Text of Accuracy Assessment of Thematic Maps Thematic Accuracy
Accuracy Assessment of Thematic Maps Thematic Accuracy
Learning Objectives What is accuracy assessment and why is it crucially important? What is the typical procedure for assessing thematic accuracy of maps? What are reference data, and how are they collected? What is contingency table analysis, and how do you calculate the common accuracy metrics? As a map maker, what can you learn from accuracy assessment?
What is Accuracy Assessment? Map Accuracy -- The proportion of agreement between a classified map and reference data assumed to be correct. Thematic Precision The level of detail that is mapped. A map distinguishing lodgepole pine, Douglas fir, Ponderosa, etc. is more precise (but probably less accurate!) than a map just showing Forest. Accuracy and precision are DIFFERENT!
Spatial Accuracy vs. Thematic Accuracy Thematic accuracy is how well the class names on the map correspond to what is really on the ground. Spatial accuracy quantifies errors in the locations of boundaries. Thematic and spatial accuracy are related but are usually treated separately.
Steps for performing thematic accuracy assessment: 1)Develop a sampling scheme 2)Collect reference data 3)Compare reference data to classified map 4)Compute accuracy metrics and deliver to map users
Sampling Schemes Reference locations must be unbiased Reference locations must be large enough to find with certainty on your classified image You must ensure that you can correctly identify the types at your reference sites You must visit LOTS of reference locations
Reference data collection sites for 2001 NLCD map, with ecoregions.
Collecting Reference Data Field collection Use GPS to find pre-determined reference sites Use explicit definitions of the classes to insure that reference data are consistent (should have done this with map classes too) Develop efficient plan for visiting sites and contingencies for unreachable sites
National Park Service image (nps.org)
Collecting Reference Data (cont.) Double Sampling Using high resolution remotely sensed data to assess the accuracy of low resolution data Air photos, Air videography, higher resolution satellites, etc. Requires some ground work to train yourself to accurately interpret the reference data May negate the assumption of accurate reference data! Relatively inexpensive, good for inaccessible areas, fast, etc.
Quantifying Accuracy Comparing Mapped Types to Reference Types Contingency Tables = Error Matrices = Confusion Matrices Traditional Accuracy Statistics are calculated from the error matrix
Contingency Table or Error Matrix Error matrix consists of n x n array where n is the number of classes Rows: reference (correct) data (n rows) Columns: mapped classes (n cols)
Error Analyses - Example Create a map with 3 thematic classes (water, forest and urban) Collect 95 ground reference data (Water 33, Forest 39 and Urban 23) Compare those locations to those places in the map Generate Error Matrix.
Contingency Table Reference data Classified image Water ForestUrbanTotal Water215733 Forest631239 Urban012223 Total27373195
Classification Accuracy Lots of ways to look at the thematic accuracy of a classification Overall accuracy Errors of omission Errors of commission Users accuracy Producers accuracy Accuracy statistics (e.g., Kappa) Fuzzy accuracy
Overall Accuracy Of all of the reference sites, what proportion were mapped correctly? Easiest to understand but least amount of information for map users and map producers (us).
Off-diagonal Elements The off-diagonal elements of a contingency table tell us the most about how to improve our remote sensing classification! Should spend lots of time examining ERRORS to figure out what went wrong
Errors of Omission The type on the ground is not that type on the classified image the real type is OMITTED from the classified image.
Producers Accuracy Map accuracy from the point of view of the map maker (PRODUCER). How often are real features on the ground correctly shown on the map?
Producers Accuracy Can be computed (and reported) for each thematic class Producer accuracy = 100 omission error Water: 100 36 = 64% Forest: 100 20 = 80% Urban: 100 4 = 96% Or PA = #correct/row total
Users Accuracy Accuracy from the point of view of a map USER (not a map maker). How often is the type the map says should be there really there?
Users Accuracy Can be computed (and reported) for each thematic class User accuracy = 100 commission error Water: 100 22 = 78% Forest: 100 16 = 84% Urban: 100 29 = 71% Or UA = #correct/column total
Kappa Statistic A measure of how accurate your map is above and beyond the accuracy expected by chance alone Considers omission, commission, and overall accuracy Frequently reported in the literature and used as a common way to compare maps
Kappa Statistic Kappa = (observed expected)/(1 expected) Observed is the overall accuracy from your contingency table (the sum of the diagonal)/(Total reference pixels) Expected is calculated from a matrix of marginal products Create a matrix by multiplying marginal totals. Expected = diagonal sum/total sum (from the marginal table) (See Campbell for calculation details)
Fuzzy Accuracy Recognizes that binary accuracy assessment (right vs. wrong) discards important information Can use scale of rightness or wrongness to get more nuanced accuracy assessment (e.g. absolutely right, good, reasonable, mostly wrong, absolutely wrong) There are many fuzzy metrics that can capture this information numerically.
Accuracy Assessment -- Summary Absolutely critical to any remote sensing based mapping project Expensive should be included in the budget from the start Requires collection of accurate reference data either in the field or from higher resolution data Analysis should include many aspects of accuracy to give users more information about the product.