Development of an Impervious-Surface Database for the 

Little Blackwater River Watershed, Dorchester County, 

Maryland 

By Lesley E. Milheim, John W. Jones, and Roger A. Barlow 

Open-File Report 2007–1308 

U.S. Department of the Interior 

U.S. Geological Survey 
U.S. Department of the Interior 

DIRK KEMPTHORNE, Secretary 

U.S. Geological Survey 

Mark D. Myers, Director 

U.S. Geological Survey, Reston, Virginia 2007 

For product and ordering information: 

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its natural and living resources, natural hazards, and the environment: 

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Suggested citation: 

Milheim, L.E., Jones, J.W., and Barlow, R.A., 2007, Development of an impervious-surface database for the 

Little Blackwater River Watershed, Dorchester County, Maryland: U.S. Geological Survey Open-File Report 

2007-1308, 11 p., available online at http://pubs.usgs.gov/of/2007/1308/. 

Any use of trade, product, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government. 

Although this report is in the public domain, permission must be secured from the individual 

copyright owners to reproduce any copyrighted material contained within this report. 

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Contents 

Introduction .........................................................................................................................................................................1

Classification Scheme .......................................................................................................................................................2

Input Data.............................................................................................................................................................................2

National Agricultural Imagery Program Dataset for Dorchester County, Maryland..........................................2

Digital Elevation Model..................................................................................................................................................2

Study Area .......................................................................................................................................................................2

Roads ................................................................................................................................................................................2 

Streams.............................................................................................................................................................................2

Digital Orthophoto...........................................................................................................................................................3

Software...............................................................................................................................................................................3

Processing Steps................................................................................................................................................................3

Data Collection................................................................................................................................................................3

Quality Assurance/Quality Control ..............................................................................................................................3

Metadata..........................................................................................................................................................................3

Results ..................................................................................................................................................................................4

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Figures 

Figure 1. Lidar-based digital elevation model (DEM) of the Little Blackwater River watershed study area 

(44.5 square kilometers). Study area delineated by red line (main map and index map). Watershed boundary 

includes a 100-meter buffer. Egypt Road project delineated by orange line. Stream gage location identified 

by blue dot............................................................................................................................................................................5

Figure 2. Impervious-surface boundaries within the Little Blackwater River watershed study area overlain 

on Nation Agricultural Imagery Program (NAIP) orthoimagery of June-July 2005., Study area is delineated 

by red line. Impervious-surface areas are depicted as green polygons and black lines. ....................................6

Tables 

Table 1. Summary impervious surface area June-July 2005 within the Little Blackwater River watershed....4
Development of an Impervious-Surface Database for the 

Little Blackwater River Watershed, Dorchester County, 

Maryland 

By Lesley E. Milheim, John W. Jones, and Roger A. Barlow 

Introduction 

Many agricultural and forested areas in proximity to National Wildlife Refuges (NWR) are 

under increasing economic pressure for commercial or residential development. The upper portion of 

the Little Blackwater River watershed – a 27 square mile area within largely low-lying Dorchester 

County, Maryland, on the eastern shore of the Chesapeake Bay – is important to the U.S. Fish and 

Wildlife Service (USFWS) because it flows toward the Blackwater National Wildlife Refuge (BNWR), 

and developmental impacts of areas upstream from the BNWR are unknown. 

One of the primary concerns for the refuge is how storm-water runoff may affect living 

resources downstream. The Egypt Road project (fig. 1), for which approximately 600 residential units 

have been approved, has the potential to markedly change the land use and land cover on the west bank 

of the Little Blackwater River. In an effort to limit anticipated impacts, the Maryland Department of 

Natural Resources (Maryland DNR) recently decided to purchase some of the lands previously slated 

for development. Local topography, a high water table (typically 1 foot or less below the land surface), 

and hydric soils present a challenge for the best management of storm-water flow from developed 

surfaces. 

