as they ascend in minutes (there are 60 minutes in each degree and 90 degrees from the equator to the pole). Each of the minutes is a mile of distance, hence a 7.5 minute topo is 7.5 miles square.

Longitude numbers, the east-west component, increase as you go from right to left, and are expressed in degrees, minutes and seconds.

Use a straight-edge and a latitude-longitude plotter, and you can locate your position on the map pretty accurately.

You also can choose to use the UTM system on your GPS. Have no fear, there are corresponding numbers on your topo map. The readout for your GPS might look like this: 19 followed by 4973.450 followed by 524.225 That means you are in Zone 19, you are 4,973.45 kilometers north of the equator and 524.225 kilometers to the east of zone 19 (each topo tells you what zone it’s in). With the UTM plotter you can draw this on your topo and accurately define your location.

Knowing exactly where you are and being able to define that point on a map guarantee you will not be lost. If you get turned around in the woods, all you need to do is ask the GPS to give you a bearing to return to your starting point. Follow that bearing with your compass, and you’re home.

You can get started with a GPS for around $120 and spend up to $400 for the more sophisticated instruments. The Map Store (827-4511) sells Magellan and Garmin brands.

. . .

All cross-country enthusiasts are reminded that the Penobscot Valley Ski Club’s 14th annual Caribou Bog Wicked Winter Ski Tour and Race will be held beginning at 11 a.m. Sunday, March 4. The 18-kilometer classic-style trail starts at Essex Woods in Bangor and finishes at Sargent School in Stillwater. Transportation will be provided back to Bangor from the end of the race. Registration is $10 for adults, $6 for teens. A free Children’s Tour begins at 1 p.m. in Stillwater (parents should be available for support). The event will benefit Shaw House and trail development in Bangor. Register at 9 a.m. at 703 Outer Essex St., Bangor, on March 4, or online at www.pvskiclub.org. For more information write to: P.O. Box 873, Bangor 04402, or call 941-5670. Rain date is March 11.

Jeff Strout’s column is published on Thursdays. He can be reached at 990-8202 or by e-mail at jstrout@bangordailynews.net.