How to Understand the True Accuracy of the Best Handheld GPS System
To begin with, you need to know that in order to offer you with best accuracy of GPS, the device should be able to collect several data points from the different satellites. The number of points required for proper accuracy of handheld GPS is 1000. Based on this information, the accuracy of GPS is described in the following two terms:
Circular Error Probable (CEP): The accuracy expressed using this term is based on the points that fall within a circle, that is, you get only the horizontal accuracy of the GPS. This is because, out of the total points used to compute the accuracy of the handheld GPS, half of the data-points fall outside the circle centered round the truth (estimated accuracy of the GPS device).
For example, if the ad says that the accuracy of GPS is 2 meters CEP, it means that there is 50 per cent probability that the measurement lies inside the circle of 2 meters. On the other hand, it also means that there is 50 per cent probability that the measurement lies outside the circle with 2 meter radius
95 Percent Confidence: The accuracy of GPS is also mentioned in 95th percentile. For example, if the ad says that the accuracy of the GPS is accurate to 10 meters, you can be sure that accuracy of the GPS may contain an error of 10 meter circle and a 5 per cent probability of the error being greater than 10 meters.
NOTE: It is essential to understand that there is no such thing as perfect accuracy of GPS. Each measurement, be it for GPS or anything else, has some probability of error. Also, please note that both CEP and 95 percentile are "estimated" and not "guaranteed" error. Hence, the accuracy of GPS as mentioned in the ads by GPS vendors may or may not be precise, taking into account, the above mentioned error possibilities. Try to go for a handheld whose error possibilities are minimum based on above explanations.
Now that you understand the fundamentals of determining the accuracy of GPS, read on to find an important factor used in GPS ads by some GPS vendors that is completely misleading. The following sections also describe some important selection criteria for the most reliable portable GPS devices while offering tips to reduce the probabilities of errors in the accuracy of GPS handhelds.