Tree Canopy Cover in Christchurch, New Zealand 2018/19

Abstract

Tree canopy cover (TCC) is an important way of describing urban forest extent and distribution and can be used to assess the ecosystem services they provide. Tree canopy cover was mapped for Christchurch, New Zealand by Orbica Ltd. Their deep-learning approach identified and delineated tree cover from aerial imagery and LiDAR data acquired over Christchurch during 2018/19. Using this tree cover map, canopy area and cover were determined for various geographic boundary types, including wards, District Plan zones, publicly- and privately-owned land, street catchments, water body setbacks, and parks.

The mapped tree canopy cover in Christchurch for 2018/19 was 13.56%, ranging between 6.51% in Hornby to 27.6% in Coastal. Tree cover in each of the District Plan’s zones ranges from 2.01% in Mixed Use zones to 23.24% in Open Space Zones. Tree cover on publicly-owned land comprises 42.96% of the total tree cover in Christchurch (publicly-owned land has 18.95% canopy cover), with the remaining 57.04% being on privately-owned land (privately-owned land has 11.17% canopy cover). Canopy cover within street catchments was 9.22%, within water body setbacks was 20.86%, and within parks and reserves was 25.07%.

The tree canopy cover map has an overall accuracy of 97.8%. However, the accuracy assessment identified an omission error rate (10.2%), roughly double that of the commission error rate (5.04%) for trees. A point sample-based approach to estimating canopy cover, suggests that 2018/19 canopy cover in Christchurch is 14.7% with a 95% confidence interval of between 12.51% - 16.90%. Taken together, the relatively high omission error rate and the 14.7% canopy cover estimate from the sample-based approach, suggest that the tree cover map resulting from the deep-learning approach may have slightly underestimated tree canopy cover. Despite this, the mapped tree cover is well within the calculated 95% confidence interval, so should be viewed with certainty.