The first ordinance passed by the Southern California city of Santa Clarita after its incorporation was the Oak Tree Ordinance. Thousands of oak trees cover the Santa Clarita Valley, and these trees are an integral part of the city’s landscape—the city logo prominently features an oak tree.
Protecting oak trees and heritage trees
The ordinance specifically protects oak trees of the Quercus species native to the area, including Valley Oak, California Live Oak, Canyon Oak, Interior Live Oak, and Scrub Oak. In addition to regulating the pruning, encroachment, and removal of oak trees, the ordinance aims to protect “heritage oaks.”
Heritage oaks are defined as the largest and oldest oaks in the city, measuring at least 108 inches in circumference for a single trunk or 72 inches in diameter for multiple trunks, at 4 1/2 feet above the ground.
Growth and Development in Santa Clarita: Adhering to the Oak Tree Ordinance
Incorporated in 1987, the city of Santa Clarita continues to experience significant growth and development. Development plans submitted to the city must undergo careful scrutiny to ensure compliance with the Oak Tree Ordinance.
Given this requirement, it became a priority for the city’s GIS group to develop a geographic layer identifying the locations of oak trees within the city. When the GIS group began participating in Los Angeles County’s inaugural aerial imagery acquisition program (LAR-IAC), the opportunity arose to experiment with remotely sensing tree locations.
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Collaborating with Engineering Systems: A New Approach to Identifying Oak Trees
After reviewing various academic and commercial options, the City’s GIS group decided to contract with the Los Angeles firm Engineering Systems. The goal was to utilize 4-inch resolution near-infrared aerial imagery to extract the locations of specific oak tree species within the city. The challenge lay in developing a spectral signature for the oak trees from the aerial imagery, which could then be automated to identify all tree locations.
Pilot Project: Mapping Oak Trees Using Spectral Signatures
A pilot project was initiated, involving the identification of oak trees by the city’s arborist for a single aerial tile. Staff at Engineering Systems used these marked locations to create a polygon layer in AutoCAD of all oak trees present. Engineering Systems then developed an application using Microsoft .NET that scanned the TIFF image—a 8000 x 8000 4-inch pixel grid comprising 64 million pixels.
The pixels within the polygons were extracted and analyzed to prepare histograms representing the frequency of individual Red, Green, and Blue (RGB) values. These histograms helped determine the peak values representing the spectral signature of the oak trees.
Challenges and Refinements: Ensuring Accurate Oak Tree Identification
The spectral signature was then used as input parameters for a second application that scanned the TIFF image tile and extracted pixels matching the signature. The process underwent several iterations, matched against field surveys, to verify that the correct species of trees were being selected.
The analysis revealed that the spectral signature was accurate in identifying more mature oak trees, but younger trees with smaller canopies were not being identified. Engineering Systems is currently working on refining the process to accurately identify these younger trees. Over 166,000 trees were located using this automated process.
Integrating the Data: A Valuable Resource for City Planning
The resulting geographic layer also identifies the diameter of each oak tree’s canopy. With the oak tree locations now georeferenced, the trees can be spatially linked to parcel numbers. This allows staff across various city departments to know when a property contains an oak tree, ensuring compliance with the Oak Tree Ordinance when development plans are submitted by developers and property owners.
