GEOG 469
Energy Industry Applications of GIS

GIS Errors, Accuracy, and Precision


Errors can be injected at many points in a GIS analysis, and one of the largest sources of error is the data collected. Each time a new dataset is used in a GIS analysis, new error possibilities are also introduced. One of the feature benefits of GIS is the ability to use information from many sources, so the need to have an understanding of the quality of the data is extremely important.

Accuracy in GIS is the degree to which information on a map matches real-world values. It is an issue that pertains both to the quality of the data collected and the number of errors contained in a dataset or a map. One everyday example of this sort of error would be if an online advertisement showed a sweater of a certain color and pattern, yet when you received it, the color was slightly off.

Precision refers to the level of measurement and exactness of description in a GIS database. Map precision is similar to decimal precision. Precise location data may measure position to a fraction of a unit (meters, feet, inches, etc.). Precision attribute information may specify the characteristics of features in great detail. As an example of precision, say you try on two pairs of shoes of the same size but different colors. One pair fits as you would expect, but the other pair is too short. Do you suspect a quality issue with the shoes or do you buy the shoes that fit? Would you do the same when selecting GIS data for a project?

The more accurate and precise the data, the higher cost to obtain and store it because it can be very difficult to obtain and will require larger data files. For example, a 1-meter-resolution aerial photograph will cost more to collect (increased equipment resolution) and cost more to store (greater pixel volume) than a 30-meter-resolution aerial photograph.

Highly precise data does not necessarily correlate to highly accurate data nor does highly accurate data imply high precision data. They are two separate and distinct measurements. Relative accuracy and precision, and the inherent error of both precision and accuracy of GIS data determine data quality.

Watch this!

The 11-second video below, created by Glenn Johnson from Penn State's Dutton e-Education Institute, demonstrates the difference between precision and accuracy. 

Click for a text description.

There are 4 archery targets with 4 arrows landing in different places. Where the arrows land demonstrates the archers level of accuracy and precision.

Accuracy and Precision: All arrows land in the bull's eye.

Accuracy and Imprecision: All arrows land on the target in all different places but none on the bull's eye.

Inaccuracy and Precision: All the arrows land in the same area but it is not in the bull's eye.

Inaccuracy and Imprecision: Two arrows hit the target outside of the bull's eye. Two arrows miss the target altogether.

Glenn Johnson