GIS 5935 - Lab 1 - Calculating Metrics for Spatial Data Quality

In GIS Fundamentals, Paul Bolstad states that "an accurate observation reflects the true shape, location, or characteristics of the phenomena represented in a GIS."  Bolstad also asserts that "there are four primary ways we describe spatial data accuracy: positional accuracy, attribute accuracy, logical consistency, and completeness."  In Part A of the Lab 1 assignment we were tasked with evaluating the horizontal and vertical precision and accuracy of waypoints collected with a handheld GPS device.  The X, Y coordinates and elevation values were provided for each waypoint.  By determining the location coordinates and elevation of the "average" waypoint were were able to determine the horizontal distance from each waypoint location to the average waypoint location.   The horizontal precision was determined by creating a buffer where 68% of the waypoint observations were found to be within 4.4 meters from the waypoint average location.  Similarly, we also used the "average" elevation value to determine the vertical 68% precision estimate of +/- 5.71 meters.  The calculated horizontal and vertical 68% precision estimates show "how close together" are the waypoint observations.

Accuracy shows "how close are the observations to the true value."  Using a reference point location we measured the horizontal distance to the average GPS waypoint location.  The horizontal accuracy was measured to be 3.24 meters.  The vertical accuracy (average location elevation - reference location elevation) was +/- 5.96 meters.  There is over a meter difference between the horizontal precision of 4.4 meters and the horizontal accuracy of 3.24 meters.   The low precision and accuracy of a recreational GPS device, such as the Garmin GPS76MAP unit used to collect the waypoints in this assignment, should be expected.  This ESRI article discusses the differences between recreational and professional grade GPS devices.

Spatial Precision and Accuracy of GPS WayPoints