Thematic Accuracy Assessment Procedures. Version 2 - USGS

2.3.2 Areas with Training Sites
Accuracy assessment sites that happen to be co-located by chance with sites that have been used
for mapping training or for classification plots are acceptable. Since the selection of sample sites
for thematic accuracy assessment is independent of the selection of vegetation classification plot
sites or mapping training (calibration or verification) sites, the probability that a training site will
be selected as an accuracy assessment site is small, except for some very rare map classes. In
these cases, the accuracy assessment sites visited may comprise a significant proportion of the
area of the mapped class and may need to be visited to obtain a minimally adequate sample size.
Even though the accuracy of these sites would be expected to be high, they should be included in
the inference area, as other causes for thematic error (field key and observer error) will also
affect accuracy. Additionally, the accuracy assessment seeks to evaluate all possible sites in the
inference area, regardless of whether some may have been visited or not.
The assessment of sites that have been used previously for mapping training does not violate the
principle of assessment independence, as long as the training site data (as well as the map class
delineated at the site) are not revealed to the observer. The key consideration is that the
application of the two different data sets be independent (Congalton and Green 2009), not
whether the sites are independent.
2.3.3 Adjustments for Access Costs
As stipulated in Chapter 1, individual observation sites are to be selected using a simple random
sampling design within each map class, with the entire project (park) area included as the
inference area whenever feasible. This usually will be the case for smaller parks and for many
medium-sized parks. Accessibility generally will be related to the size and shape of the park, the
density of roads and trails, the amount of difficult terrain (steep slopes, dense vegetation) to
traverse, and the frequency and size of travel barriers (e.g., cliffs, rivers, private lands). Where
access costs are high due to distance and/or difficult terrain, the inference area may be reduced in
size to more accessible parts of each map class. In large to very large parks, reducing the size of
the inference area by applying a cost surface (access buffer) to limit the data collection to more
accessible sites in order to make costs of data collection reasonable will be almost certainly
necessary. The inherent assumption in this approach is that the less accessible areas that are not
considered for sampling will have similar accuracy properties as the more accessible areas that
are included in the inference area. The inference area included in the sampling design must be
carefully described in the accuracy assessment report, so that the user understands to which park
areas the accuracy assessment results apply by statistical inference and to which areas applying
the results must be done by assumptions.
Not all map classes will be evenly distributed near or away from routes of easy access such as
roads and trails. In mountainous western parks, roads may be denser in low elevation areas and
high elevation (e.g., alpine) areas may be uniformly less accessible. In the Appalachian
Mountains, trails and some roads often run along stream valleys or ridge crests, so that
vegetation types that occur in these settings are more accessible, whereas vegetation occurring on
steep middle slopes (e.g., boulder fields) may be less so. Some minimum degree of map class
representativeness is desirable for each map class.
Except in situations of exceptionally difficult access, it is recommended that the most accessible
50 th percentile of every abundant (50 hectares or more in area) map class should be included in
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