Geoscience Reference
In-Depth Information
Table 19.1 The University of Maryland
Mid-Atlantic RESAC Impervious
Surface Percentage per Pixel Classes
Impervious Class
0
1-10
11-20
21-30
31-40
41-50
51-60
61-70
71-80
81-90
91-100
Note:
Classes are represented in the raster data
as 10, 20, etc., such that class 1-10 = 10,
11-20 = 20, etc.
19.2.2
Data
Subpixel impervious surface cover data derived from TM imagery were provided by the
University of Maryland's Mid-Atlantic Regional Earth Sciences Application Center (RESAC)
impervious surface mapping effort (Mid-Atlantic RESAC, 2002). The Mid-Atlantic RESAC process
utilized a decision tree classification system to map 11 different levels of impervious surface
percentage per 30-m pixel (Table 19.1) (Smith, 2001). Reference data were obtained using photo-
grammetrically derived GIS planimetric vector data provided by Baltimore County, Maryland. The
vector data included anthropogenic features such as roads, parking lots, and rooftops but did not
include driveways associated with single-family homes. The lack of compiled driveways was a
limitation of the truth set and has the potential to be a source of error.
The Dead Run subwatershed was delineated using USGS Digital Raster Graphics (DRG) and
“heads-up” digital collection methods. The compiled Dead Run subwatershed was subsequently
utilized to clip both the Mid-Atlantic RESAC raster data and the Baltimore county impervious
surface planimetric data. This produced a spatially coincident Dead Run 30-m subpixel estimate
GRID and a Dead Run impervious surface truth vector file (Figure 19.2). All data were processed
in the UTM Zone 18, NAD83 projection. The respective data sets were independently registered
(prior to our study) and no attempt was made to coregister the data via image-to-image methods.
19.2.3
Spatial Processing
GIS raster overlay techniques were utilized to compute the reference values for percentage of
impervious surface for each 30-m grid cell within the Dead Run subwatershed. The process was a
modified form of zonal analysis. Here, however, the zones are the individual 30-m classified pixels
as opposed to individual land LU/LC zones. This method was a variation of the overlay processes
reported by Prisloe et al. (2000) and Smith (2001) and included the following analysis procedures:
•
A vector-to-raster conversion of the Dead Run impervious surface reference data was performed to
produce a high-resolution (3-m) impervious surface grid cell (0 = nonimpervious, 1 = impervious).
•
A comparison of the classified 30-m Dead Run data with the 3-m impervious surface reference
data was performed using an overlay process, which calculated the number of reference data cells
spatially coincident with the classified data (Plate 19.1). The count of coincident reference data
cell percentage for each Dead Run grid cell was tallied.
