What you need to know about GPS maps depends on your navigation needs.
If you want simple highway directions, skimming the GPS receiver's manual
will probably give you all you need to know. But if you're going off road and
need more navigation help, understanding the basic principles of
GPS mapping and location will be useful.
A GPS receiver will tell you your location, but usually you also want to
know your position in relation to somewhere else. And this can be done with
a map, either the paper kind or an electronic one shown on a display screen. Since
a flat map must represent the spherical earth, accurate directions require
some type of conversion system.
Unlike world globes we're familiar with, the earth isn't a
perfect sphere. For instance, it's actually fatter at the equator than
the circumference through the poles. And in order to accurately represent
the earth's size and shape, mathematical descriptions called, "geodetic
datums", are used. Datums are the mapping language for GPS and it's important
that the datum settings of your GPS receiver match those of the map used.
GPS receivers use the World Geodetic System 1984 datum (WGS-84) for a
common reference, but they can convert this to other datum if needed.
The GPS Coordinate Systems
To find a position on a map, you need a coordinate or grid system.
Like the datum used, most GPS receivers will work with several different
The most common GPS coordinate system uses longitude and latitude.
Latitude is shown on a map as lines parallel to the equator, both north and
south of it. Each line of latitude is described in degrees from the equator
with the equator as 0 degrees and the poles as 90 degrees North
and 90 degrees South. Lines of longitude run parallel to, and east and west
of the prime meridian, which is a line running north and south through the
Royal Greenwich Observatory in England. Longitude is measured from 0 to 180
degrees either east or west of the prime meridian. To provide greater location
precision, degrees are divided into 60 minutes per degree and 60 seconds
per minute. A typical location description would look like:
W 110° (degrees) 23' (minutes) 06" (seconds) N 44° 57' 43"
The Universal Transverse Mercator (UTM) is a popular GPS
coordinate system, commonly used on United States Geological Survey (USGS)
maps. The advantage of UTM is that it eliminates the cumbersome conversions
required when using latitude and longitude. Position coordinates are simply added
and subtracted without the need to change into degrees, minutes, and seconds.
UTM divides the earth into 60 north-south zones which are 6° wide. Each zone is
divided by bands running east-west 8° wide. These zones also
contain grids with the Equator and the central meridian for each zone
serving as the zone's base reference or origin point. Locations using
UTM are referenced with a zone number, a band letter and meters from
the zone's origin. An example of UTM position coordinates is:
Zone 23 S (band letter) 0192367 E (grid position) 3392435 N.
Because the grid origin is set at a large number instead of zero, the
grid numbers are always positive and described with the terms "easting"
Using an Electronic GPS Map
There's no improvement of accuracy or detail with an electronic
map compared to a paper one. But it is convenient to have any map you
choose contained in a small GPS unit or PDA. Getting a new map is as
easy as replacing a memory card or downloading one from the Internet.
With an electronic GPS map, you can also vary the scale to get as detailed
as necessary to plot your route information. The appearance of a
electronic GPS map is similar to a paper map. However an electronic
map lets you choose which coordinate system you prefer to use. You also
can choose whether you want directions referenced to either true north
or magnetic north. True north takes the North Pole as its 0° reference
and magnetic north uses the Magnetic North Pole. Since the position of
the Magnetic North Pole varies, the map software must have these adjustments
programmed or allow for manual adjustment.
Electronic maps give you powerful features, but a paper map and compass are
always a good backup. And if you use mapping software, you can print out a custom map
showing only the section with your planned route.