Urban Tree Canopy Assessment: Glossary

Active Sensor

An active sensor is a measuring instrument that generates a signal, transmits it to a target, and receives a reflected signal from the target. Information concerning the target is obtained by comparison of the received signal with the transmitted signal.  Radar systems used to track airplanes are an example of an active sensor. 

Automated Feature Extraction

The identification of geographic features and their outlines in remote-sensing imagery through postprocessing technology that enhances feature definition, often by increasing feature-to-background contrast or using pattern recognition software. (ESRI GIS Dictionary, http://support.esri.com)

Bottom-Up Canopy Assessments

Bottom-up approaches uses data collected on the ground, frequently a plot-based sampling scheme to measure tree canopy cover.  In this approach, the amount of tree canopy cover that falls within study plots is extrapolated and taken to represent the urban tree canopy cover as a whole.  This on-the-ground method may be most appropriate for very small communities, such as a homeowner’s association or a school district’s properties.  Plot data can be collected using the US Forest Service’s i-Tree tools and methods at www.itreetools.org. 

Chesapeake Bay Program (CBP)

The Chesapeake Bay Program (CBP) is a partnership between federal and state agencies, non-profit organizations, and academic institutions whose aim is to protect and restore the Chesapeake Bay.

Chesapeake Executive Council

A legislative body serving Maryland, Pennsylvania, and Virginia. The Executive Council establishes the policy direction for the restoration and protection of the Chesapeake Bay and its living resources. A series of Directives, Agreements and Amendments signed by the Executive Council set goals and guide policy for the Bay restoration.  The CEC consists of the Governors of Maryland, Pennsylvania, and Virginia, the Administrator of the U.S. Environmental Protection Agency, the Mayor of the District of Columbia and the Chair of the Chesapeake Bay Commission.

Color Infrared (CIR)

An example of multispectral data that includes part of the visible light spectrum as well as the near infrared.  CIR is especially useful for vegetation mapping.

Ecosystem Services

The benefits that people obtain from ecosystems.  These benefits may be environmental, social, or economic.  Examples of environmental outcomes include the protection of streams, reduced stormwater runoff, reduced ozone concentrations, and increased carbon sequestration.  Social outcomes may include improved human health, buffers for wind and noise, increased recreational opportunities, and neighborhood beautification.  Economic outcomes can include reduced heating and cooling costs and increased property values.

Existing UTC

Any piece of land in the city that was covered by UTC at the time of satellite data acquisition.

Forest Opportunity Spectrum (FOS)

The Forest Opportunity Spectrum provides a framework for organizing data, as well as for asking and answering urban forestry related questions.  This framework can assist decision-makers as they decide what their tree canopy goal will be and what actions they can take to achieve that goal. 

Forest Opportunity Types

FOS allows forest opportunity types to be user defined.  For example, Baltimore City identified six major FOS types:  regional forestry, riparian forests, large parks, abandoned industrial areas, neighborhood areas, and roads (which includes street trees).  Forest Opportunity Types are most often defined by and associated with specific issues identified by government agencies, non-profits, businesses, and community groups.  FOS types overlap in geography, goals, and ownership.

Geographic Information Systems (GIS)

Acronym for geographic information system. An integrated collection of computer software and data used to view and manage information about geographic places, analyze spatial relationships, and model spatial processes. A GIS provides a framework for gathering and organizing spatial data and related information so that it can be displayed and analyzed.  (ESRI GIS Dictionary, http://support.esri.com)

Geoprocessing

A GIS operation used to manipulate GIS data. A typical geoprocessing operation takes an input dataset, performs an operation on that dataset, and returns the result of the operation as an output dataset. Common geoprocessing operations include geographic feature overlay, feature selection and analysis, topology processing, raster processing, and data conversion. Geoprocessing allows for definition, management, and analysis of information used to form decisions.  (ESRI GIS Dictionary, http://support.esri.com)

IKONOS

A commercial satellite that collects high-resolution imagery at 1- and 4-meter resolution.  It offers multispectral (MS) and panchromatic (PAN) imagery. IKONOS launched on September 24, 1999, and provides imagery beginning January 1, 2000.  Imaging, Inc. distributes IKONOS imagery under the product name CARTERRA.

LIDAR

LIght Detection And Ranging sensors are active sensors that collect extremely detailed elevation data by way of a laser.  By emitting pulses from the laser, then sensing the time it takes for the pulse to return, the height of objects on the ground can be inferred.  A relative surface DEM generated from LIDAR data can greatly complement imagery when performing a UTC assessment as it allows for features that have similar spectral and textural properties, to be differentiated based on height.  LIDAR can be particularly useful in separating trees from shrubs and buildings from parking lots.

