Basemap
Most units contain a basemap, a rough guide to North America. A basemap will typically have highways and larger roads, as well as larger bodies of water. A basemap is meant for graphic display and basic orientation; it will not provide detailed maps suitable for in-depth navigation purposes. A basemap will show you that you're in New York, for example, but it won't have most streets or many point-of-interest listings.
Compact Size
With a few exceptions, handheld GPS receivers come in two shapes: there are compact units, roughly the size of a fat cellphone; and there are full-sized units, with larger screens and sometimes extra features, but are taller and heavier. Both types are still thin and fit easily in a backpack; you should check individual unit pages for specific dimensions.
Color
Units have monochrome or color screen displays. In addition to looking sharper, a color screen also makes it easier to differentiate map details such as waypoints and contour levels.
Expandable
Expandable units take either SD or microSD data cards for loading mapping data. Some units instead have a limited amount of internal memory for the same purpose. All handhelds have enough memory to store waypoints and a track log without adding anything. An expandable-memory receiver lets you add large map segments to your system.
Autorouting Capability
Autorouting units will take detailed street map software made by their manufacturer. If you add this software, your handheld will be able to give you driving directions, and beep when it's time to turn. If the unit lacks autorouting capability, you may still be able to load the maps for display purposes, but you won't be able to calculate driving directions.
Topographical Maps Available
Topographical data provides elevation contours, campground and parks information, often trail data, and basic lake data. For those taking a walk in the woods or climbing a mountain, you'll want to invest in topographical software. Topo software won't provide maps to calculate driving directions; for that, you'll need an autorouting system.
Marine Charts Available
If you're a boater, you'll want to add a marine chart for your region. Detailed marine charts provide full depth contours, nav aids, buoys, ports plans, tide info and more. Some topo software will cover the shoreline, but for in-depth navigation on the water, you'll want to add a marine chart.
Buoyancy
While all handheld GPS receivers sold at Tiger GPS are fully waterproof, some units are designed to actually float if dropped in the water!
Barometric Altimeter
A device with a barometric altimeter uses changes in air pressure to determine your altitude. It could also be used for indicating changing weather. Combined with a topographical map, you'll be certain how far up the mountain or how deep in the canyon you are.
Electronic Compass
While any GPS receiver can tell you which direction you're moving, a unit with a compass can tell you which way you're facing and give you magnetic bearing while standing still.
High-Frequency Receiver
Some newer units have a SiRFStarIII or equivalent receiver. This new technology acquires satellite signal much faster and maintains it, even in tough environments. Where an older GPS might take two minutes to find its satellite fix, and must be used with a clear view of the sky, the high-frequency chips can find a fix in 30 seconds and sometimes even work inside the house!
Touch-Screen
Some newer units have a touch-screen interface, allowing you to use your finger or a stylus to input commands on the screen.
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