Important Features to consider when purchasing a GPS unit for your car.

Here's what experts say to look for when considering a GPS unit. Your considerations will vary a bit depending on whether you are looking for a unit for use in the car, a camping/hiking model or one to use for both purposes.

• Quantity and quality of maps is critical. Furthermore, they must be up to date (or updateable) unless you will only drive in extremely mature cities like New York or San Francisco. Built-in and detailed road maps of your area of interest are an essential feature of a good automobile GPS receiver. The ability to upload maps with a USB cable or memory cards is also desirable. Some units have SD card slots.

• Automatic destination routing is a standard feature on any good car navigation GPS. Some inexpensive units can only handle point-to-point routing, while more sophisticated auto GPS models can handle multi-segment trips.

• Look for text-to-speech capability. Instead of just giving an instruction to "turn left at 100 feet," models with text-to-speech can pronounce the street name ("turn left at 100 feet on Main Street"), giving you better information and reducing the need to look at the screen.

• Make sure the unit you select is capable of being mounted neatly and securely on your dashboard. Nearly all models come with the proper hardware, but it may be an additional cost accessory with some. Experts point out that you must be able to mount the unit where it won't interfere with airbag deployment. Otherwise, the unit could be blasted into you or a passenger. Note that some states do not allow windshield mounting.

• Consider multimedia features. Many units play MP3s and can store photos. Some models play through integrated speakers, while others have an FM transmitter to play through your car stereo. Although wireless FM is not high fidelity, it's satisfactory for car use and the MP3 format. However, some models cannot play music and navigate at the same time.

• Decide if you want to be able to get live traffic reports and alerts. FM TMC, MSN Direct service, Tele Atlas and Clear Channel have subscription-based live-traffic services. Compatible GPS units use a built-in or external receiver -- or a Bluetooth-compatible phone -- to download this info and incorporate it into routing. Check coverage, however. Each service is only available in a limited, but growing, number of major markets.

• Make sure the screen size is appropriate for your eyesight and vehicle space. The vast majority of car GPS units have 3.5-inch displays. Some, like the Garmin nuvi 660, have larger displays.

• Evaluate weight and bulk. If you want to use the unit in two cars or as a portable MP3 player, portability is important. If not, you may still wish to carry the unit to reduce the likelihood of theft.

• Consider a unit with a large POI database (five million or more). Points of interest range from tourist attractions, hotels and restaurants to gas stations and ATMs.

• Weigh convenience features. A touchscreen is common. Many units simplify text entry by only allowing you to type letters or names in the database. As you start to type, these units may also give you options to select.