Thematic Accuracy Assessment Procedures. Version 2 - USGS

Method 3 (Method 4 with a post hoc review of limited floristic data collection) is a less desirable
methodology for the NPS Vegetation Inventory, but may be employed if the following
conditions are met:
-Field data collected for post hoc review are limited to a list of dominant and/or frequent species
in each stratum (e.g., not species cover estimates, photographs, etc.). This measure is intended to
save field time and also direct limited field time toward carefully assessing criteria needed to
follow the field key. It will limit the ability to “overrule” reference data values made in the field
to the more significant errors. “Reasonable but wrong” calls will often stand. Exhibit I is an
example of a field data assessment form that meets these criteria.
-Neither the identity of both the map class (sample data value) nor the reference call that has
been made by the field observers can be revealed to the reviewing expert prior to review.
Revealing the map class identity is a clear violation of independence in a hypothesis test.
Revealing the observers’ reference data values will likely bias the reviewer’s judgment. The
reviewer knows that he/she is reviewing only reference data values that are mismatches with the
sample data values and is likely to avoid the same reference data when there is ambiguity. The
reviewer must be given an unbiased opportunity to agree with, as well as disagree with the field
observers’ calls. If the reviewer lacks sufficient information to make a judgment about the best
reference data call, then no alternative value need be offered, and the field observers’ reference
data values will be regarded as correct.
-Data used in mapping (e.g., properties of a vegetation image and other remotely sensed data)
may not be used to influence either reference data value.
If these procedures are followed, then only field data for sites with mismatches between the
sample data value and reference data value need be reviewed and the expert reviewer’s
assessment from the [limited] field data may be regarded as the most correct reference data
value, when there is disagreement between the field observer and the reviewer.
3.2 Field Methodology
3.2.1 Field Observer Skill Level Considerations
While the NPS Vegetation Inventory accepts Method 4 as a best practice, a minimal amount of
preparation and oversight is necessary to assure that observer skill and that the accuracy gap
between use of Method 4 and Methods 1, 2, and 3 is minimized, within a reasonable amount of
project control.
Field observers should be hired and/or vetted and trained, as needed to represent an adequate
skill level to:
Identify all species named in the field key, as well as likely look-alikes. For example, if northern
red oak (Quercus rubra) is named in the key, then observers should be able to recognize this
species whenever it is encountered, to avoid errors of omission of northern red oak in the key. If
the similar-appearing scarlet oak (Quercus coccinea) is also present in the study area, observers
should be able to recognize it and differentiate it from northern red oak, regardless of whether or
not scarlet oak is named in the field key (in order to avoid errors of commission in situations in
the key that call for recognizing northern red oak). Familiarity with most of the flora is
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