Manual Interpretation

Human interpretation is generally considered the most accurate method of extracting features from imagery, however it is extremely time consuming.

Multispectral Data

Data that spans several parts of the EM spectrum is referred to as multispectral data.  Color infrared (CIR) imagery is an example of multispectral data.  It displays light from part of the visible spectrum as well as near infrared (NIR). 

Near Infrared (NIR)

Having a NIR (near infrared) band can assist in distinguishing tree and vegetation types (broadleaf vs. conifer vs. grass), impervious surface types (concrete vs. asphalt), and other features (forests vs. forested wetlands).  NIR can also be used to assess vegetation condition.  This makes NIR data invaluable for natural resource management.

Nutrient Pollution

Although nutrients, such as nitrogen and phosphorus, are essential to all plant life within the Chesapeake Bay, an excess of these same nutrients can be harmful. This is called "nutrient pollution".  Excessive nutrient levels in aquatic systems can lead to harmful algal blooms, reduced sunlight for submerged aquatic vegetation, and low oxygen conditions that can kill fish and other aquatic life.  As the use of the land has changed and the watershed's population has grown, the amount of nutrients entering the Bay's water has increased tremendously leading to many environmental problems in the Bay and its tributaries. 

Passive sensor

Passive sensors record waves of electromagnetic (EM) energy that are either emitted or reflected from an object.

Possible UTC

Where is it biophysically feasible to plant trees?  This is the first step in the assessment process.  It is not concerned with costs, logistics or the fact that tree planting may not be appropriate or desirable in some locations.  For the Baltimore UTC assessment, all land that was not covered by water, a road, or a building was considered a “possible” planting location.

Potential UTC

Where is it economically likely to plant trees?  Which areas have regulatory constraints that conserve tree cover or have incentive supports for adding tree cover?  Which areas are most cost-effective for achieving water quality or other goals? 

Preferable UTC

Where is it socially desirable to plant trees?  For example, where will tree cover make neighborhoods more attractive?  Where will tree cover address other issues such as cooling and cleaning the air?

PROW (Public Right Of Way)

Any sidewalk, planting strip, alley, street, or pathway, improved or unimproved, that is dedicated to public use.  The term includes any strip of land over which public facilities such as highways, railroads, or power lines are built.

Radiometric Resolution

Radiometric Resolution is the number of brightness levels that the remote sensing technology can sense.  The higher the radiometric resolution, the better the sensor will be able to distinguish objects with similar spectral properties.  Most remote sensors, such as Landsat, yield 8-bit data (28) where each pixel has a possible value of 0-255.  Newer sensors are capable of collecting data at a much higher resolution.  For example, the IKONOS and QuickBird satellites gather 11-bit (211) data, allowing for improved feature recognition when compared to traditional 8-bit data.

Resolution

see Spatial Resolution, Radiometric Resolution, Temporal Resolution, and Spectral Coverage

Riparian Zone

This is the area of vegetation around streams.  In less urbanized systems, the riparian zone is extremely important for water quality.  This area of vegetation captures and processes pollutants before they can make it into surface waters.  In urban areas, however, riparian zones are often less effective at removing pollutants.  One reason is that urban streams tend to be deeply incised, causing the riparian zone to be disconnected from the stream below.  Secondly, the streams in many urban areas have been functionally replaced with storm sewers.

Smart Growth

This term has many definitions depending on the context.  According to the US Environmental Protection Agency:
Smart growth is development that serves the economy, the community, and the environment. It changes the terms of the development debate away from the traditional growth/no growth question to "how and where should new development be accommodated."  Smart Growth answers these questions by simultaneously achieving:

• Healthy communities -- that provide families with a clean environment. Smart growth balances development and environmental protection -- accommodating growth while preserving open space and critical habitat, reusing land, and protecting water supplies and air quality.
• Economic development and jobs -- that create business opportunities and improve local tax base; that provide neighborhood services and amenities; and that create economically competitive communities.
• Strong neighborhoods -- which provide a range of housing options giving people the opportunity to choose housing that best suits them. It maintains and enhances the value of existing neighborhoods and creates a sense of community.
• Transportation choices -- that give people the option to walk, ride a bike, take transit, or drive.

