Computerized Topo Maps
How Good Are They?

Basic GPS Concepts and Terms

By Dr. Brad DeLong
Author of 4-Wheel Freedom: The Art of Off-Road Driving

Please visit our Sponsor, CLICK on the Banner above

Basic GPS

GPS devices can seem intimidating because of all the menus, screens, and commands listed in the user manuals. But like most other electronic devices we use these days, they're capable of doing a whole lot more than we'll ever need them to do. You only have to learn a few basic functions to get a lot of use out of a GPS. If you're shopping for one, be sure you get a 12 parallel channel one, since they're more accurate, faster, and track better under tree cover. Other than that, the choice is a personal one. Look at some of the ones available here on the 4X4BOOKS.com website and see which ones appeal to you. In these articles, we'll use "GPS" to mean a GPS device or receiver.

I used a fairly simple Garmin GPS II+. Simple by this year's standards - a couple of more sophisticated generations are available now, but the Garmin GPS II+ is a good basic machine to test the map functions we're interested in.

Basic GPS units contain no mapping within the GPS receiver. The next level of GPS receivers contain a database of points for positions of locations such as towns and cites or campgrounds or other locations. But they do not contain linear data such as roads or contours. The latest generation of Mapping or Charting GPS receivers do contain complete maps including roads, cities, water features, and more. Some allow downloading of additional mapping detail from the manufacturers' CD-ROMS. The new line of GARMIN mapping GPSs (e-map, 12 Map, III+, Street Pilot, Street Pilot Color, and NavTalk) allow downloading of topographic map detail into the receiver. The GARMIN MapSource CDs contain topographic map detail similar to that found on the 1:100,000 scale UGSS topo maps.

GPS terms

Acquisition. A GPS tells you where you are by reading radio data transmitted from a number of GPS satellites circling around us all the time. A GPS needs a direct line-of-sight to at least four satellites at a time to calculate its latitude, longitude, and altitude above sea level. There are 24 satellites total, put up there by our government, each circling the earth twice a day. A GPS "acquires" the satellites and displays the user's position on its screen.

A 12 parallel channel GPS will acquire the data it needs to get started faster than the older units. Twelve "parallel" channels means the receiver can track up to 12 satellites simultaneously. There are some 12 channel units that track less than 12 satellites at once and are not "parallel" channel receivers. Most of the better GPSs today are 12 "parallel" channel units. If you've taken the GPS a distance (several hundred miles) from where you last used it, it will take longer to acquire the satellite data and get started.

If a GPS unit can acquire only three satellites, instead of four, it can still express position in terms of latitude and longitude, but it can't read the altitude. Altitude is probably the least accurate function of the GPS, anyway, even if it can pick up four satellites. The GPS is inherently inaccurate by distances ranging up to 1000 feet or so. This is because the government purposely detunes GPS for civilian use, to create inaccuracy for anyone trying to use the civilian system to guide missiles toward our country. Usually a GPS will be accurate within 100 feet or so, but you can never be quite sure how close it's coming to your real position.

The better the GPS can "see" the satellites, the more accurate it will be. It loses accuracy, and sometimes loses the signal altogether, in dense forest, deep canyons, and in buildings. In vehicles, it will be more accurate if you attach an external antenna, which you can stick on the windshield with a suction cup mounting that many manufacturers offer.

Latitude and longitude. These are the imaginary lines on the surface of the earth that serve as reference points for our location. Latitude lines run parallel with the equator, starting at 0º, running north to 90º north latitude, and south to 90º south latitude. The north 45th parallel, which is 45º north, is halfway between the equator and the North Pole. (It happens to run through Idaho, just north of McCall in the central portion of the state, which is an hour or two south of where we live.)

Longitude lines, called meridians, run north and south between the North Pole and the South Pole. They define east and west positions. They aren't parallel, because they all come together at each pole. The meridian designated zero is called the "prime meridian," and runs through Greenwich, England. The meridians of longitude run east and west from the prime meridian until they reach 180º, in the Pacific Ocean. The International Date Line roughly follows the 180º meridian, and runs east of New Zealand, jogging around in the north to separate Russia from Alaska. Sunday in Alaska is Monday in Russia. The US is west of the prime meridian, so longitude in the US is designated as West. Longitude in Israel is East.

Latitude and longitude are classically expressed in degrees, minutes, and seconds. There are 60 seconds in a geographical minute, and 60 minutes in a degree. For instance, the location of Deadhorse Saddle, in north central Idaho, is: 46º 44' 12.1" N 115º 37' 41.1" W, given as latitude/longitude. The latitude, then, is 46 degrees, 44 minutes, and 12.1 seconds North of the equator. The longitude is 115 degrees, 37 minutes, and 41.1 seconds West of the prime meridian.

Latitude and longitude can also be expressed in decimal numbers, instead of in minutes or seconds. GPS devices and the computer topo maps have options which let you select how to express position, usually in the Navigation Setup screen or the equivalent. The selection menus would look like this, using the position of Deadhorse Saddle as an example:

It's helpful to set both the GPS and the topo maps to the same option, to avoid confusion. Remember that minutes and seconds start at 00 and run through 59. When they hit 60, they kick up to the next number. For instance, the next second after 45º 59' 59" is 46º 00' 00".

The continental United States fits into a block running approximately from a latitude of N 24º 00' 00" in the south, to a latitude of N 49º 00' 00" in the north, and from a longitude of W 67º 00' 00" in the east, to a longitude of W 125º 00' 00"in the west.

Waypoints. The GPS unit will let you save your current position in the machine by pushing one or two of the buttons. This information is called a waypoint. If you save a series of waypoints, you can have your machine store them in a route.

