Thematic Accuracy Assessment Procedures. Version 2 - USGS

2.0 Sampling Design
2.1 Sampling Design and Data Collection Objectives
The sampling design (Stehman and Czaplewski 1998) for accuracy assessment within the NPS
Vegetation Inventory adheres to the scientific principles that govern sampling and statistical
analysis, while also striving to be practical and cost effective. Specifically, the methodology
should satisfy the following objectives (Stoms et al. 1994):
1. The methodology should be scientifically sound. In order to accomplish this need, the method
should be repeatable, and the sampling design should permit the adequate representation of the
population about which statistical inferences are to be drawn.
2. The methodology should be economically feasible in view of both time and cost constraints.
3. The methodology should be applicable to all areas that are part of the project. Although there
may be some regional variation in the implementation of the accuracy assessment, these
variations should be based on the same theoretical foundation, so that the results of individual
project assessments are comparable.
The objective of collecting observations for the thematic accuracy assessment is to draw
inferences about the magnitude of discrepancies between the true attributes of a point or area and
its representation on the map. Thus, the accuracy assessment objective contrasts with the
objective of classification plots (releves), which is to provide raw data for the thorough
description of vegetation types as efficiently as possible and is often accomplished by subjective
sampling (Mueller-Dombois and Ellenberg 1974).
With these types of objectives, the randomness of the sample sites should be emphasized, and the
number of samples sites will be heavily influenced by statistical constraints.
Appendix C describes an example of a sampling design in steps for Shenandoah National Park
(Young et al. 2009).
2.2 Observation Area (Minimum Mapping Unit) Size
While the issue of observation area size is of concern for field methods (Chapter 3), it also places
some constraints on the sampling design and is treated here. As stipulated in the previous
chapter, the thematic accuracy assessment observation area will be equivalent in size to the size
of the minimum mapping unit designated for that map class.
The default (standard) minimum mapping unit (MMU) size for the NPS Vegetation Inventory is
0.5 hectare. While this size suffices for many vegetation types, mappers are often able to map
vegetation that occurs in smaller stands because these stands contrast well with surrounding
stands in imagery. An example of this is a small herbaceous wetland that is surrounded by an
upland evergreen woodland. It is often advantageous to delineate these smaller stands when they
are easily recognized and reasonably few in number because: (1) they represent vegetation types
important to management and (2) designating them as separate map classes can increase the
accuracy of surrounding or adjoining classes. This is because these small stands then will not
occur as unmapped inclusions that the field observer may fail to recognize as different and that
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