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.
Quantile: Each class contains an approximately equal number (count) of features. A quantile
classification is well-suited to linearly distributed data. Because features are grouped by the number
within each class, the resulting map can be misleading, in that similar features can be separated into
adjacent classes, or features with widely different values can be lumped into the same class. This
distortion can be minimized by increasing the number of classes.
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