It has been commented that Personal GPS Tracking Devices bears a resemblance to sanitation plumbing of a hundred years ago: basically an outdoor activity. But often we want to know where we are when we don’t have the luxury of a line-of-sight relationship with four or more satellites. In tunnels or mines or caves, for some examples–even in deep valleys or on urban streets–GPS operates at a disadvantage. One way to locate your position, assuming you knew it at some time in the past, is an Inertial Navigation System (INS). An “INS” is a mechanical apparatus whose heart is a set of spinning gyroscopes; it can detect very small changes in its position and report these to an operator. Marine navigators can place position data into such systems while still in harbor, sail for three months, and return to the point of origin, with the “INS” indicating the original settings within a hundred yards or so. Certainly it is possible for a vehicle that is mapping a road system to have an INS onboard to see it through tunnels, or valleys between skyscrapers. The Ohio State University Center for Mapping has developed a van for delineating highways that combines a number of position locating techniques and systems with photographic or videographic information, showing pavement condition, signs, intersections, and the like. The information can be installed in a GIS. Later, a point on an interactive computer map can be selected and the user can “drive” along the road, seeing the features.
Magnetic GPS Combined with Other Systems
To see how that helps us remove most of the error from a GPS fix, let’s focus on both a single point on Earth’s surface (a true point, “T”), and its representation in the GPS receiver (the measured, or observed, point, “O”). Suppose we take a Personal GPS antenna, and place it precisely at that known point “T”–a point that has been surveyed by exacting means and whose true position is known to within a centimeter. We call such an antenna-receiver configuration GPS ase station. In postprocessing GPS, (more correctly called postmissionprocessing GPS), the data from both the base and the rover are brought together later in a computer, and the appropriate correction is applied to each fix created by the rover. In the projects that follow, you will use postmission processing to correct GPS files. For mapping and Portable GPS Tracker activities this approach yields better results. Proof of the Pudding.
The above discussion became pretty theoretical. Is differential correction worth it? You be the judge. The files we used to open this Overview are shown again below. Here we added all the fixes that were displayed before, but here now each fix was differentially corrected. In other words, all the fixes with their obvious errors are now within the area of one small circle– that presumably includes the true location of the antennae. The ticks again are 10 meters apart, so you can get an idea of the amount of error reduction and the accuracy you can expect from differentially corrected data.