The San Pasqual River Valley, located near San Diego, California, is rich in cultural and biological resources. San Pasqual is the site of one of the most famous battles in the Mexican-American War (1846-1848). Mule Hill, a predominant feature of the River Valley, received its name as the soldiers from the defeated United States Army of the West spent four days defending a small knoll while living on mule meat and water. Artifacts are still periodically found such as a soldier's button or a dropped musketball. Today there is a State historic park and a museum dedicated to the Battle of San Pasqual. In addition to its historic significance, steep slopes and hillsides containing biologically sensitive habitat surround the River Valley.

The San Dieguito River Valley Park Joint Powers Authority (SDJPA) is a small agency responsible for creating a natural open space park in the San Dieguito River Valley (www.sdrp.org). The park will someday extend from the ocean at Del Mar through San Pasqual to Volcan Mountain, north of Julian-a total length of 55 miles.

Staffs from the SDJPA and the City of San Diego have been working on the location and design of an environmentally sensitive hiking and equestrian trail through the San Pasqual Valley. Once completed, the 9.4-mile Mule Hill trail will serve as an educational and recreational feature for the public. In several locations, alternatives existed and City staff needed to make recommendations to the San Diego Planning Commission on the preferable trail alignment. The goal of this project was to be able to support the staff conclusions and be able to accurately portray the information to the City of San Diego decision makers.

City of San Diego Turns to ArcView and ArcView 3D Analyst

Staff members from the City of San Diego knew that raw information on the length of trails and two-dimensional maps of trail locations would not give the visual impact needed to accurately support their recommendations. City staff turned to ArcView and the ArcView 3D Analyst extension to provide a three-dimensional cartographic visualization of the proposed trail alternatives.

The River Valley makes a sharp 90-degree turn in the stream course as the river bends south. This area, referred to as the narrows, represented a significant constraint in planning the Mule Hill trail. A trail running along the valley floor would have to cross into the floodway causing impacts to wetland habitat and a continued maintenance problem as a result of periodic flooding. The other possibility was to connect the proposed trail to an existing fire road 335 feet above the River Valley bottom. In the design of the trail, three alternative factors were considered. First, City and SDJPA standards limit trail grades to no greater than 10 percent at any given point; second, impacts to biological resources must be minimized; and third, switchbacks are less preferable due to the higher cost of maintenance.

Two Alternatives Identified

Staff members from SDJPA identified one alternative that ran through scrub oak chaparral, a very sensitive biological habitat. A second trail alternative was identified by City of San Diego staff members that minimized the length of the trail, reduced direct impacts to habitat, and avoided impacts to the scrub oak habitat. The second alternative, however, would require a series of switchbacks through equally sensitive coastal sage scrub habitat. The challenge to City of San Diego staff would be to identify which trail alternative was superior and be able to support a recommendation to the Planning Commission. Once again, they turned to ArcView and the ArcView 3D Analyst extension to help analyze and support their recommendation.

Three-Dimensional Visualization

A major barrier to the use of GIS has been the lack of suitable digital data. Regionally, San Diego County is fortunate with an increasing abundance of spatial data. Using ArcView 3D Analyst, a color infrared digital orthophoto was draped over an elevation grid generated from an existing digital elevation model (DEM) to create a three-dimensional visualization of the landform. The two proposed trail alignments were converted into three-dimensional shapefiles using the elevation grid and overlaid on the three-dimensional color infrared orthophoto. It was necessary to offset the trail alignments by 15 feet for better visualization of the two trail alignments. The resulting three-dimensional image was rotated and magnified to obtain the most ideal visual representation of the trail alignments. The resulting three-dimensional image scene was converted to a JPEG file and ultimately printed as a poster exhibit for the City of San Diego Planning Commission.

ArcView 3D Analyst was also used to depict a profile of the two trail alignments to determine their respective grades. The profile view depicted the changes in elevation along the various lengths of the proposed trails. A line representing a 10 percent grade was displayed on the profile layout as a reference for City trail standards. Portions of the trail that fall below this line had a grade greater than 10 percent and thus did not meet City of San Diego or SDJPA standards. All GIS work was done on a desktop Hewlett-Packard Kayak PC running on a Windows NT 4.0 operating system.

Results

City of San Diego staff recommended supporting the proposed SDJPA trail alignment. While the coastal sage scrub trail would have been approximately 817 linear feet shorter than the SDJPA alternative, thus minimizing impacts to sensitive habitat, it would have also required seven switchbacks and exceeded the City's standard trail grade for more than 1,500 feet. In addition, it was determined that the sage scrub trail would have bisected the breeding habitat of a federally listed endangered bird. Combined with the three-dimensional visualization of the trail alignments and the GIS analysis, City staff members felt confident in their recommendations to the San Diego Planning Commission. On January 4, 2001, at a special hearing, the Planning Commission unanimously voted to approve the Mule Hill trail.

For more information, contact Keith Greer, City of San Diego Planning Department (tel.: 619-236-7258, e-mail: kgreer@sandiego.gov).