A spatial data coordination group was formed by the Dorchester County Soil and Conservation 

District to collect data to aid decisionmakers in watershed management and on the possible impacts of 

development on this watershed. Determination of streamflow combined with land cover and 

impervious-surface baselines will allow linking of hydrologic and geologic factors that influence the 

land surface. This baseline information will help planners, refuge managers, and developers discuss 

issues and formulate best management practices to mitigate development impacts on the refuge. 

In consultation with the Eastern Region Geospatial Information Office, the dataset selected to be 

that baseline land cover source was the June-July 2005 National Agricultural Imagery Program (NAIP) 

1-meter resolution orthoimagery of Maryland. This publicly available, statewide dataset provided 

imagery corresponding to the closest in time to the installation of a U.S. Geological Survey (USGS) 

Water Resources Discipline gaging station on the Little Blackwater River. It also captures land cover 

status just before major residential development occurs. This document describes the process used to 

create a database of impervious surfaces for the Little Blackwater watershed. 

1
Classification Scheme 

The USGS and Maryland DNR discussed impervious-surface identification requirements on 

August 10, 2006, at the Maryland Department of Natural Resources in Annapolis. After consultation 

with the Dorchester County Soil and Conservation Service, the group decided to delineate roads, both 

residential and nonresidential rooftops, residential and nonresidential driveways, including parking 

areas and other impervious surfaces greater than 50 square meterssuch as tennis courts. 

Input Data 

To map land use according to the classification scheme specified, the following input data were 

used. 

National Agricultural Imagery Program Dataset for Dorchester County, Maryland 

This June-July 2005 1-meter resolution agricultural (leaf-on) imagery (NAIP_Dorchester.sid) 

shown in figure 2 was used to visually identify land use and land cover within the Little Blackwater 

River watershed (http://dnrweb.dnr.state.md.us/gis/data/data.asp). 

Digital Elevation Model 

An ArcGRID containing the 2-meter resolution Lidar digital elevation model (DEM) 

(abovebnwr) for a subset of Dorchester County, Maryland (fig. 1.) was used to identify the watershed 

boundary. This dataset is the mosaic of the Lidar DEM subsets available from the Maryland Department 

of Natural Resources. The Lidar DEM subsets used are: ab1331, ab1332, ab1333, ab1334, ab1335, 

ab1336, aa1331, aa1332, aa1333, aa1334,aa1335, aa1336, z1331, z1332, z1333, z1334, ab1341, 

ab1342, ab1343, ab1344, ab1345, ab1346, aa1341, aa1342, aa1343, aa1344, aa1345, aa1346, z1341, 

z1342, z1343, z1344, ab1351, ab1352, ab1353, ab1354, ab1355, ab1356, aa1351, aa1352, aa1353, 

aa1354, aa1355, aa1356, z1351, z1352, z1353, z1354, ab1361, ab1363, ab1365, aa1361, aa1363, 

aa1365, z1361, and z1363. 

Study Area 

These data (BufferedStudyArea.shp) identify the watershed boundary plus a 100-meter buffer 

(fig. 1.) for the Little Blackwater River. The file was generated from the digital elevation model above 

the USGS stream gage located approximately 1.5 miles downstream of the confluence of the Maple 

Dam Branch and the Little Blackwater River (lat. 38°30'06" N, long. 76°05'39" W) using the ArcMap 

9.0 Spatial Analyst Hydrology toolkit. It resembles earlier watershed boundaries for this area except for 

the area east of the airport. 

Roads 

These data (SHA_cline_SA.shp) identify the centerlines for roads within the watershed. The 

original shapefile is available from the Maryland State Highway Administration website 

(http://www.marylandgis.net/SHAdata/list.asp) and was trimmed to the study area. 

Streams 

These data (SHA_streams_SA.shp) identify the streams within the watershed. The original 

shapefile is available from the Maryland State Highway Administration website 

(http://www.marylandgis.net/SHAdata/list.asp) and was trimmed to the study area. 

