The physical framework of a community is called its infrastructure. These utilitarian workhorses of a city can be divided into green and gray. Green infrastructure are areas covered with trees, shrubs, and grass; gray infrastructure are areas of buildings, roads, utilities, and parking lots. Green infrastructure is porous, allowing water to soak into soil which naturally filters pollutants before entering rivers. Gray infrastructure is impervious, forcing water to runoff and which must be managed and cleaned before entering rivers.

A planimetric map of a Washington DC neighborhood shows a neighborhood’s gray infrastructure including buildings and roads (left). Classified high-resolution satellite imagery adds a green infrastructure data layer (trees and other vegetation) with its associated environmental benefits (right).

Unlike gray infrastructure, the functional role of trees, as green infrastructure in cities is not adequately documented. Without quantifying its value, trees are not factored into the budget process. The size, shape, and location of a city's green infrastructure can be measured and the public utility functions they perform can be accurately calculated.

While both gray and green infrastructure are important in a city, communities that foster green infrastructure wherever possible are more livable, produce fewer pollutants, and are more cost effective to operate. However, balancing the gray with the green can be a serious challenge for a local government manager.

To establish a healthy balance of gray and green infrastructure, communities can now:

• Quantify the presence of green infrastructure and its function for air and water improvement.
• Once quantified, designate green infrastructure as a public utility (just as gray infrastructure is) in the budget process
• Establish a tree canopy goal or target as part of every development and management project to utilize its functional potential
• Adopt public policies, regulations, and incentives to increase and protect green infrastructure

With the advent of geographic information systems (GIS) that most cities currently use, staff can integrate the value of green infrastructure, as well as model the impacts of development scenarios into daily planning and management.

Using Satellite Imagery and GIS to Measure Infrastructure
While municipalities commonly use geographic information systems (GIS) to map and analyze their gray infrastructure, they typically do not integrate trees and other elements of the green infrastructure into their day-to-day planning and decision making processes. Reasons for this include 1) the lack of understanding of the ecological and economic value of trees and other environmental features, and 2) the absence of a means to readily use this information in commonly-used GIS systems.

This project addresses both of these impediments. Data documenting the environmental characteristics of trees is now available thanks to data provided by researchers from the U.S. Forest Service, the Natural Resources Conservation Service, the Environmental Protection Agency and Purdue University. This project creates and uses an accurate digital data layer--a green data layer that will fit easily within the city's GIS. Those working in planning, urban forestry and on related natural resource issues can now readily utilize this data to integrate green infrastructure into land use planning.

Two types of satellite imagery are useful for determining tree cover in cities. The Landsat satellite has been circling the earth since 1972 and therefore can provide a good view of the historic changes that have occurred. In the last few years, new satellites provide high resolution imagery, where individual trees with 6 foot crowns can be viewed. Landsat data is best used to understand change trends and to support general public policies. In contrast, high resolution satellite data is used to create a digital representation of a city's green infrastructure. This green data layer integrates well with other GIS data layers and is most useful for daily land use planning and management.