Guidelines for Developing and Evaluating Tree Ordinances

Measurement of canopy cover at edge of pavement (CCEP)

Street tree canopy is especially problematic to assess because of differences that exist in street widths and configurations. After reviewing existing methods and experimenting with various methods, we recommend canopy cover at the edge of pavement (CCEP) as a standard for assessing street tree canopy. CCEP can be measured on almost all types of roads either from aerial imagery or by ground survey. Furthermore, CCEP values are related to the amount of shading that streets receive, and the "canopied" effect that is obtained when trees arch over streets. In the two photos below, the street on the left has a low CCEP value whereas the street at the right has a high CCEP value. Both streets are in Vacaville, CA.

Measuring CCEP from aerial images

Variations of the line intercept method are used to measure CCEP from aerial images. Using the hybrid line/point method, the evaluator lines up a single row of dots or a finely divided ruler along the visible edge of the pavement. The evaluator counts the number of dots or ruler divisions that fall on tree crowns and the total number of dots in the sampled segment. Percent CCEP is then calculated as:

% canopy cover = 100 x (points falling on tree canopy/total number of points along sampled lines)

While this method is easiest to apply on relatively straight sections of road, it is possible to apply the method on moderately curved roads by using a flexible plastic rule held on edge.

On very curved roads and those with very low or high canopy cover, it may be more efficient to use the standard line intercept method. In this method, the lengths of all tree canopies that intersect the line of the edge of pavement are measured with a ruler, planimeter, or digitizer. Percent CCEP would be calculated as for the line intercept method, i.e.,

% canopy cover = 100 x (length covered by tree canopy / total length of sample)

Measuring CCEP via ground survey

CCEP can also be readily evaluated by using a foot survey. To estimate CCEP in a foot survey, the evaluator walks along the edge of the pavement. At evenly spaced points, e.g., every 3 steps, the evaluator notes whether canopy is present directly over the point at the edge of the pavement. Hand tally counters can be used to keep track of the total number of sample points and the number of sample points with tree canopy overhead. A length of 1/2 inch diameter PVC pipe, lightweight measuring pole, densitometer (a small instrument with a level that allows one to sight a point directly overhead), or similar tool can be used to help project a line vertically from the edge of pavement upward to increase the accuracy of the evaluation. The percent CCEP is calculated using the following formula:

% CCEP = 100 x (points with canopy cover/total number of points)

One advantage of measuring CCEP as part of a ground survey is that it allows one to examine correlations between CCEP and other tree or site characteristics. The evaluator can collect data on site or tree characteristics at the same time that canopy is being assessed. The level of CCEP can be affected by tree species selection, tree age, planting position, and pruning practices, and it may be of use to know which factors are the most important in your community.