Remote Sensing Classification Accuracy
1. Select Test Areas Selecte test areas in an image to evaluate the accuracy of a classification Test areas should be representative categorically and geographically Sampling methods: uniform wall-to-wall, random, stratified random sampling Sample size: 50 - 100 pixels each category
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2. Error Assessment A classification is not complete until its accuracy is assessed Error matrix KHAT statistics
Error Matrix Also called confusion matrix and contingency table Compares the ground truth and the results of the classification for the test areas Can be used to evaluate the result of classifying the training set pixels and the results of classifying the actual full-scene
Error Matrix Diagonal cells are correctly classified pixels Classified Reference Data Data Water Sand Forest Urban Corn Hay Row Total Water 480 0 5 0 0 0 485 Sand 0 52 0 20 0 0 72 Forest 0 0 313 40 0 0 353 Urban 0 16 0 126 0 0 142 Corn 0 0 0 38 342 79 459 Hay 0 0 38 24 60 359 481 Col Total 480 68 356 248 402 438 1992 Diagonal cells are correctly classified pixels correctly classified pixels 1672 Overall accuracy = ------------------------------- = ------- = 84% total pixels evaluated 1992
Error Matrix Classified Reference Data Data Water Sand Forest Urban Corn Hay Row Total Water 480 0 5 0 0 0 485 Sand 0 52 0 20 0 0 72 Forest 0 0 313 40 0 0 353 Urban 0 16 0 126 0 0 142 Corn 0 0 0 38 342 79 459 Hay 0 0 38 24 60 359 481 Col Total 480 68 356 248 402 438 1992 In this case, the non-diagonal column cells are omission errors e.g. omission error for forest = 43/356 = 12% The non-diagonal row cells are commission errors e.g. commission error for corn 117/459 = 25%
Error Matrix Omission error = 1 (100%) - producer's accuracy Classified Reference Data Data Water Sand Forest Urban Corn Hay Row Total Water 480 0 5 0 0 0 485 Sand 0 52 0 20 0 0 72 Forest 0 0 313 40 0 0 353 Urban 0 16 0 126 0 0 142 Corn 0 0 0 38 342 79 459 Hay 0 0 38 24 60 359 481 Col Total 480 68 356 248 402 438 1992 correctly classified in each category producer's accuracy = ---------------------------------------------- the total pixels used in the category (col total) Omission error = 1 (100%) - producer's accuracy
Error Matrix Commission error = 1 (100%) - user's accuracy Classified Reference Data Data Water Sand Forest Urban Corn Hay Row Total Water 480 0 5 0 0 0 485 Sand 0 52 0 20 0 0 72 Forest 0 0 313 40 0 0 353 Urban 0 16 0 126 0 0 142 Corn 0 0 0 38 342 79 459 Hay 0 0 38 24 60 359 481 Col Total 480 68 356 248 402 438 1992 correctly classified in each category user's accuracy = ------------------------------------------------------- the total pixels used in the category (row total) Commission error = 1 (100%) - user's accuracy
KHAT Statistics A measure of the difference between the actual agreement between reference data and the results of classification, and the chance agreement between the reference data and a random classifier
KHAT Statistics ^ observed accuracy - chance agreement k = -------------------------------------------------- 1 - chance agreement The KHAT value usually ranges from 0 to 1 0 indicates the classification is not any better than a random assignment of pixels 1 indicates that the classification is 100% improvement from random assignment
KHAT Statistics r r N × S xii - S (xi+ × x+i) ^ i=1 i=1 k = ----------------------------------- r N2 - S (xi+ × x+i) i=1 r - number of rows in the error matrix xii - number of obs in row i and column i (the diagonal cells) xi+ - total obs of row i x+i - total obs of column i N - total of obs in the matrix
KHAT
KHAT Statistics KHAT considers both omission and commission errors
Readings Chapter 7