| GPS Q&A
By Catherine Mansfield Q. What is the difference between real-time differential correction and post-processed differential correction for GIS data capture, and why would I choose to use one of these methods? J.M. Salem, OR A. Differential correction refers to the process used to remove the error on the GPS signal that results from Selective Availability and other systematic errors. This error can affect the accuracy of a GPS position over 100 meters.
Typically GIS applications require more accurate position data than this, so differential correction is used to remove systematic errors. The errors can be removed either as you work in the field or at a later stage in the office.
For real-time differential correction you require in addition to your GPS receiver another radio receiver used to receive the corrections broadcast from a Differential GPS service provider. The GPS receiver applies the differential messages to compute and uses a real-time corrected position for the captured features and attributes. This file can be transferred to the PC-based processing software and is immediately ready for export to your GIS.
For post-processed differential correction, you collect the raw Selective Availability corrupted GPS data into a file, along with the field collected features and attributes. When you return to the office you can differentially correct the GPS positions with information collected by a nearby GPS base station. The differential correction program interface is usually a simple point and click operation - this is the base file and this is the rover file, now run differential correction. The resultant post-processed corrected file has the effects of Selective Availability (and other correctable errors) removed and is ready for export to your GIS.
Real-time differential correction is very useful for navigational purposes and saving time in the office processing the data. For GIS applications, this means features can be easily relocated in the field for maintenance or GIS database update. GIS database update is important if the information is to be kept relevant. Once the initial data capture has been performed it may be necessary to return to the features periodically to check their condition. Taking existing GIS data into the field and using real-time differential GPS makes it very quick and simple to relocate features.
Postprocessing differential correction is useful if you want to obtain better position accuracy and reduce the cost of equipment in the field.
The increase in accuracy results from the reduction of latency inherent in real-time differential GPS and the increase in the power of the post-processing algorithms. In post-processing differential GPS there are no latency induced errors since the base and rover data files can be matched up perfectly by the post-processing program. The other contributor to an increase in post-processing accuracy is that special filters may be implemented. The reduction in field equipment cost results from not having to carry the extra radio receiver and batteries to power the extra equipment. Less field equipment will give somewhat greater reliability at a lower cost.
The use of real-time or post-processed differential GPS techniques, in the end, will depend on your application's requirements. Back |
