Vegetation Classification and Mapping Project Report - USGS

Accuracy Assessment and Additional Map Classes
assessment crew, who sampled in the
fall following a drier summer monsoon
season, often found these skeletons on
sand dunes and were instructed to assess
the underlying vegetation. Summer
monsoon rainfall in 2005, the year before
accuracy assessment, had resulted in
prolific growth of Russian thistle where it
occurred in the seed bank. The skeletons
that the accuracy assessment crew viewed
probably remained from that infestation
event. Half of the polygons mapped as
Russian Thistle Sand Dune Vegetation
were assessed in the field as another
map class; however, Russian thistle was
commonly found at these sites. Conversely,
polygons in the field identified as Russian
Thistle Sand Dune Vegetation in the
photointerpretation frequently supported
native vegetation and were field identified
as Rubber Rabbitbrush / Blue Grama
Shrub Herbaceous Vegetation or Fourwing
Saltbush / Galleta Shrubland.
The vegetation at PEFO has few distinct
boundaries between types, particularly
those types that are composed of grasses
with shrubs. There is a gradient in grass
cover as well as shrub cover. The grass
species and the shrub species are not
readily distinguishable using aerial
photography; in fact, some species are not
even readily distinguishable even when
you are beside the species in the field.
Compounding this is the mosaic pattern
that often occurs with shrub distribution.
The cover of shrubs can vary at a grain
finer than the minimum-mapping unit
(.5 ha). These mosaics of varying shrub
density make the assignment of map
classes diffcult between the field and a
photo-interpretation assessment. This
results in field assessments that may be
based on dominant species cover whereas
the photo-interpretation assessments
might be based on diagnostic or indicator
species as the only distinguishable
community characteristic on the aerial
photo.
Sparse vegetation at PEFO is often so
sparse that indicator species used to
distinguish the community may not be
apparent during accuracy assessment but
may still be present in the map polygon.
Frequently vegetation at PEFO, even
in the more densely vegetated areas,
has less than 25% cover. This makes it
diffcult to determine on the ground as
well as from the aerial photo when a
species is co-dominant or dominant since
the determination relies on assessing
relative differences of less than 5%
cover. Patchiness in vegetation cover, as
discussed above, further confounds the
determination. The accuracy-assessment
field team assessed a minimum mapping
unit within the polygon whereas the
photointerpreter delineated the entire
polygon.
The vegetation at PEFO displays a
surprising amount of variability in
appearance, considering that it is largely
composed of perennial shrubs and grasses.
When dormant, grasses are brown and
diminished in volume compared to their
non-dormant aspect. Shrubs also can lose
leaves and fade in color during drought.
Variation in annual rainfall and variation in
the seasonal timing and amounts of winter,
spring and summer monsoonal rains
greatly influence the visual aspect of the
PEFO vegetation. These climatic factors
influenced the ground appearance of the
plant communities during the fieldwork
of 1996 and 2003, the photointerpretation
data collection in 2004-2006, and the
accuracy-assessment data collection in
2006. Additionally, the photointerpretation
team was mapping conditions present on
aerial photography acquired earlier than
the field observations, which introduces
some potential of a mismatch between
ground conditions and the imagery. These
changes are diffcult to account for in the
development of the map and in assessing
the map accuracy.
The photosignatures of some map
classes, such as in the badlands, were
masked by the strong spectral signature
of bare soil. Where this happened, the
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