Vegetation Classification and Mapping Project Report - USGS

4 Accuracy Assessment and Additional Map Classes
The Southwest Biological Science
Center (SBSC) team completed the
vegetation map database by coordinating
field collection of accuracy assessment
observations; developing the accuracy
statistics for the base map classes;
developing the group, macrogroup, and
management map classes; and developing
accuracy assessment statistics for those
map schemas. In this step, the team
used the accuracy assessment guidelines
and protocols developed by the USGS-
NPS Vegetation Mapping Program
(Environmental Systems Research
Institute et al. 1994). They developed the
accuracy assessment by comparing the
performance of base map class polygons,
as mapped, with field-collected data at
the location of those polygons.
4.1 Methods
To conduct the accuracy assessment, the
team: (1) developed a sampling design,
(2) collected field data with on-theground
field sampling (reference data),
and (3) developed map performance
statistics. As is common practice when
measuring uncertainty in mapped
classes, a sample of polygon locations
representing each map class (the sample
data) was compared to the reference data.
The higher-level group, macrogroup, and
management map classes consisted of
aggregations of base map classes. They
were assessed using the same reference
data as for the base map. Finally, the team
developed summary statistics describing
the characteristics of the base, group,
macrogroup, and management map
classes.
4.1.1 Field Sampling Design
The map database, with base map
classes, was made available to the SBSC
team for accuracy assessment in two
separate shapefiles. The initial shapefile
represented the northern half of the park,
and the final shapefile consisted of the
entire park, including the southern half.
Although it is ideal to develop a sampling
design using a complete map database,
the full map database was not available
until the late summer months. Prior to
developing the sample design, the SBSC
team checked the topology and data
structure of each map section for node
and label errors.
The team generally limited accuracy
assessment sampling to within the park
boundaries. With the approval of park
managers, the team did not field assess
land use and non-vegetated map classes.
These map classes are well known and
often easier to identify on the aerial
photography than the vegetated classes,
and the field team had limited fieldseason
time to sample the vegetated
map classes. All vegetated map classes,
including park specials and those based
on photointerpretation alone, were
included in the accuracy assessment
sampling design.
The SBSC team used the “scenario”
table recommended by the Vegetation
Mapping Program (ESRI et al. 1994)
to estimate the number of sample sites
needed for each map class in order to
achieve a statistical measure of class
accuracy with a confidence level of 90%
and an acceptable sample error of 10%.
Each scenario was based on the number
of polygons and total area of a map class.
The team assigned each map class to one
of five different scenarios to determine
the target number of sampling sites
needed (table 9). Depending upon its
scenario assignment, a map class could
include between 1 and 30 sampling sites.
The team developed the sampling
design for the park in two stages. To
compensate for the northern half
containing only a portion of all park
polygons, the team reduced the initial
number of sample sites for that section
by multiplying the portion of the total
map that the section represented by the
target number of sample sites determined
through the scenario table. This allowed
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