Thematic Accuracy Assessment Procedures. Version 2 - USGS

4. Define the most appropriate source of higher accuracy to be used for reference (field)
observation data against which to compare the map data, taking into account the
additional needs of cost and user relevance, along with accuracy.
5. Reinforce the differences between minimum mapping unit based and polygon based
designs in accuracy assessments and that the former approach is required in the NPS
Vegetation Inventory. See Appendix A.
6. Improve and make more efficient individual map class sample size allocation, given the
increased understanding of the nature of vegetation distribution within National Park
units.
7. Where appropriate, allow flexibility in minimum mapping unit sizes, given the increased
understanding of vegetation spatial scales within parks and the scale of parks themselves.
8. Provide specific procedural guidance on sampling design.
9. Provide specific procedural guidance on response design (field methodology).
10. Provide updated procedural guidance on analysis (computational methods).
1.3 Key Requirements and Best Practices
Producers of ecological classifications and maps, project managers, and accuracy assessment
practitioners for the NPS Vegetation Inventory should be familiar with the following key
requirements (RQ) and best practices (BP) of the program.
(RQ) Since the spatial component (a Geographic Information System layer) of the vegetation
database will be compiled as a series of park-specific projects, a separate thematic accuracy
assessment will be conducted for each project. (BP) Single projects will usually be limited to
individual parks, but, in some cases, multiple closely situated or contiguous parks will be
mapped as a single project by the same team of producers.
(RQ) Thematic accuracy will be assessed using field observation by a qualified observer as the
reference source of higher accuracy to which the map is compared.
(BP) The primary purpose of the thematic accuracy assessment is to inform users of the
limitations of the individual vegetation map classes and of the relationship of the errors
(confusion) between classes. While evaluation of overall map (project) accuracy against a
minimum threshold is also often required in order to assess project performance, this need is
more quickly addressed in the earlier and less extensive process of map validation (see Table 1,
Figure 1, Appendix E, and http://science.nature.nps.gov/im/inventory/veg/index.cfm).
(RQ) As a quality control process that is intended to be independent from and external to the
mapping process (e.g., Thomas et al. 2004), the thematic accuracy assessment is undertaken after
work on the map and associated products (ecological classification and field keys) has ceased.
(RQ) As a process independent from mapping process, a thematic accuracy assessment must be
carried out, even if every possible area the size of a minimum mapping unit was observed during
6