Vegetation Classification and Mapping Project Report - USGS

USGS-NPS Vegetation Mapping Program
Colonial National Historical Park
Tree-of-Heaven Forest (0%), and Tidal Freshwater Marsh (60%). Four accuracy assessment
points that fell in areas mapped as Coastal Plain Loblolly Pine – Oak Forest were not
classified in agreement with the map. These errors can be attributed to below minimum
mapping unit inclusions of other vegetation types within the target polygons and incomplete
inspection of polygons. Since the compositional data collected in accuracy assessment
support the original mapped class, a misunderstanding of the key or descriptions seems a
likely cause of error. Only 25% of the accuracy assessment points that fell into Coastal Plain
Calcareous Ravine Forest were in agreement with the map. This is an understandable error
since confirmation of this vegetation in the field can depend on the visibility of several
indicator species that are only apparent early in the growing season. Modifications were
made to the local descriptions noting that, late in the season when some herb species are not
evident, Coastal Plain Mesic Calcareous Ravine Forest may be confused with Mesic Mixed
Hardwood Forest. The one polygon of Successional Black Walnut Forest that was classified
as Successional Mixed Scrub was transitional in composition and structure and could not be
confidently reclassified based on accuracy assessment data. The error of omission for
Successional Tree-of-Heaven Forest is 0% since it is mapped as only one polygon and the
polygon was identified as Successional Tuliptree - Loblolly Pine Forest during accuracy
assessment. Examination of accuracy assessment data revealed that only a small portion of
the polygon was observed and that tree of heaven (Ailanthus altissima) may have been
misidentified as black walnut (Juglans nigra). Four polygons mapped as Tidal Freshwater
Marsh were identified as other tidal marsh map classes in accuracy assessment. Vegetation at
some of these locations was transitional between concepts and difficult to classify from
accuracy assessment data.
The error of commission exceeds 80% for all classes in the final vegetation map except
Coastal Plain / Piedmont Floodplain Swamp Forest (Green Ash - Red Maple Type) (50%) and
Mesic Mixed Hardwood Forest (77%). One area mapped as Coastal Plain / Piedmont Small-
Stream Floodplain Forest and one area mapped as Non-Riverine Saturated Forest were
incorrectly identified as Coastal Plain / Piedmont Floodplain Swamp Forest (Green Ash – Red
Maple Type) in the accuracy assessment. These map classes can be difficult to distinguish in
the field, particularly in ecotonal or disturbed areas, and on photography since all are
deciduous wetland forests that have similar photosignatures. Three areas mapped as Coastal
Plain Mesic Calcareous Ravine Forest and three areas mapped as Successional Tuliptree –
Loblolly Pine Forest were identified as Mesic Mixed Hardwood Forest in accuracy
assessment. On the ground, Coastal Plain Mesic Calcareous Ravine Forest may be confused
with Mesic Mixed Hardwood Forest, particularly late in the growing season when some of the
indicator species are not visible. The two map classes are both deciduous upland forests with
similar canopy composition and similar landscape positions, and thus could easily be
confused in photo interpretation. Confusion between Successional Tuliptree - Loblolly Pine
Forest and Mesic Mixed Hardwood Forest both on the ground and in photointerpretation is
also understandable since some stands of these map classes can be compositionally similar,
and both map classes can have virtually identical photo signatures. The error of commission
could not be calculated for Semipermanent Impoundment or Sucessional Mixed Scrub since
these classes were not included in the accuracy assessment. Both classes are present in the
contingency matrix to indicate when accuracy assessment observations identified sample
points as these classes.
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