ry in two ways:
•You can tell the receiver to record its
coordinates when you are at that
•You can find the location on a map
(the internal map or another one) and
enter its coordinates as a waypoint.
through glass, but are absorbed by
water molecules (wood, heavy foliage)
and reflect off concrete, steel, and
rock. This means that GPS units have
trouble operating in rain forests, urban
jungles, deep canyons, inside automobiles and boats, and in heavy snowfall
— among other things. These environmental obstacles degrade positional
accuracy or make it impossible to get
a fix on our location.
more frequent battery changes.
Size and Weight — Small lightweight
handhelds, large display chartplotters,
and panel mounted aviation models.
This capability lets you use the
GPS receiver in a number of different
ways. You can record any specific
location that interests so that you can
find it again at a later time.
You can also combine a series of
different waypoints to form a route.
One way to use this function is to
periodically record waypoints as you
travel so that you can backtrack, or
follow the same route again in the
future. Route-mapping lets you examine a map at home and record a series
of waypoints along the roads or trails
that lead to your destination.
If the receiver has a data port,
you can download routes to a computer, and then upload them again
when you plan to follow those routes.
Computers can do a lot more with
GPS location data than average
receivers, because computers have
much more memory and much faster
processing capabilities. You can also
update computer maps easily, so you
can include any surveying adjustments or changes in an area.
At its heart, a GPS receiver is just
an accurate way to get raw positional
data, which can then be applied to
geographic information that has been
accumulated over the years. This idea
is incredibly simple, but it has seemingly endless applications. The considerable contributions of GPS to aviation,
maritime navigation, military operation, surveying, and recreation have
secured its place among the most revolutionary inventions of all time.
Some GPS receivers have the ability to store attribute information in
addition to position information.
Examples of attribute information are
the condition of a street sign, the name
of a road, or the condition of a fire
hydrant. Position and attribute information can be stored in a Geographic
Information System (GIS) to help users
manage their assets more efficiently.
GPS signals work in the
microwave band. They can pass
Before Purchasing a
Antenna Configuration — If you are
going to use the unit mainly in the
open and in a car, you need a unit
with a built-in antenna and the capacity
to attach an external antenna, a fixed-mount unit with a mountable external
antenna, or an aviation antenna.
Before investing in GPS equipment, it is important to clearly define
your needs in terms of accuracy level
required and end results expected.
Identifying your requirements ahead
of time will help you determine which
type of receiver to purchase, and specific features you will need in order to
accomplish your objectives. It will
help you avoid purchasing a receiver
that you will be disappointed with
later because it can't perform the way
you expect it to.
Low-end consumer GPS units
(from Garmin and Eagle/Lowrance)
are in the $100.00 to $200.00 range.
All these units have parallel 12-chan-
The greatest thing about Garmin
units is that they have a bi-directional
serial port that allows them to hook
up to your computer. Though
many GPS units can only transmit their current positional information, the Garmin units also
allow you to transfer their waypoint databases, route tables,
and other useful information.
The wide availability of programs supporting the Garmin
transfer protocol makes these
units good choices for comput-er-based use.
Consider these issues when
selecting a GPS unit:
DGPS Capability — Do you need
the best accuracy possible? If so,
combining a Differential GPS receiver
with your GPS unit will give you the
best accuracy possible. All Garmin
GPS units are DGPS-ready and some
fixed-mount marine units even have
the DGPS receiver built in.
If you're shopping for a unit, pay
attention to features like form factor
(handheld versus mounted), external
antennas, mapping, and computer-controllability. A good strategy is to
clearly outline your project requirements and then contact several GPS
equipment manufacturers with your
specifications. As you research available equipment and ask questions,
you will gain an understanding of what
kinds of equipment are currently available and will meet your needs. NV
Battery Life — If you are going
to be using the unit away from
an auxiliary power source, consider the weight of carrying extra
batteries. Units with color displays tend to have a decreased
battery life compared to
grayscale displays, requiring
Figure 3. Etrex GPS receiver.