Use of auxiliary data to improve the precision of estimators of thematic map accuracy

Collecting reference samples for accuracy assessment is expensive, so that statistical procedures that improve the precision (reduce variance) of estimators of thematic map accuracy while incurring little additional cost are desirable. Auxiliary data available from existing landcover maps, aerial photographs, videography, or AVHRR, for example, can be used in combination with ground visits to enhance the precision of map accuracy estimates. This precision gain is achieved by employing a regression estimator commonly used in survey sampling. Practically important gains in precision can be achieved with no additional complication in the sampling design and without additional field visits. The only additional cost incurred is that of obtaining the auxiliary data. The regression estimator can be used with simple random or systematic sampling if auxiliary information is available on the entire region represented by the land cover map. Alternatively, a more practical design for accuracy assessment would be double sampling, which requires that the auxiliary information be obtained for only a subsample of the map region. Appropriate estimators and standard errors are presented for using the regression estimator with either simple random or double sampling.