GIS Data used to create the County Profiles
Conservation of the Landbase l Fire Risk l Forest Cover l Forest Patches l Population Data (1990) l Population Data (2000) l Public Drinking Water Source Areas l Riparian Buffer Restoration l Wildland Urban Interface
Conservation of the Landbase
The maps of conserved lands feature the GIS layers, part of the Conservation Lands Database, generated by the Department of Conservation and Recreation Natural Heritage Division. These data include conserved lands from federal, state, local, and private holdings, including conservation easements. For further information, please contact David Boyd at the Natural Heritage Division.
Fire Risk
The maps of Wildfire Risk Assessment feature a GIS layer generated by VDOF and published March 2003. DOF examined which factors influence the occurrence and advancement of wildfires and how these factors could be represented in a GIS model. DOF determined that historical fire incidents, land cover (fuels surrogate), topographic characteristics, population density, distance to roads, and density of roads were critical variables in a wildfire risk analysis. DOF gathered these data layers, often creating them, and used them in a raster-based weighted aggregate model.
Forest Cover
The maps of forest cover features the 2000 Forest Cover GIS layer generated by DOF from satellite image classification. VDOF generated this layer to show the forestland in Virginia that meets the United States Forest Service, Forest Inventory and Analysis Program's definition of forestland. Forestland is defined as lands with at least a stocking of 10% cover of live forest trees of any size, or formerly having such tree cover, and not currently developed for non-forest use. The minimum area for classification of forestland is usually 1 acre with a minimum width of 120 feet stem-to-stem. Forested strips must be at least 120 feet wide for a continuous length of at least 363 feet in order to meet the acre threshold. Unimproved roads and trails, and clearings in forest areas are classified as forest if less than 120 feet wide or smaller than 1 acre.
The map was developed through the classification of Landsat 7 ETM+ satellite imagery, acquired in the years 1999 and 2000, and refined using various ancillary data sets and algorithms. Classification training and accuracy assessment was done using over 7,000 Forest Inventory and Analysis (FIA) ground data plots.
Forest Patches
The maps of forest patches feature the 2000 Forest Cover GIS layer generated by DOF from satellite image classification. Part of the layer processing included intersection with the VDOT roads layer, so forest patches represent continuous forest blocks unbroken by other land use or roads. The map shows these patches color-coded by size to demonstrate the degree of forest fragmentation within the county.
Population Data (1990)
The maps of 1990 population are created using US Census Bureau 1990 Block GIS data. Population density was calculated by DOF based on total population and block size. Densities were categorized into Low, Moderate, or High based on a 1995 assessment by DOF field staff of what thresholds constitute low, moderate, or high in their work area. These vary by county and can be found in DOF's 1995 Virginia Forest Land Assessment Report.
Population Data (2000)
The maps of 2000 population are created using US Census Bureau 1990 Block GIS data. Population density was calculated by DOF based on total population and block size. Densities were categorized into Low, Moderate, or High based on a 1995 assessment by DOF field staff of what thresholds constitute low, moderate, or high in their work area. These vary by county and can be found in VDOF's 1995 Virginia Forest Land Assessment Report.
Source Drinking Water Areas
The Source Public Drinking Water Areas maps feature the GIS source area layers generated by the Virginia Department of Health. For public drinking water derived from surface water, this dataset includes all upstream source areas as delineated from USGS 7.5' Topographic Quads. For public drinking water derived from wells or springs, a 1-mile fixed radius from the intake is determined to be the wellhead protection zone. For further information, please contact Christopher D. Adkins at the Virginia Department of Health.
Riparian Buffer Restoration
The maps of Riparian Buffers feature the 2003 Riparian Buffer GIS layer generated by Penn State Land Analysis Laboratory. Buffers were mapped using a GIS automated buffer inventory algorithm that compared the USGS National Land Cover Dataset and USGS 1:100k National Hydrography Dataset. The algorithm determined where buffers exist by sampling at regular intervals along an axis orthogonal to the stream segment. This method, therefore, produces a layer that describes whether buffers exist on one, both or no sides of a waterway. These data are useful at the watershed and county scale, however are not suitable to be used at the site-level owing to the nature of the analysis method.
Wildland Urban Interface
The maps showing the Wildland/Urban Interface feature a GIS layer derived by intersecting areas of moderate to high population (from 2000 Census Block Data) with the layer of forest cover (from 2000 Forest Cover). This layer was then overlaid over the Wildfire Risk Assessment to demonstrate the spatial relationship between the two.
Need additional information?
Contact the GIS Manager.
Last modified: Friday, 07-Mar-2008 21:24:07 UTC