Estimating canopy cover using aerial photography for a mixed conifer zone, northern, New Mexico

Abstract

In 2010 an evaluation was conducted to determine if aerial photography could be used to accurately estimate canopy cover. Canopy cover data were estimated from 12 randomly selected sites. The sites were located in the Goat Hill subsidence area, at the Chevron Mining Inc. molybdenum mine in northern New Mexico. With aerial photography (NAIP, 2009), the 12 selected sites were separated into three canopy groups (A, B, C) based on the homogeneity of the associated canopy cover density. Each group had at least three study sites with three 20m parallel transects located 5m up slope from center, on center, and 5m down slope from center. A densiometer was used to read canopy cover every two meters along each transect (10 readings per transect, 30 per study site). A total of 360 canopy cover readings were created for the 12 sites. Data for each group were collected in 2007 and 2010. A mean canopy cover was calculated for each canopy group by combining the study site data for the two years. Polygons were digitized for each group using the aerial photo as the land area base. The data within the polygons were combined into two separate classes using the Natural Breaks (Jenks) method of classification. Both values were tallied and the sums were compared. The percentage canopy cover data were derived by dividing the sum of each value by the total value for each class. This resulted in an estimated canopy cover percentage for each class. The field data canopy cover percentage and the aerial photography estimated percentage were compared. The percentages derived from group A, B and C were 50%, 13%, 60% respectively. Estimated aerial photography canopy cover percentages for class A, B, and C were 51%, 13% and 56% respectively. A regression analysis was conducted between both data sets, yielding a correlation (R2) of.99. The correlation suggests that aerial photography can be used to estimate canopy cover within the subsidence area and potentially elsewhere on the mine. This technique has potential application for areas that may be difficult to access on foot, but are accessible with aerial photography.