| GPS Q&A
By John Bohlke Q.Can I expect to get the GPS accuracy as stated in product literature or by a salesperson 100 percent of the time? W.N. Renton, WI A. For many reasons GPS accuracy, as stated in literature or by a salesperson, will not be achievable 100 percent of the time. Based on how GPS manufacturers typically advertise product accuracy, the customer should expect their horizontal results to meet the accuracy 68 percent of the time. The amount of vertical accuracy will be about twice as much as the horizontal accuracy. Fortunately the results that do not meet the specification are not significantly worse and the product literature may also list an accuracy that represents a 95 percent confidence. Most specifications also include a list of conditions necessary to achieve the accuracy. Those conditions may include requirements for the minimum number of satellites, good satellite geometry or the minimum amount of time spent on a point or maintaining satellite lock.
The customer will meet the specified accuracy less than 68 percent of the time if the conditions are not suitable for GPS. Reliable accuracy is less likely near obstructions such as trees or buildings. The obstructions often interrupt, reflect or reduce the strength of the GPS signals thereby reducing the number of points that will meet an advertised accuracy specification. Poor satellite geometry, minimal satellite coverage, atmospheric conditions and signal errors introduced by the Department of Defense may also affect consistent performance.
If the user records multiple GPS measurements at a point, most processing software will determine the consistency of those measurements when estimating the accuracy. All of the measurements could have been skewed by the same amount due to any of the undesirable field conditions mentioned previously. Therefore, the software may offer a misleading estimation of accuracy. Since the variables affecting the accuracy are fairly unpredictable, a GPS manufacturer is unable to guarantee consistent results 100 percent of the time.
In order to establish confidence in GPS accuracy the user should take measurements on known points and compute the amount of error between the known and computed values. A sensible approach to establish control points involves occupying points a couple of times and/or processing data from multiple points that were occupied simultaneously. A network adjustment will compute a good estimation of the accuracy for that type of data. Q. What results can I expect from a sub- meter, decimeter or sub-centimeter product? A.C. San Jose, CA A. Sub-meter, decimeter and sub-centimeter are common specifications in the GPS industry but are often misunderstood. Detailed specifications typically list the actual accuracy value rather than simply claiming sub-meter accuracy for instance. After all, sub-meter may mean that the product can only produce 0.99 meter results. Generally, the specified accuracy can be achieved 68 percent of the time under ideal GPS conditions.
A sub-meter product usually has varying levels of accuracy within 1 meter. The results will depend upon the field conditions, amount of time spent on a point and/or the amount of time spent locked on the satellites. For example if a sub-meter product boasts a 0.5 meter accuracy, the user should expect 68 percent of the results to be within 0.5 meters and 95 percent of the results to be within 1 meter. These results assume the user is following the manufacturer's procedure and requirements.
A decimeter product is different than a decimeter-level product. Decimeter accuracy will establish positions within 10 centimeters whereas decimeter-level accuracy may only be capable of achieving an accuracy of several decimeters. As with other accuracy specifications, the same percentage of reliability applies to decimeter products. Decimeter products generally rely on a weaker portion of the GPS signal and therefore require a more rigorous procedure and better field conditions than other accuracy specifications.
Surveyors establishing control points require sub-centimeter GPS equipment. Unlike sub-meter and decimeter products, sub-centimeter products require a network survey that includes control points. A network adjustment will identify bad baselines and distribute error accordingly. Proper network procedures will consistently yield sub-centimeter results.
Overall, the user should read the fine print when examining GPS accuracy. A manufacturer can not provide alternate specifications for poor field conditions because conditions such as the amount of tree coverage, potential for signal reflection or changes in the atmosphere can not be quantified. For the same reasons, the manufacturer can not guarantee a practical accuracy 100 percent of the time for the same reasons. Ultimately it is up to the user to test the product in their local environment in order to gain confidence in the results and understand the product's capabilities. Back |
