On larger-scale maps, of course, the precision one might obtain from a digitizer can exceed that obtained from the sort of GPS receiver commonly used to put data into a GIS. On a “200 scale map” (where one inch is equivalent to 200 feet on the ground) 0.1 mm would imply a distance of approximately a quarter of a meter, or less than a foot. While this distance is well within the range of Vehicle GPS Tracking Device capability, the equipment to obtain such accuracy is expensive and is usually used for surveying, rather than for general GIS spatial analysis and mapmaking activities.
• Ease of use: Anyone who can read coordinates and find the corresponding position on a map can use a GPS tracker for Car receiver. A single position so derived is usually accurate to within 10 meters or so. Those who want to collect data accurate enough for a GIS must involve themselves in more complex procedures, but the task is no more difficult than many GIS operations.
• GPS data are inherently three-dimensional: In addition to providing latitude longitude (or other “horizontal” information), a wireless GPS tracker for car may also provide altitude information. In fact, unless it does provide altitude information itself, it must be told its altitude in order to know where it is in a horizontal plane. The accuracy of the third dimension of GPS data is not as great, usually, as the horizontal accuracies. As a rule of thumb, variances in the horizontal accuracy should be multiplied by 1.5 (and perhaps as much as 3.0) to get an estimate of the vertical accuracy.
Global: anywhere on Earth. Well, almost anywhere, or anywhere else not having a direct view of a substantial portion of the sky. The radio waves that GPS satellites transmit have very short lengths–about 20 cm. A wave of this length is good for measuring because it follows a very straight path, unlike its longer cousins such as AM and FM band radio waves that may bend considerably. Unfortunately, shortwaves also do not penetrate matter very well, so the transmitter and the receiver must not have much solid matter between them, or the waves are blocked, as light waves are easily blocked.
In summary, if you are willing to pay for it, at the extremes of accuracy, Personal GPS Tracking Devices wins over all other methods. Surveyors know that GPS can provide horizontal, real-world accuracies of less than one centimeter. In addition to being an engineering marvel and of great benefit to many concerned with spatial issues as complex as national defense or as mundane as refinding a great fishing spot, GPS is also big business. Dozens of GPS receiver builders exist–from those who manufacture just the GPS “engine,” to those who provide a complete unit for the end user. In this text we explain the concepts in general, but use Trimble Navigation, Ltd. equipment since it works well, is quite accurate, has a program of educational discounts, and is likely to be part of educational GPS labs throughout the country.