Accuracy vs. Precision: The Target Analogy for GPS and GNSS Users

Trimble GeoXH 6000Accuracy vs. Precision:  The Target Analogy by Mark Young provides a quick reference that explains the differences.  Accuracy is the degree of truth while in some contexts precision may mean the degree of replication or the objects “reproducibility.”

The analogy below is used to explain the difference between accuracy and precision by comparing three targets that were repeatedly shot with an arrow. Accuracy describes the closeness of arrows to the bulls eye at the target center. Arrows that strike closer to the bulls eye are considered more accurate. The closer a systems’ measurements to the accepted value, the more accurate the system is considered.

To continue the analogy, if a large number of arrows are shot, precision could be measured by the size of the arrow cluster. (When only one arrow is shot, precision is the size of the cluster one would expect if this were repeated many times under the same conditions). When all arrows are grouped tightly together, the cluster is considered precise since they all struck close to the same spot, even if not necessarily near the bulls eye. The measurements are precise, though not necessarily accurate.

High Accuracy Low Precision
GPS Data Collection
High Precision Low Accuracy
GPS Data Collection
High Precision High Accuracy

GPS Data Collection devices have varying levels of accuracy and precision.  For instance, a Trimble GeoExplorer Series has both high accuracy and high precision while a Trimble Juno 3 Series has low accuracy and high precision.  For questions regarding this post please contact me @geojobegis.

Mark Young is a GIS and GPS Tech for GEO Jobe. Follow him on Twitter: @myoung84

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