Vegetation Classification and Mapping Project Report - USGS

Results and Discussion
Plots
A total of 67 vegetation plots were visited over two trips (Figure 5, page 17). The first trip in
October 2008 resulted in 46 vegetation plots, and the second trip in January 2009 afforded 21
vegetation plots. Thirty-nine of the plots were directly visited while 28 were recorded from “offset”
locations. Appendix C lists the 54 vegetation species observed in the field during this
project.
Mapping Classes
There were a total of 26 mapping classes used in classifying all polygons that were digitized
(Table 6, page 22). Of the 26 mapping classes, there were 11 Virgin Islands Sub-formations, four
Alliances, and 11 Associations that are described in the Vegetation Classification (Appendix A).
Twenty of the 26 mapping classes had one or more vegetation plots representing them (Table 6,
page 22). Four of the six mapping classes lacking vegetation plots were directly visited and
confirmed although a formal vegetation plot data collection did not occur (Beach, Developed
Area, Salt Pond, and Coastal Southgate Rock Cliff Alliance). One of the six mapping classes
lacking vegetation plots was walked through on the way to other vegetation plots (Pisonia
subcordata Woodland Association). One of the six mapping classes lacking vegetation plots was
not directly visited and was only identified to the Virgin Islands Sub-formation (Gibney et al.
2000) level (Semi-deciduous Forest).
Polygons
A total of 51 polygons were digitized (Figures 5 and 6, pages 17 and 18) based on interpretation
of the imagery used (see Photo-interpretation Key Appendix B). Thirty of the 51 polygons had
one or more vegetation plots occurring in them (Figure 5, page 17). Sixteen of the 21 polygons
lacking vegetation plots were directly visited and confirmed although a formal vegetation plot
data collection did not occur (six polygons of Beach, four polygons of Developed Area, one
polygon of Salt Pond, and five polygons of Coastal Southgate Rock Cliff Alliance). One of the
21 polygons lacking vegetation plots was walked through on the way to other vegetation plots
(Pisonia subcordata Woodland Association). Four of the 21 polygons lacking vegetation plots
were not visited (one polygon of Pisonia subcordata Woodland Association, two polygons of
Semi-deciduous Forest, and one polygon of Gallery Semi-deciduous Forest). Nine of the total 51
polygons were smaller than the minimum mapping unit of 400m 2 (one polygon of Laguncularia
racemosa Shrubland Association, one polygon of Coastal Southgate Rock Cliff Alliance, three
polygons of Beach, and four polygons of Developed Area). A vegetation classified map was
designed identifying all of the map classes representing polygons (Figure 6, page 18). Some
polygons graded gently into one another and where to place the polygon boundary in the ecotone
was difficult to determine and was made with the mapper’s best judgment.
Accuracy Assessment
SFCN did not conduct a separate accuracy assessment of the classification accuracy as 30 of the
51 polygons had vegetation plot data collection. In addition, Beach (6 polygons), Developed
Area (4 polygons), Salt Pond (1 polygon) and Coastal Southgate Rock Cliff Alliance (5
polygons) were directly visited and confirmed although a formal vegetation plot data collection
did not occur. Thus 92% of the polygons either had vegetation data collection or were directly
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