Thematic Accuracy Assessment Procedures. Version 2 - USGS

a sampling scheme, the individual sites are represented as dimensionless points and would
coincide only in unlikely cases in which the same x and y values are selected more than once.
However, due to the need to observe a sufficiently large area (the size of the MMU for that map
class) around the point in order to determine the reference (field) class value, the selection of a
sample size of points that is relatively large compared to the map class area is likely to produce
some site locations that are close enough to one another so that the observation areas associated
with these points overlap in space, particularly so in map classes that have limited total area.
For the purposes of thematic accuracy assessment, the inference population can be regarded as a
potentially infinite set of individual observations in time, as well in space. Errors in
correspondence between map and observer include factors other than mapping error, such as
field key error, reasonable user interpretation error, and vegetation that was not described in the
ecological classification step. In that context, observations of overlapping observation areas,
even those that overlap completely, could be made independent by simply having different and
independent field teams observe and identify them. However, because of the costs associated
with access and the difficulty in recruiting qualified field observers, this approach would be
largely impractical for the NPS Vegetation Inventory. Field teams most often will need to gain
access to and observe sites sequentially by their proximity to one another to save access time.
If significant portions of one or more observation areas overlap spatially and the same field
observer is visiting them in short sequence, it would be unreasonable to expect the observer to be
able to make the field calls independently from one another, since the observer’s decision on the
initial call naturally would tend to influence the decision on a subsequent call that includes much
of the same observation area and vegetation. In order to maintain independence of observations,
the observation areas of individual observations should be independent from one another (nonoverlapping),
although, as is the case in Subsection 2.3.4, it may be acceptable for independence
of field calls by the same observer if adjacent observation areas overlap by a very small area
(e.g., less than 10%). It is not only permissible, but is often necessary, for polygons to be
populated with multiple sample sites.
As a general rule, individual observation sites should be located so that their areas overlap
minimally or not at all (sampling without replacement). In a site allocation methodology, this
might involve generating the requisite sample size for a map class, then to use an unbiased
(random selection) means of eliminating individual sites whose observation areas overlap those
of other sites. The eliminated sites are then replaced by sites generated from a subsequent round
of site allocation, with priority for inclusion in the sample being given to sites generated in
earlier rounds. If the map class area is very limited relative to the number of sites and associated
observation areas that must be accommodated, it may be more efficient to generate a larger
number of sites than is necessary with each round of site allocation and eliminate those that
encroach upon sites produced in earlier rounds of allocation. The rules developed for priority of
allocation will affect the location and, in some cases, the maximum number of sites that can be
accommodated in a map class. This is because subsequent selections of site locations often will
be limited in their accommodation within the map class without overlap with other sites because
of specific placements of sites generated in an earlier allocation round.
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