Virtually all GPS receivers provide output to external devices (plotter and computers) in a format called NMEA 183. While the electrical characteristics of NMEA 183 are compatible with a computer's serial connection, they are not precisely the same; NMEA's voltages are lower. This has posed problems when trying to connect to a computer (or PDA) that does not support a direct serial connection. This is sometimes called a "legacy" connection or port by those who think that USB is the only connection a computer needs. The problem is that there is no reliable (or inexpensive) way to convert NMEA to serial with enough voltage to make serial-to-USB converters work. Until the Garmins of the world begin to offer GPS receivers with output in USB, we have no choice but to use navigation devices with serial ports.
The Earthmate USB Receiver
Fortunately, DeLorme has developed an affordable, true USB GPS receiver called the Earthmate. With an MSRP of $129.95, you would expect a "dumb" receiver that takes five minutes or more to start up, but the new Earthmate uses its host computer to do some of the heavy lifting that a $300 GPS device would do internally. A GPS receiver that does not contain a sizable non-volatile memory and a good clock must download all the necessary data to operate every time the unit powers up. The Earthmate does not have internal power or advanced internal memory, yet this little GPS receiver offers 30-second cold boot-up times comparable to more expensive units. Consider that the Earthmate is also WAAS-(wide area augmentation system) enabled and this little unit is really a good buy.
Installing USB devices on some computers can be an irksome task. We installed the Earthmate on PCs running Windows XP and Windows 98 without any problems. One word of caution, though: DeLorme designed the Earthmate to be used primarily with its own mapping programs. We tested it with Street Atlas 2003 (a fine street navigation program) and Topo USA 4.0 (DeLorme's vector and raster topographic navigator). The unit worked as advertised. The setup software offers the user the opportunity to use the receiver with other, non-DeLorme mapping programs with a regular serial input. Choosing this option allows you to use the new Earthmate with programs such as Maptech's Terrain Navigator (tested successfully) as well as with DeLorme's programs.
A potential problem arises when installing the receiver on a laptop. To use the Earthmate with non-DeLorme software, you must install the device as a virtual serial device. This is not a problem unless the BIOS in the computer has assigned all the virtual serial ports available. In many computers, the BIOS will not support more that four serial devices. In the case of one of our test machines, our USB-to-serial installation did not work because the four serial ports were assigned to a touch screen, a modem, and two physical serial ports. Here is the important distinction: While the machine had two serial ports available, they were assigned to the physical plugs on the back of the computer. The laptop simply could not recognize the third "logical" serial port. Therefore, we could not use the USB-to-serial feature of the Earthmate software. However, we were successful with a newer Windows XP computer.
The Earthmate is simple to use with DeLorme software. Just select the device in the GPS selector and click start. In our experience, the unit provided location information within one minute of cold start-up. The Earthmate is WAAS-compatible, which means that in addition to listening to the 24 GPS satellites, the unit also listens to a pair of geo-stationary satellites that broadcast GPS correction information compiled from about 25 ground stations. Most recreational WAAS-enabled GPS receivers are capable of 3- to 6-foot horizontal accuracy and 10-foot elevation accuracy.
Herein lies our only complaint with the DeLorme system, and it is not the fault of the Earthmate. DeLorme specializes in vector maps. These are data files that consist of instructions for the computer to draw maps. Hence, a vector map file is not the map itself. The process of converting a raster, or printed, map to vector is fraught with the potential for errors. These can include scanning/tracing errors or simple errors of rounding numbers such that a curved street ends up looking like a fractured polygon. We have used DeLorme's Street Atlases and topographic products in eight western states (and a couple of eastern ones) and have generaly experienced good accuracy. However, there are some areas of Southern California, for example, where DeLorme is inaccurate. The addition of WAAS to the Earthmate tends to highlight the imprecision of some of DeLorme's vector maps. When using Topo USA's raster maps, we noted that the new Earthmate's accuracy was of very high precision on the USGS topographic quads.
DeLorme's Earthmate GPS receiver is a new and vastly improved piece of technology. And as the Earthmate is compatible with third-party mapping programs and is WAAS-enabled, it is an excellent value for the price.