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Estimating Our Urban Tree Canopy: I-Vue and I-Canopy

When we do an inventory of our trees, it helps us to know the species, the mortality and condition of our trees.
The next step is to translate this into an estimate of the percentage of canopy coverage our community has achieved.

But much more in possible –

In the last 20 years, the analysis of the benefit of city trees as a group or the “Urban Tree Canopy” (UTC) has improved substantially -- we can quantify how urban trees contribute to the environment, in saving energy, in reducing pollution and CO2 and in managing storm water.  Many of these tools have been refined and made accessible to the public in a set of computer tools called “I-Tree” developed by the USDA and US Forest Service in partnership with many other organizations.

Some tools in I-Tree, like I-Vue and I-Canopy rely on the advances in satellite mapping and refinements like LiDAR imaging – which we take for granted now that even our cell phones can pinpoint our location.  Google Earth is a marvel of clarity and resolution.  (See several images of Audubon Park in Column 2).  You can even see newly planted trees in Curlew Park.

The first of these LandSat maps for Tree Cover Analysis dates to the National Land Cover Database (2001) which maps the entire United States at 30 meter resolution by satellite imagery.  This mapping enabled us to begin to quantify the % of impervious surfaces, and types and intensity of development, as well as the % of Tree Coverage.  I-Vue was recently used to generate an extensive analysis of the Metro Louisville Urban Forest, and showed that overall Urban Tree Canopy coverage was 27%, below the target goal of 40%, and below that of other southern cities, like Nashville.

If we apply I-Vue to Audubon Park area, outlining the city closely along the boundary streets, we get the following information:

1.    The total image area is 260.2 acres, of which 35.3 acres is impervious surface (13.6%) (building and pavement), and the overall existing Tree Canopy measured at 88.7 acres, or 34.1%.

2.    The developed area (single family homes) is 142.3 acres (54.7%), and has 23.1% Impervious coverage and Tree Canopy Coverage of 20.8%. 

3.   The area that qualifies as “forested” is 114.8 acres (44.1%) (defined as 25-100% coverage by trees at least 19 foot in height) includes our parks, and naturalized areas along easements between houses, and some double lots.  The forested areas are 51% Tree Canopy and 51.6% Canopy Green Space (plantible areas, usually grass).

By the I-Vue analysis, we do not meet the goal of 40% coverage based on the 2001 data.  Notice that our parks make an important contribution to increasing our canopy coverage from 20.8% (residential developed areas) to 34.1% overall.

Using I-Canopy to estimate Urban Canopy Coverage

Another method of estimating Urban Tree Canopy is to use a new web-based tool called I-Canopy.  This tool uses 2011 Google imaging and generates random aerial sampling within the Audubon city outline (a mapping shape file).  Using 400 random points, and classifying each point as Tree, Grass, Pavement, Building, and Water (for swimming pools), the estimated Tree Coverage is 45.9%, with impervious surface (pavement and buildings) of 33.1%.  Part of the increase of impervious surface results because the Audubon City ESRI shape file includes Hess Lane.

Clearly, we are somewhere close to the desired range, if we can compensate for losing our ashes!  These are all mature trees, with extensive canopies, and even though in poor condition, they still provide significant ecological benefits.  We need to overcome our inertia and piece-meal planning to create a city wide plan to maintain and improve our canopy.

The updated I-Street Inventory analysis with actual information about our existing trees will give us actual dollar estimates of the ecological value of our own trees.

A Google Image of the Henderson and Oriole Park
Green Islands in the City

In Curlew, as of 6/3/2010, you can see the new planting sites...

The random sampling of Google Maps (like those above) has the advantage that it includes areas that are otherwise inaccessible to a direct inventory, like backyards and easements.  It can be difficult to classify some points  (like shadows of trees) but using enough points helps offset any classification errors.  On the Curlew Park image above, almost all the trees to the right side of the park that form the canopy are ashes.  The park will lose about 50% of its canopy coverage when the ashes are gone.

Anne Bobigian,
Feb 4, 2012, 5:02 PM