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February 24, 2005
Geospatial Applications: Choosing Handheld GPS Units
There is a great variety of hand-held Global Positioning System (GPS) units available. How does one decide which unit to buy? People use hand-held GPS units for many reasons that require different features. GPS units are available for a wide variety of prices. Some units operate independently, while others work only in conjunction with a computer. I suggest comparing units for specific features in addition to price. Buyers should compare GPS units for features such as battery life, differential correction options and if the unit can or can’t be connected to a computer to transfer data, as well as other features. The most common type of GPS units are those that operate independently; however, there are several other types available, including flash card, Bluetooth and secure digital units. These units need to be connected to a computer to operate. The number of channels a GPS unit receives indicates the number of GPS satellites that can be used simultaneously to calculate its position. Most units initially use four satellites to calculate positions and then add additional satellites as more signals are received. Typically, hand-held GPS units are capable of receiving signals from 12 GPS satellites at one time. Battery life is important for the hand-held and Bluetooth units. Compact flash and secure digital units use power from the attached computers, so the significant feature is how much power they use. Manufacturer specifications listing battery life refer to continuous use. Area calculation is a feature in a GPS unit that allows users to calculate the number of acres in areas marked with the GPS unit. The area calculation feature is not relevant with flash card, Bluetooth or secure digital units because they operate in conjunction with geographic information system (GIS) software programs on a host computer, which normally have the capacity to calculate area. Another important feature is whether the GPS unit can use differential correction signals to increase its accuracy. The differential correction signal used in hand-held GPS units is the Wide Area Augmentation System (WAAS). Differential correction is available through the Federal Aviation Administration at no cost to the user. The WAAS differential correction signal provides GPS accuracy to less than 10 feet in hand-held units. XTrac is a feature used by some flash card, Bluetooth or secure digital units. XTrac extends the use of GPS units to areas where signals are difficult to receive, such as under tree canopies or near tall buildings. XTrac is a software program on the unit’s computer chip that increases the receiver sensitivity. It does so by acquiring signals from GPS satellites with weaker signals, as well as the stronger satellite signals, before it calculates its position. For example, a normal GPS receiver will acquire signals from four satellites with the strongest signals to calculate its position. GPS units with XTrac also will acquire signals from two or more weaker satellites before outputting a position. The satellites with weaker signals usually are lower on the horizon where the radio signals may be deteriorated by the earth’s ionosphere, and other objects that clutter the horizon such as trees and buildings. Another feature used to compare GPS units is the ability to transfer GPS data from hand-held units to computers. Transferring data facilitates using the GPS data in GIS computer programs. Again, this feature is relevant only in the hand-held units. Some hand-held GPS units are capable of storing background maps in built-in memory. The maps are proprietary and are available online or on a CD. Available maps include roads, rivers, towns, elevation and background maps. It generally is not possible to load your own maps into handheld GPS units. The display size is important for hand-held units. Larger display sizes make it easier to see information on the screen, but normally increase the size of the unit. Two-way communication is only an option in hand-held GPS units. Two-way communication allows users to pass GPS locations back and forth between units. By using the two-way feature of these units, the geographic position of the companion unit is displayed on the first unit. Flash card GPS units are used with devices such as hand-held, laptop and tablet computers. The main advantage of a flash card GPS unit is that it plugs directly into other devices and takes power from the devices, so it doesn’t require an external power source. Bluetooth GPS units use a wireless radio technology that allows the GPS unit to transfer GPS position data to a computer without a wire or cable connection. The fact that Bluetooth GPS units can connect to hand-held computers without a cord makes them suitable for in-vehicle navigation. Bluetooth GPS units have internal batteries and many of them are rechargeable. Secure digital GPS units are much like flash card units except they plug into the secure digital slot of a hand-held, laptop or tablet computer. The choice to use a secure digital or compact flash card depends on the type and availability of expansion slots on the computers that will be used with the GPS unit. For more information about handheld GPS units, go to the NDSU Geospatial Technology Education Web site at http://134.129.78.3/geospatial/default.htm. This site includes charts comparing the features of GPS units from various manufacturers, as well as links to the manufacturers’ Web sites. ### Source: John
Nowatzki, (701) 231-8213, jnowatzk@ndsuext.nodak.edu |
Market Advisor: |
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North Dakota State University |
