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area on hillside properties with respect to Measure PP standards. Staff believes the two options for measuring slope include: • Option One: Calculate 25-percent slope as a specific value based on the distance between contour lines on a topographic map. • Option Two: Calculate 25-percent slope as an average value, such as the WIS formula of the City's Hillside Planned Development District (Attachment 3) even though this method, which is an average of slopes less than 25 percent and potentially greater than 25 percent, may include slopes over a 25-percent grade. Staff Recommendation: Option One. Option One is consistent with the Measure PP provision that, "No grading to construct residential or commercial structures shall occur on hillside slopes 25- percent or greater..." This is the more conservative of the two options as it will only allow development on slopes less than 25-percent. 1.b. Methodology for Defining Contour Intervals The contour interval is the difference in elevation between adjacent contour lines. Contour lines are the horizontal lines, curved or straight, that join points of the same elevation (height) above a given level, such as mean sea levels. Contour lines and intervals can graphically show land features such as valleys, hills, creeks, ridgelines, etc., and can show the relative steepness of slopes by the spacing between the adjacent contour lines. Figure 2, on the following page, shows how the contour intervals are reflected as contour lines, and how the spacing between the contour lines — wide or narrow — can graphically indicate the relative steepness (grade) of the slope. 5 Note: Typically expressed as NGVD (National Vertical Datum of 1929), the system that has been used by surveyors and engineers for most of the 20th Century as the basis for relating ground and flood elevations. 5