Route. A route is a series of waypoints which describes the path you've traveled while saving waypoints. You can save a route, then follow it again at a later date, or you can follow it backward to get back to your home base. My Garmin GPS II+ will save 500 waypoints in as many as 30 routes. You can download the waypoints and routes from your GPS into your computer topo map program and display them on the maps on your computer screen.

Track. As you travel, your GPS will automatically keep track of where you've been. It will keep saving the track until it runs out of tracking points (my Garmin will save 1021 tracking points before it runs out of room), or until you purposely clear the track log so you can start a new track. You can download the track from the GPS into the computer topo map, and display the track on the map. You can then save this track as a bunch of waypoints, which you then can upload back into another GPS so another person can follow your track later.

You can also save the track in your GARMIN GPS into a route, still in your GPS. The GARMIN units will condense the number of tracking points down to a smaller number, and save the track as waypoints in a route. It's more accurate to download the track from your GPS as a track, not as a saved route, because the track will have more points and will be more accurate than the track which you've converted to a route.

Datum. There are various ways of representing the earth on maps. Different maps use different systems. The system that a various map uses is called its "datum." Two common ones in the US, for instance, are the NAD27 CONUS datum, devised in 1927, and the WGS 84 datum, developed in 1984 and the system upon which the GPS satellite system is based. Your GPS also uses a particular datum to treat the data it receives from the satellites. If you're using a paper map, it's important to look on the map and see what its datum is, then set the GPS to the same datum in the Navigation Setup page. If a computer topo map program uses a specific datum, then the GPS should be set to the same datum when downloading or uploading.

Magnetic variation. Magnetic variation, also called "magnetic declination" is the difference, in degrees, between the bearing to magnetic north shown on your compass and the bearing to true north. The true North Pole and the magnetic north pole are not the same. The magnetic north pole lies either east or west of the true North Pole, depending on where you are in the world, and its location gradually changes from year to year.

East variation is positive. West variation is negative. If you go to the Navigation Setup page of your GPS receiver, you can set up the heading to show a true north bearing, a user-input magnetic bearing, or an automatically calculated magnetic bearing. Here in north central Idaho, where I live, my Garmin automatically says the magnetic variation is E015, which means East 15º. The DRG topo maps for the area say the magnetic variation is East 19º, and Fugawi calculates it as East 17º. So the exact value depends on what source you rely on.

Magnetic variation is important if you're using a bearing to determine the direction of something. If you're just using your GPS to follow a route, it doesn't matter if your device is showing a true bearing or a magnetic bearing. It's just going from waypoint to waypoint. Since a waypoint is a fixed location defined by specific values of latitude and longitude, the GPS will find it.

However, if you're using your GPS to determine your location by reading a bearing from one of the waypoints it contains, then its important whether that bearing is a true heading or a magnetic heading. If it's a magnetic heading, then you have to adjust it to fit the true bearings on the map you're using to find where you are. For instance, using a value of East 15º for the magnetic variation we have here in Idaho, with your GPS set to a magnetic heading, assume you read a bearing of 90º to a waypoint in your GPS, along with your distance from that waypoint. Now you can go to your map and locate that waypoint and find your position relative to the waypoint. But you have to adjust the bearing from magnetic to true. A 90º magnetic bearing on the GPS is equal to a 105º true bearing on the map. If you don't make this adjustment you'll be way off - by 15º, or by 30º if you go the wrong direction, either of which is a big error, especially if you're lost, or on a life and death rescue mission in the wilderness.

Similarly, if you're using your map to determine a bearing to a certain location or object, intending to follow that bearing with your compass or GPS, then you need to convert from the true bearing of the map to the magnetic bearing of the compass or GPS (if you have your GPS set for a magnetic heading). In the example above, suppose your desired objective is on a 90º heading from your present location on the map. To get to it, with a magnetic variation of East 15º, you have to follow your compass on a magnetic heading of 75º.

The point is, you should coordinate the heading of your GPS (true or magnetic) with your maps. If you're using computer topo maps, then you can set both the GPS and the topo maps to magnetic headings or both to true headings.

Computerized Topo Maps Home Page
Part I: Introduction | Basic GPS w/ Terms | Interfacing with GPS | Contour Interval & Scale | Detail & Quality
Part II: Introduction | Installation and Technical Support | Ease of Use | Entering Waypoints and Routes
Part III: Introduction | Downloading Tracks from your GPS | Actively Tracking from your GPS | Printing Maps
General Comparison of the Programs

Product and Ordering Information from 4X4BOOKS.com
Topo USA 2.0 | 3-D TopoQuads | Wildflower TOPO! and TOPO! GPS | TrailSmart | ALL TOPO MAPS | Fugawi

4X4NOW Feature Page
4X4NOW Trail Reports | Moab 4WD Trails
4X4NOW "How-To"  | 4X4NOW Buildups
4X4BOOKS | MAPNOW

get Garmin at GPSNOW - Order by 5pm and have it Tomorrow!
Choosing | for Laptop/Pocket PC | Basic | Handheld Mapping | 2-way Radio | Automotive | Motorcycle | RV/Truck/Bus | Land & Water
Accessories
eTrex H | Legend | Vista | Venture HC | Summit HC | Legend HCx | Vista HCx
GPS 18 DLX | 60
Mobile 10 | Mobile 10 for smartphones
GPSMAP 60 | 60Cx | 60CSx | 76S | 76Cx | 76CSx | 276C | 376C | 378 | 478
MapSource City Navigator North America | US Topo | BlueChart
nüvi 200 | 270 | 350 | 360 | 370 | 650 | 660 | 670 | 680
Quest | Quest 2
Rino 110 | 120 | 130 | 520 | 530 | 520HCx | 530HCx
StreetPilot 2820 | 7200/7500 | c330 | c340 | c530 | c550 | c580
Astro GPS dog tracking system
zūmo 450 / 550
© 4X4NOW