Vegetation Classification and Mapping Project Report - USGS

USGS-NPS Vegetation Mapping Program
Colonial National Historical Park
land-use class name. Representative photographs of most of the vegetation classes are provided
after the descriptions.
Photographs were taken by VADNH staff unless otherwise indicated. A bibliography for the
sources cited in the global vegetation descriptions from the USNVC is also provided in
Appendix I.
Fields used in the local and global descriptions are defined in Appendix J. Each USNVC
association is also crosswalked to the state classification for Virginia (Fleming et. al 2006). The
state conservation rank (S Rank), global conservation rank (G Rank), and classification
confidence for state and global classifications are included in the descriptions. Definitions of the
conservation ranks are listed in Appendix E.
The dichotomous key to the vegetation classes of Colonial National Historical Park is presented
in Appendix K. The dichotomous key should be used in conjunction with the detailed vegetation
descriptions.
Accuracy Assessment
Positional Accuracy
The final horizontal positional accuracies of the leaf-on and leaf-off mosaics are 1.047 meters
and 0.943 meters, respectively, both of which meet Class 1 National Map Accuracy Standards
(FGDC 1998b). A copy of each spreadsheet, containing the x and y coordinates for each ground
control point and the accuracy calculation formula, is included in the NCSU-CEO data archive.
Thematic Accuracy
The results of thematic accuracy assessment for the original vegetation map were calculated with
a contingency matrix (Table 12). The Kappa Index for the original vegetation map was 72.8% ±
5.7%, with an overall accuracy of 74.3%. This accuracy falls below the USGS-NPS VMP
requirement of 80%. Nine classes have errors of omission that fall below 80%, and six classes
have errors of commission that fall below 80%. Errors of omission are errors in classifying the
accuracy assessment observation and represent the probability that the ground samples have been
classed correctly. Errors of commission are map errors and denote the probability that the
mapped vegetation associations represent the associations actually found on the ground
Since the overall thematic accuracy did not meet the 80% USGS-NPS VMP standard, all cases
where accuracy assessment observations differed from the mapped data were reviewed to
determine which errors were acceptable or understandable and which errors could be easily
corrected to increase the accuracy of the map. Examination of the errors associated with each
observation shows that they can be attributed to below minimum mapping unit inclusions of
other vegetation types within the target polygons, problems with field keys and/or descriptions
that lead to user error, accuracy assessment points falling within ecotones or transitional areas,
seasonal constraints on vegetation identification, compositional similarities among map classes
making them difficult to discern in the field, or to the misclassification of polygons during initial
photo interpretation.
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