Module 1 - Special Topics in GIS - Calculating Metrics for Spatial Data Quality

Data quality is a critical aspect in GIS analysis.  Both accuracy and precision must be addressed.  Both may be positional, temporal, or thematic in nature.  Accuracy is defined as the degree that measured values approach the "true" or reference value.  Precision is not the same as accuracy, but rather is a measure of how close repeat measurements are to each other.  

The map at left depicts 50 repeat measurements (black dots), made using a GPS device, of a single location in Hillsborough County, Florida, near Tampa. The spatial average of the measurements (horizontal only) is shown by the green circle at the center, while degrees of spread, or estimates of precision, are shown by the blue circular zones.  The central zone contains 50% of the measurements, while the outer-most zone contains about 95% of the measurements.  This is a measure of the precision of the data gathering.

Horizontal accuracy of the measurements is indicated by the distance between the average of the measured points (green circle) and the reference location, which is know to be truer to the real position on the ground (red triangle).  The fact that the average of the measured points lies about 3 meters southeast of the reference location indicates a bias in the measured point toward the southeast: a systematic error in accuracy.

Here is a table of the horizontal and vertical accuracy and precision for these GPS measurements.

-Horizontal precision (68%):                                                            4.4 meters

-Horizontal Accuracy

  (Distance between average location and

  reference (“true”) point):                                                                3.2 meters

-Vertical precision (68%):                                                                 5.7 meters            

                            

-Vertical Accuracy

 (Difference between average elevation and

 reference (“true”) point elevation):                                                   6.0 meters

As seen in the table above, if you’re comparing horizontal to vertical for either precision or accuracy, vertical error is larger in both cases.  If you compare horizontal precision to horizontal accuracy, and then vertical precision to vertical accuracy, the pairs of values are fairly similar.  The 68% estimate for vertical precision is slightly better than the vertical accuracy.