Spatial Resolution

Spatial Resolution is the “pixel size” associated with the data. For reference, it generally takes at least 4 pixels to identify a feature.  So, while Landsat imagery, with its 30 square-meter resolution, may be adequate for measuring large areas of intact forest it will do a poor job of identifying street trees in urban areas.  This is why the Chesapeake Bay Guidelines suggest a minimum resolution of one-meter or better.  As spatial resolution increases so does the storage size of the data.

Spectral Coverage

Spectral Coverage is another consideration for data acquisition.  Certain features and properties of land cover may be more distinguishable in different bands of the electromagnetic spectrum.  For instance, the inclusion of a NIR (near infrared) band is optimal for classifying vegetation data as the majority of EM energy reflected by vegetation is in the NIR portion of the spectrum. 

Strategic Urban Forests Assessment (SUFA):

UTC assessment process using high-resolution remote sensing imagery. A vegetation mask is created from the NIR-to-Red, (Band4:Band3) ratio image. A texture image of the resulting ratio image is produced to separate UTC vegetation from non-UTC vegetation pixels (separate trees from other vegetation). The resulting image provides for quantification of existing UTC and non-UTC vegetation.

Stormwater Runoff

Surface water that fails to infiltrate the soil after a rainstorm.  In developed watersheds it flows off roofs and pavement into storm drains which may feed directly into streams; Stormwater carries pollutants from urban areas directly into local waterways.  By slowing, intercepting, and treating rainfall, trees can help reduce the volume of pollution-carrying stormwater runoff.

STRATUM (Street Tree Resource Analysis Tool for Urban forest Managers)

STRATUM is a street tree management and analysis tool for urban forest managers that uses tree inventory data to quantify the dollar value of annual environmental and aesthetic benefits.  Using an existing inventory of street trees, this software allows managers to evaluate current benefits, costs, and management needs.

Temporal Resolution

Temporal Resolution represents the time frequency for the data.  This component of data quality recognizes that it is not just the image quality that matters, but also when the information was acquired.  The Chesapeake Bay Program Guidelines recommend that the data used in UTC assessment be less than five years old.  In some communities, where rapid change or development is taking place, a much higher temporal resolution may be required (i.e. data that is less than one year old) to accurately reflect the extent of current tree canopy. 

Three Ps

When moving from a canopy assessment to an implementation plan, it is useful to separate the process into a sequence of steps.  This allows the task to be broken into manageable components and prevents each step from being bogged-down by details that belong in later stages of the process.  The Three Ps, Possible, Potential, and Preferable, provide a useful sequence for structuring the goal setting and implementation process.  (See Possible, Potential, and Preferable for more information). 

Top-Down Canopy Assessments

Top-down approaches use remote sensing data, such as satellite imagery, to quantify the extent of tree cover.  For most communities, a top down approach is recommended.   This guide focuses on a top-down approach for several reasons.  First, the Chesapeake Bay Program guidelines are based on tree cover and extent which are readily assessed using top-down methodologies.  Second, percent cover is easy to conceptualize and communicate.  Third, remote sensing makes it easy to track progress over time.  Lastly, these methods are well documented and have been used successfully here and elsewhere.

Urban Forests

Urban forests include the trees in our yards, parks, public spaces, and along our streets.  Though we don’t often think of them as forests, they provide many forest benefits, such as cleaner air and water.  In addition to environmental benefits, urban forests increase property values, reduce home energy costs, block UV radiation, buffer wind and noise, provide shade and beautify our neighborhoods. 

Urban Sprawl

The unplanned, uncontrolled spreading of urban development into areas adjoining the edge of a city.

Urparian

Urparian describes the vegetated areas around roads and sidewalks.  The term comes from combining urban and riparian to form a single word.  In less urbanized systems, the corridor around streams (the riparian zone) is extremely important for water quality.  This area of vegetation captures and processes pollutants before they can make it into surface waters.  In urban areas, however, riparian zones are often less effective at removing pollutants.  One reason is that urban streams tend to be deeply incised, causing the riparian zone to be disconnected from the stream below.  Secondly, the streams in many urban areas have been functionally replaced with storm sewers.  In this context, the soil and vegetation around roads and sidewalks is the new riparian zone.  By increasing tree canopy in the urparian zone, we can return some of the environmental benefits of riparian areas to urban systems. 

Urban Tree Canopy (UTC)

Urban tree canopy (UTC) is the layer of leaves, branches, and stems of trees that cover the ground when viewed from above.

Watershed

This is the area that drains to a common waterway, such as a stream, lake, estuary, wetland, or the ocean.  The Chesapeake Bay Watershed stretches across six states and includes all of the areas that eventually drain into the Bay