After data is collected by the GPS receiver, the data is compared with that collected by the base station for the same time interval. Differential correction is the mathematical method used to refine the estimates from the GPS receiver based on data from the base station. With medium-grade receivers, the estimates can be refined from errors measured in tens of meters to, in many cases, errors of less than two meters. With some such receivers, differential correction is managed by processing data on a computer after it is uploaded from the GPS receiver. In some higher grade receivers, differential correction is done in real time, i.e. at very nearly the time of collection.
Available accuracy varies to less than a centimeter.
Such data is used for the creation of maps, i.e. images of spatially-based data. With the aid of image-processing software designed for manipulation of GPS data, the data can be used to generate images that overlay onto existing maps. Thus, for example, land use maps can be updated by overlaying images of recent data onto existing images. In conjunction with processing of data for accurate representation, the use of a data dictionary makes possible the creation of accurate, annotated maps from GPS data. With the availability of high accuracy in real time, i.e. so that coordinates of a location are immediately available, very precise navigation is possible.