Bluetooth Hands-Free Calling

This feature uses Bluetooth technology embedded in
your GPS device to communicate with your bluetooth enabled cell phone
so that you can make and receive calls using voice commands direct to
your GPS.  You have to make sure that your cell phone carrier is
compatible with your GPS unit.  If your GPS also has a travel guide
with phone numbers you might also be able to dial restaurants and
hotels directly from the guide.

Integrated Traffic Receiver

This feature allows you to receive automated
traffic alerts if your GPS device has a traffic receiver and you
subscribe to a service that will send the traffic alerts to your GPS. 

Turn-by-Turn Voice Direction

This feature uses text to speech technology so that
your GPS unit will provide voice turn-by-turn instructions to you. 
Without this you must read the navigational instructions on the GPS
display.

Screen Size

The standard screen size for portable and pocket
GPS units is now 3.5” and you should probably not buy one with a
smaller screen size.  If you are going to be viewing your GPS from a
wide angle (like mounted on your car dash at an angle) as opposed to
directly in front of you or if you have poor eyesight you might
consider a bigger screen size but they are quite a bit more expensive.

Points of Interest

This is the number of important navigational
features that are in your GPS database.  Points of interest are usually
for a given geographic region so if you are going to be using your GPS
in Europe for instance you will care about how many important landmarks
your database has specifically for Europe.

Waterproof

This feature means your GPS will not be damaged by
water and is important for marine, motorcycle, hiking and camping,
cycling, and fitness GPS units.

Automatic Rerouting

This feature refers to the ability of your GPS to
provide and alternate route if your primary route is not desirable (for
traffic jams for example).  This feature integrates with traffic
receivers so that the GPS can automatically reroute you if there are
traffic problems on your primary route.

Preloaded Information

A GPS unit needs a database of  maps and points of
interest to be a valuable navigational help.  If your GPS unit comes
preloaded you can usually hit the power button when you get it home and
it will start working.  If it is not preloaded with data then you will
need to connect your GPS to your computer and download the important
information into the GPS. 

Wide Area Augmentation System (WAAS)

This system uses additional reference stations to
augment the standard global positioning system.  These additional
points of reference include both ground based and space based reference
positions.  The additional reference positions allow the GPS unit to
calculate positions far more precisely and the accuracy can be within 1
to 2 meters most of the time.  In addition to providing more accuracy,
WAAS also allows the GPS unit to function when it might have a partial
view of the sky and therefore the satellites of the global positioning
system.

Antitheft

There are various technologies that are designed to
reduce theft of your GPS.   One of the most common is a locking system
whereby you have to enter a personalized code to get the GPS to
operate.

SD and CF Cards

SD and CF Card slots allow flash memory cards to be
plugged into your GPS for adding additional information to your GPS
database and for storing information from your GPS device as well.

Battery Life

For portable and pocket GPS units the quality of
the battery system is very important and is measured by the battery
life or the amount of time you can use your GPS before you have to plug
it in to recharge the batteries.  You should look for battery life of
6-8 hours.

MP3 Player

Like all electronic devices, GPS units are also
beginning to provide multimedia entertainment capabilities in addition
to core navigational functionality.  Some of the popular GPS units now
have built in MP3 players and plenty of storage so you can use your GPS
to play music.

Weight

For portable GPS units the weight is very
important.  The smallest units weigh only a few ounces now and can be
worn on your wrist like a watch.  Pocket GPS units for the car weigh
around 5 Oz and portable units for the car weigh about 10 Oz.

Size

The size of the GPS unit is measured in terms of
the width (w), the height (h) and the depth (d).  Pocket GPS units are
typically less than 1” in depth and portable units are a couple of
inches in depth.