Thematic Accuracy Assessment Procedures. Version 2 - USGS

1. Sample size can be made proportional to the abundance and frequency of the map class.
With this approach, the rarer classes would receive few to no observations. This is unacceptable,
since all map classes are potentially individually important to park management, and, thus, it is
desirable to have a reasonably precise estimate of accuracy for each class. Conversely, the
sample size that might be generated for the most abundant classes would be an inefficient use of
field time, since the rate of confidence interval width reduction per observation decreases with
increasing sample size, representing a decreasing benefit per unit cost (see Figure 2 in Chapter
4). Furthermore, travel time required to reach all sites in a sample of randomly allocated sites of
given size is greater for abundant (spatially extensive) map classes than it is for rarer map
classes, which generally have a more limited distribution (i.e., sampling is more efficient in rarer
map classes and often significantly so).
2. Maximum and minimum sample sizes can be established, taking into account statistical
as well as cost constraints and probable class abundance and frequency.
Because it is important for the user to understand the error properties of each map class,
regardless of its abundance, an ideal scenario might place an equal number of observations in
each map class. This number should be large enough so that the point estimate of accuracy for
the class is reasonably reliable, but small enough to avoid excessive time and cost. Because an
area representing a minimally adequate stand size needed to evaluate a vegetation type must be
observed at each site and because of the logistical site location constraints discussed earlier in
this chapter, some rarer map classes (those that cover less total area) may not accommodate this
ideal number of observations.
The 1994 guidelines recommended the second approach, with a sample size of either 30 (for
abundant and fragmented classes), 20 (for abundant, but less fragmented, or less abundant, but
more fragmented classes), or 5 or fewer (for rare or very rare classes) (Environmental Systems
Research Institute et al. 1994). In the practice of allocating sampling sites in NPS parks using
these three alternative samples sizes, it was found that many to most map classes in smaller parks
(e.g., Cogan 2007, Patterson 2008) and some less abundant map classes in larger parks are too
sparsely populated to make meaningfully precise estimates of accuracy. To correct that
deficiency and, to take into account the site locating constraints and minimum mapping unit
sizes, a blend of the two approaches described above is now employed to assign numbers of
observations to individual map classes according to one of three scenarios (see also Table 4):
Table 4. Standard sample size allocations for NPS Vegetation Inventory thematic accuracy assessment,
based on map class area.
MAP CLASS TOTAL AREA *
NUMBER OF OBSERVATIONS PER MAP CLASS
> 50 hectares 30 *
8.33 to 50 hectares 0.6 per hectare*
< 8.33 hectares 5*
* - as measured before buffering for cost surface (access buffer) or for map class boundary buffer.
** - or as many spatially independent (non-overlapping) observation sites as map class area,
MMU size and other considerations will allow.
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