Spatial Accuracy and Precision

After a brief respite, I return to the GIS academic gauntlet with a look at the many facets of error in spatial analysis- primarily those involved in measurement precision and accuracy.  

  

Horizontal Accuracy - 3.8 meters
Horizontal Precision - 4.3 meters

The above map and measurements are based on a set of 50 point locations, recorded with a GPS unit in the same location at different times.  The precision of the measurements is a measure of how close each measurement is to the others taken in the same place, as there is some inherent error in commercially available GPS unit readings, and each of the 50 above seem to have given a relatively different location.  The average of the 50 is the larger yellow point in the center, and the blue bands around it correspond to the areas containing 50%, 68% and 95% of the points, respectively.  Because 68% of values in a normal distribution fall within one standard deviation of the mean, we can say that the average of the values within the 68% band gives a measure of horizontal precision- 4.3 meters.  The horizontal accuracy is a measure of the point distances from the true, or accurate location, which is obtained from a benchmark located where the measurements were actually taken on the ground.  The distance from that benchmark's known coordinates to the average point location is 3.8 meters, and so the horizontal accuracy of the GPS readings is determined to be 3.8 meters.