Geoscience Reference
In-Depth Information
Table 12.1
Example Error Matrix
the error matrix. This value is the most commonly reported accuracy assessment statistic. User's
and producer's accuracies are ways of representing individual category accuracies instead of just
the overall classification accuracy.
One of the assumptions of the traditional or deterministic error matrix is that an accuracy
assessment sample site can have only one label. However, classification scheme rules often impose
discrete boundaries on continuous conditions in nature. In situations where classification scheme
breaks represent artificial distinctions along a continuum of land cover (LC), observer variability
is often difficult to control and, while unavoidable, it can have profound effects on results (Congalton
and Green, 1999). While it is difficult to control observer variation, it is possible to use a fuzzy
assessment approach to compensate for differences between reference and map data that are caused
not by map error but by variation in interpretation (Gopal and Woodcock, 1994). In this study, both
deterministic error matrices and those using the fuzzy assessment approach were compiled.
12.3 METHODS
Accuracy assessment requires the development of a statistically rigorous sampling design of
the location (distribution) and type of samples to be taken or collected. Several considerations are
critical to the development of a robust design to support an accuracy assessment that is truly
representative of the map being assessed. Important design considerations include the following:
•
What are the map classes and how are they distributed? How a map is sampled for accuracy will
partially be driven by how the categorical information of interest is spatially distributed. These
distributions are a function of how the features of interest have been categorized — referred to as
the “classification scheme.”
•
What is the appropriate sample unit? Sampling units are the portions of the landscape that will be
sampled for the accuracy assessment.
•
How many samples should be taken? Accuracy assessment requires that an adequate number of
samples be gathered so that any analysis performed is statistically valid. However, the collection
of data at each sample point can be very expensive, requiring that sample size be kept to a minimum
to be affordable.
