Methodology and Interpretation

Percent impervious surfaces
The percent total impervious area is calculated using road density as the independent variable in a linear regression model, as described in:
May, C.W., Horner, R.R., Karr, J.R., Mar B.W., Welch, E.B. 1997. Effects of urbanization on small streams in the Puget Sound Lowland Ecoregion. Watershed Protection Techniques. 2:4. pp. 483-493.

Standard Deviation: Classes show the amount a feature's attribute value varies from the mean value of the distribution. Class breaks are generated by successively adding or subtracting the standard deviation from the mean. A two-color ramp is best used to emphasize values above or below the mean. It is particularly useful in viewing spatial variability of a parameter.

Metric input GIS data:

• United States Hydrologic Units (8-digit HUCs) - Metadata

• United States Coastal Change Analysis Program (CCAP) - Metadata

• United States Census 2000 (c2k) - Metadata

• United States Roads (Wessex / GDT) - Metadata