Vegetation Classification and Mapping Project Report - USGS

Accuracy Assessment and Additional Map Classes
base. These fine-scale maps allowed each
field team to locate specific polygons on
the ground and to visualize the size and
shape of the polygon they were assessing.
The field team navigated to each site and
then examined the polygon, using the field
maps as a guide to the polygon’s shape.
They recorded site data (table 10) and
generally took at least one photo from
the site waypoint. They determined the
plant community found on the ground
and the corresponding base map class
using the plant community/map class key
in an area of at least 0.5 ha in size (e.g.
a circle within a 40-m radius) from the
geographic coordinates of the designated
sampling site. The evaluator assigned
confidence in the base map classes using
a categorical scale: exact, good (some
problems), poor, or none that fit; and
documented the reasons for a confidence
level that was less than exact. If additional
map classes or plant communities were
present in the polygon, the evaluator listed
alternate map classes. During the accuracy
assessment fieldwork, the field crew and
the SBSC project leaders refined the plant
community/map class key and some of the
plant community descriptions.
4.1.3 Field data processing and accuracy
assessment statistics
Data from 662 sampling locations were
entered into a Microsoft Access database.
The SBSC team rigorously quality checked
the data including comparing the entered
data with the original data sheet.
The SBSC team developed accuracy
assessment statistics by comparing
the reference map class (the map class
determined in the field during accuracy
assessment) to the map class in the
vegetation map database (the map class
assigned by the photointerpreter) at the
sampled waypoint (table 11). In some
cases, if the field crew could not access a
sample location, they made observations
from a removed location. When this
occurred, the geographic coordinates
of the target sample site, as determined
during development of the sampling
design, were the geographic location for
the reference data. The team listed all sites
where there was a difference in the sample
and reference comparison (an apparent
error). The field data sheet for each
apparent error was re-examined and the
alternate map class accepted if it matched
the reference data.
The team developed a contingency
table (also known as an error matrix)
to summarize the reference and sample
data comparisons for each map class. The
SBSC team used methods described in the
Vegetation Mapping Program guidelines
(Environmental Systems Research Institute
et al. 1994) to calculate user and producer
accuracy (see Glossary) for individual
map classes, as well as overall accuracy,
and a kappa index, which accounts for
correct classifications due to chance
(Foody 1992; The Nature Conservancy
and Environmental Systems Research
Institute 1994a). The kappa index was
calculated for map classes that were
assigned target sites; map classes that
were observed in the field, but were not
Table 10. Summary of data collected within 2006 accuracy assessment observation sites.
Type of information
Site documentation
Map class
Confidence in map class
Plant community
Confidence in plant community
Vegetation description
Items noted
Site identification, georeferencing, photo, elevation
Base map class of the site
The evaluators confidence in base map class assignment
Association, alliance, or park special at site
The evaluators confidence in plant community assignment
Cover class, dominant species by strata
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