Project Methods
Assessment of Classification Accuracy
Introduction
Periodic assessment of thematic accuracy during map production is a critical part of any mapping project. Mapmakers want to know how reliably a given habitat type can be classified, this is called "producers accuracy." Map users want to know what percentage of the polygons labeled with a specific habitat type is classified correctly, this is called "users accuracy" (Congalton, 1991; Verbyla, 1991). Such periodic assessment is necessary to monitor and maintain acceptable standards of quality following creation of each draft. Most importantly, once final products are produced, the reliability of results must be estimated and reported.
Thematic accuracy of the Puerto Rico and U.S. Virgin Islands habitat maps was evaluated for the three most general habitat categories: unconsolidated sediment, submerged vegetation, and coral reef/hard bottom. Accuracy was estimated at each of two locations within the project area that included the full complement of habitat types, depth ranges, and water conditions representative of Puerto Rico and the U.S. Virgin Islands. For this reason, the accuracy of maps measured at these two locations is assumed to be representative of map accuracy elsewhere in the project area. This approach, which focused in two small areas, enabled a statistically robust evaluation of thematic accuracy to be conducted without the logistic difficulty of collecting data for accuracy assessment over the entire project area.
In addition, since a novel mapping approach was used to enhance production time and provide additional project deliverables, it was necessary to ensure that maps produced using the ArcView Extension had comparable accuracy to maps produced using more routinely used techniques. To accomplish this goal, the thematic accuracy of ArcView maps was compared to the accuracy of maps produced using published and well known photogrammetric techniques.
Goals of the accuracy assessment:
- Compare the thematic accuracy of maps produced from on-screen digitizing using the ArcView Extension to those produced by digitizing directly from hard copy photos using a stereoplotter.
- Evaluate the ArcView derived products more thoroughly, including areas with different reef environments/ water conditions representative of sites throughout the project extent.