REMOTE SENS. ENVIRON. 37:35-46 (1991) A Review of Assessing the Accuracy of Classifications of Remotely Sensed Data Russell G. Congalton Department of Forestry and Resource Management, University of California, Berkeley T h i s paper reviews the necessary considerations and available techniques for assessing the accuracy of remotely sensed data. Included in this review are the classification system, the sampling scheme, the sample size, spatial autocorrelation, and the assessment techniques. All analysis is based on the use of an error matrix or contingency table. Exam- ple matrices and results of the analysis are pre- sented. Future trends including the need for assess- ment of other spatial data are also discussed. INTRODUCTION With the advent of more advanced digital satellite remote sensing techniques, the necessity of per- forming an accuracy assessment has received re- newed interest. This is not to say that accuracy assessment is unimportant for the more traditional remote sensing techniques. However, given the complexity of digital classification, there is more of a need to assess the reliability of the results. Traditionally, the accuracy of photointerpretation has been accepted as correct without any confir- mation. In fact, digital classifications are often assessed with reference to photointerpretation. An obvious assumption made here is that the photoin- terpretation is 100% correct. This assumption is rarely valid and can lead to a rather poor and unfair assessment of the digital classification (Biging and Congalton, 1989). Therefore, it is essential that researchers and users of remotely sensed data have a strong knowl- edge of both the factors needed to be considered as well as the techniques used in performing any accuracy assessment. Failure to know these tech- niques and considerations can severely limit one's ability to effectively use remotely sensed data. The objective of this paper is to provide a review of the appropriate analysis techniques and a discussion of the factors that must be considered when perform- ing any accuracy assessment. Many analysis tech- niques have been published in the literature; how- ever, I believe that it will be helpful to many novice and established users of remotely sensed data to have all the standard techniques summa- rized in a single paper. In addition, it is important to understand the analysis techniques in order to fully realize the importance of the various other considerations for accuracy assessment discussed in this paper. Address correspondence to R. G. Congalton, 145 Mulford Hall, Department of Forestry and Resource Management, University of California, Berkeley, CA 94720. Received 15 October 1990; revised 14 April 1991. oo34-42s7/91/$3.50 TECHNIQUES Until recently, the idea of assessing the classifica- tion accuracy of remotely sensed data was treated 35