GPS Hints

This chapter is designed to help you collect the best possible data from your GPS receiver; either for yourself, or for plotting on a gpsMap.

Experience will refine and improve your maps, but these tips should help you get started. We'll reveal some tricks to improve the accuracy of your GPS waypoints and trails without spending a lot on equipment.

After a quick overview of the Global Positioning System, we'll discuss GPS receivers, data collection, coverage, accuracy, Selective Availability, data-refinement methods, and downloading your GPS data to a personal computer.

Earth Station

The Global Positioning System

The Global Positioning System (GPS) brings a new dimension to the outdoor world. Some have even called it "The Next Utility". GPS can enhance the safety, efficiency, and convenience of all of the ways we get around.

The Global Positioning System is controlled by stations on the ground, which are linked to 24 orbiting satellites (and 2 spares) about 12,000 miles up. The satellites orbit every 12 hours, broadcasting to any GPS receiver within their "line of sight". Your receiver calculates a location based on the timing of signals from 4 or more satellites.

We'll cover some technical issues here, but for a full description of how GPS works, good references are already available. "GPS Land Navigation", by Michael Ferguson (Glassford Publishing, Boise 1997) is an excellent book on recreational navigation - with and without GPS, maps, or compass. There are also lots of online resources.

Blue Green

GPS Receivers

A GPS receiver can display your position on Earth, your altitude, speed, direction of travel, and the time of day. All you need is an inexpensive receiver, and an unobstructed view of the sky.

Many factors can determine how accurate a GPS location is, so it's best to be familiar with the system, your receiver, and the places you'll be going.

GPS receivers can store a list of your favorite places, help guide you, even "backtrack" if you get lost. Better receivers can record your journey as a "track".

Receivers with a computer interface let you "download" your collected locations and tracks to a personal computer using interface cables and software. You can also "upload" data to prepare for a trip.

Recreational GPS receivers can cost less than $100, and are available at outdoor outfitters, electronics-specialty stores, or by mail-order. Features and prices vary widely, so shop around a bit. If possible, try it before you buy it, or make sure the seller has a good return policy.

The type of receiver you get can have a great impact on the quality of the position and tracking data you collect. A multi-channel "parallel" receiver, which can listen to many satellites at once, will be quicker and more accurate than a single-channel "multiplexing" receiver listening to one satellite at a time.

You may want to ask about the options available: an external "amplified" antenna, DGPS receiver, external power cables, and personal computer interface cables and software.

Built-in maps are nice, but have limited detail. Maps can add $100 or more to the price of a GPS receiver, and are not needed to make a printed gpsMap.

Our screen shots are from a GARMIN GPS-III Receiver (pictured under "The Global Positioning System"). For information about specific GPS receivers and how to use them, ask your outdoor supplier, electronics store, or contact the receiver manufacturers or look up their web pages.

Fall Colors

GPS Receiver Settings

GPS receivers have lots of user-settable options, so you can work the way you like. Factory settings may be OK to start with, but you'll soon find something that needs changing. User settings for different GPS receivers vary widely, so be sure to check the manual for the details.

The Time Offset (from UTC) is settable so the receiver can show the local time of day. You may already know what coordinate system you prefer to work in, or you may have a chance to start fresh, testing each. Angular coordinates: degrees, minutes, seconds are sometimes called "DMS" or "". This is usually the default setting, since DMS coordinates are widely used. Decimal degrees ("ddd.dddd") are easier to calculate with.

The Universal Transverse Mercator (UTM) coordinate system is preferred by many GPS users. Most USGS maps have closely-spaced UTM markings, which can help locate your position more precisely than the coarser degree marks. The Map Hints chapter has more about coordinates and UTM.

GPS & Datum

A vital concern for good accuracy is setting your GPS receiver to use the same "Datum" as the map you are using. Most USGS Maps use the North American Datum of 1927, which may be abbreviated "NAD27 CONUS" on your GPS receivers. A datum is a mathematical Earth Model, covered in the Map Hints section.


- Batteries +

A GPS receiver can drain its batteries within a few hours of operation, depending on the model and settings you choose. Besides the cost, there's also the waste factor, and the toxic materials many batteries contain.

Some newer GPS receivers let you choose between alkaline, NiCad, Lithium, or other batteries. All have their advantages and disadvantages. The new rechargeable alkaline batteries ("Renewal" and others) claim up to 100 uses, and cost about $5 per set of 4. A special charger is required for each type of rechargeable battery.

Of course, the best choice is to supply external power when you can. Adapters for a 12 volt "cigarette lighter" outlet are available for most GPS units. Some power cables can double as a computer interface cable.

NEXT: Logging, Coverage, Waypoints

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