2
Digital Orthophoto 

These 4-meter resolution digital orthophotos (Cambr215.tif, Cambr315.tif, 1998), available from 

the Maryland Department of Natural Resources website 

(http://dnrweb.dnr.state.md.us/gis/data/data.asp), were used as ancillary data for interpretation. 

Software 

The following software packages were used to create the database. 

• Platform: Windows XP 

• Environmental Systems Research Institute (ESRI) ArcGIS Desktop 9.0 with Spatial Analyst 

Extension 

• Habitat Digitizer Extension, Version 4 Created by Ken Buja (Ken.Buja@noaa.gov) of the National 

Oceanographic and Atmospheric Administration 

(http://ccma.nos.noaa.gov/products/biogeography/digitizer/index.html) 

Processing Steps 

Data Collection 

ArcMap 9.0 was used to collect the study area’s impervious surfaces from the agricultural 

imagery (NAIP_Dorchester.sid). Compilation was performed using the ArcMap editor and the Habitat 

Digitizer Extension. Each area was visually identified by linear patterns, texture, and color and digitized 

along the perceived visual boundary. Visual interpretation was performed within ArcMap at a scale of 

1:1,500 and at a screen resolution of 1280 x 1024 pixels. As each area was completed, it was classified 

with a unique numeric identifier (UNIQUE_ID) and a description. Additional comments were entered to 

clarify or question the assigned classification, as needed. Boundaries were dissolved and clipped to the 

study area to create the final impervious-surface shapefile. The polygon areas were calculated and added 

to the attribute table. Polygons smaller than 50 square meters were then deleted. 

Quality Assurance/Quality Control 

The final impervious-surface dataset was displayed over the original imagery to visually verify 

that all appropriate impervious surfaces were included. The area attribute was examined to ensure that 

all polygons smaller than the minimum mapping unit of 50 square meters were removed. 

Due to time and funding constraints, no formal statistical or sampling quality assurance/quality 

control measures were performed. Instead, the classified polygons were compared with the actual land 

use and land cover observed through an onsite visit in October 2006 to confirm the identified classes 

and evaluate questionable classifications. This comparison resulted in the updating of some polygon 

attributes to provide a greater consistency in the final set of classes. Recent development in the area 

precluded the confirmation of some sites interpreted as undeveloped land cover types, given the date of 

the source imagery. 

Metadata 

Metadata were generated using ArcCatalog and the Federal Geographic Data Committee 

(FGDC) ESRI template and are available as part of the shapefile contents. 

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Results 

The process resulted in a shapefile (LBRW_ImpSfc.shp) containing the impervious-surface 

areas for the Little Blackwater River watershed as depicted in the June-July 2005 agricultural imagery. 

The impervious-surface polygons are shown in figure 2 over the NAIP imagery. The impervious-surface 

area for the 50-square-meter minimum mapping unit was compared to the total area originally gathered. 

The summary of this comparison is shown in table 1. 

Table 1. Summary impervious surface area June-July 2005 within the Little Blackwater River 

watershed. 

[Abbreviations: m , square meters; km , square kilometers] 

Layer 

ImperviousSurface Area 

(m2

) 

Total Study 

Area (m2

) 

Impervious 

Surface Area 

(km2

) 

Total Study Area 

(km2

) 

Percent 

Impervious 

Surface 

Original digitized data 

2

Polygons < 50 m removed 

1485547 

1483945 

44471400 

44471400 

1.486 

1.484 

44.47 

44.47 

3.340 

3.337 

2 2

4
Figure 1. Lidar-based digital elevation model (DEM) of the Little Blackwater River watershed study 

area (44.5 square kilometers). Study area delineated by a red line (main map and index map). Watershed 

boundary includes a 100-meter buffer. Egypt Road project delineated by an orange line. Stream gage 

location identified by a blue dot. 

5
Figure 2. Impervious-surface boundaries within the Little Blackwater River watershed study area 

overlain on Nation Agricultural Imagery Program (NAIP) orthoimagery of June-July 2005. Study area is 

delineated by red line. Impervious-surface areas are depicted as green polygons and black lines. 

6