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DISCUSSION As noted above, while Measure PP included a specific purpose and General Plan Policy, it does not have technical definitions for terms involved in mapping hillsides, such as "structure," "ridgeline" and "slope." Furthermore, while many cities in the Bay Area have hillside protection ordinances, Measure PP's restrictions more uniquely apply to large hillside areas. For instance, Danville, Moraga, San Ramon, and Orinda all have hillside protection ordinances, but those apply to specific mapped areas, minimizing difficulties in defining where ridgelines begin and end (discussed further below). Nevertheless, in preparing the mapping for the Southeast Hills, the methodology developed by staff incorporates the most current and detailed mapping resources to be most reflective of the purpose of Measure PP's interest in protecting ridgelines and hillsides extending over a long distance. The following discussion presents a step-by-step summary of the methodology and techniques the City used to undertake the mapping priority project presented in this report. 1. Identify 25 Percent Over/Under Slopes. An initial task involved mapping slopes over and under 25 percent. Slope is generally defined as rise (vertical distance up a hill) divided by run (horizontal distance across the landscape), multiplied by 100 to generate a percentage (see Figure 1). Most local communities with hillside protection ordinances recommend using mapping with contour intervals less than 5 feet. The City has very granular topographical mapping of the Southeast Hills that uses 1-foot contours, dating from 2014, which was used as part of the mapping presented in this report. The granularity of the topography was next averaged-out over 3-foot by 3-foot areas, consistent with standard Geographic Information Systems (GIS) slope mapping protocol. This approach is thus protective in that even small variations in slope using 1-foot contour intervals are mapped, but the "noise" of minor variations in topography is reduced by averaging out slopes over 9-square-foot areas. Exhibit C is the resultant map of slopes over and under 25 percent using the methodology described above. Figure 2 illustrates sample slopes on Longview Drive, a road that climbs steeply west from Foothill Road, up Pleasanton Ridge. Figure 1: Calculation of Slope /Rse Veical Change Rise Slope = = X 100 = Slope Horizontal Change Run /Rise 2. Define Ridgeline. There is no formal definition of "ridge" or "ridgeline" in Measure PP or the General Plan, but Chapter 18.76 of the Municipal Code for the Hillside Planned Development District defines ridge as "a connected series of major and minor hills" and a ridgeline as "a ground line located at the highest elevation of the ridge running parallel to Mapping of Southeast Hills Planning Commission Page 5 of 10