College of Agriculture and Natural Resources
Center for Land Use Education and Research

What We're Measuring

 

Introduction

Given the importance of the public debate on development patterns and their impacts, it’s critical that studies like Connecticut’s Changing Landscape be clear about their methods and subsequent interpretations. This section of the website is offered as a quick, visual guide to help the viewer understand the basics of remote sensing, the limitations inherent in using Landsat imagery, and how our definition of “developed” land might differ from that used by other studies. Please see also The Project section for fact sheets and other information on proper uses of the data.

All images cover the same geographic area

80 meter pixel imagery
Landsat MSS

30 meter pixel imagery
Landsat TM/ETM

4 meter pixel imagery
IKONOS

0.25 foot pixel image
High resolution aerial

A Quick Remote Sensing Primer

Remote sensing is the art and science of detecting, identifying, classifying, and analyzing the earth’s surface using special sensors onboard aircraft and satellites. The image below is a Landsat TM image of the greater Connecticut area (Long Island Sound is the dark area near the bottom).

TM Scene with CT

All objects reflect energy (light) differently, giving them a unique “spectral” signature. As sunlight strikes an object (buildings, trees, water, etc.), the energy that is reflected is collected by sensors aboard aircrafts or satellites. This creates both a database and a remotely sensed image.

The height of each bar indicates the amount of light reflected for that color. The color of highest reflectance is what we see.

Understanding 30-meter Resolution

Resolution is the smallest possible level of detail of a given sensor (for you cartographers, the “minimum mapping unit.”) No feature on our land cover maps can be smaller than our minimum unit, which is 30 meters – that’s about 10,000 square feet, or ¼ acre. Thus, small features are often missed, and each square, or pixel, represents our best estimate as to the predominant land cover of that pixel.

To the right is a high resolution aerial photograph, with a 30 meter resolution grid superimposed. Pixel #1 is a homogenous pixel almost covered with trees, and would likely be interpreted by our project as a forested pixel. Pixel #2 is a “mixed pixel” with both vegetation and a large house; because the house covers a good portion of the pixel, this one would likely be interpreted as a “developed” pixel. Pixel #3, however, has a smaller structure, and might be interpreted as either “forest” or “developed,” depending on how the computer interprets the data.

Images to Land Cover

Computer programs and human expertise then turn the database/image into a land cover map. Land cover is what’s on the surface of the earth (forest), as opposed to land use, which is what is planned, practiced or permitted (park, wildlife sanctuary).

While the image itself is useful and instructive, it is still a picture. The resultant land cover data, however, can be quantified to show overall land cover patterns. To the right are examples from Hartford.

 

Satellite Image (Hartford)

 

Land Cover Map
(Hartford)

The pictures below summarize the process.

We have zoomed way in on a Landsat TM image (#1) so that each individual pixel is visible. Each pixel covers a 30 meter (100 foot) square on the ground.

1

 
 
 

For illustration purposes, a high resolution image (#2) is shown of the same area overlain with a 30m grid. The grid represents the size of the pixels in the Landsat image (#1).

2

 
 
 

Computer algorithms are used to classify each pixel into a land cover class based on the most prevalent spectral signature (#3).

3

 
 
 

The result is a land cover map (#4).

4

 
What we Mean by Developed

Terms like “urban area,” “developed land” and “development” can mean different things to different people. We want to be sure that you understand what our data shows, because the “developed area” estimated by our remote sensing techniques may be very different from the “developed area” as shown in your town comprehensive plan.

Below are two maps of Marlborough, CT. The left map shows a fairly typical town map of land use/land cover, interpreted from aerial photographs. The first four colors (orange, red, black and pink) could be called “developed” areas. The map on the right is our satellite-derive land cover map. The red and yellow areas constitute the "developed” portion of our map. While you can see that both maps are pretty similar, the town map shows more developed area. This is because of the different techniques used.

Land Use/Land Cover Map
derived from aerial photographs

Land Cover Map
derived from satellite images

Pixels vs. Parcels

Below are four different maps of a 117 lot residential area in Marlborough, CT. Each accurately measures “developed” land correctly, but using different techniques that arrive at very different results. Building footprints and roads, except in the “Planimetric” map to the left, are used for reference only.

"Planimetric" map

Remote sensing derived land cover map

10%
developed

30%
developed

Technique: Created from digitizing aerial photographs, showing outlines of houses, driveways, roads and outbuildings.

Technique: Land cover depicted as 30 meter (~100 ft per side) pixels representing predominant reflective signature of each pixel.

Possible uses: An accurate way of determining the amount of impervious surfaces for runoff modeling, estimating impacts on water resources.

Possible uses: Good for town, regional or statewide planning purposes. Generally shows what is on the land within the limits of the resolution of the data.

Generalized land cover map

Parcel map

82%
developed

100%
developed

Technique: Developed area drawn by hand around developed areas as “eyeballed” from planimetric, aerial photo, ground survey or other type of image.

Technique: Parcel (property boundary) data depicted as “developed” if a structure is on it or if it can not be used for residential use (e.g. it’s all wetlands, open space set aside, etc.)

Possible uses: Common type of map seen in town plans. Good for general planning purposes but highly subjective.

Possible uses: Shows land that is currently or potentially committed to developed uses. Good for build-out analyses or to identify developed vs. developable land.

Land Cover and Change Categories

Land Cover Category Descriptions

 

Class

Description

  Developed High-density built-up areas typically associated with commercial, industrial and residential activities and transportation routes. These areas can be expected to contain a significant amount of impervious surfaces, roofs, roads, and other concrete and asphalt surfaces.
  Turf & Grass A compound category of undifferentiated maintained grasses associated mostly with developed areas. This class contains cultivated lawns typical of residential neighborhoods, parks, cemeteries, golf courses, turf farms, and other maintained grassy areas. Also includes some agricultural fields due to similar spectral reflectance properties.
  Other Grasses Includes non-maintained grassy areas commonly found along transportation routes and other developed areas, and within and surrounding airport properties. Also likely to include forested clear-cut areas, and some abandoned agricultural areas that appear to be undergoing conversion to woody scrub and shrub cover.
  Agricultural Field Includes areas that are under agricultural uses such as crop production and/or active pasture. Also likely to include some abandoned agricultural areas that have not undergone conversion to woody vegetation.
  Deciduous Forest Includes southern New England mixed hardwood forests. Also includes scrub areas characterized by patches of dense woody vegetation. May include isolated low density residential areas.
  Coniferous Forest Includes southern New England mixed softwood forests. May include isolated low density residential areas.
  Water Open water bodies and watercourses with relatively deep water.
  Non-forested Wetland Includes areas that predominately are wet throughout most of the year and that have a detectable vegetative cover (therefore not open water). Also includes some small water courses due to spectral characteristics of mixed pixels that include both water and vegetation.
  Forested Wetland Includes areas depicted as wetland, but with forested cover. Also includes some small water courses due to spectral characteristics of mixed pixels that include both water and vegetation.
  Tidal Wetland Emergent wetlands, wet throughout most of the year, with distinctive marsh vegetation and located in areas influenced by tidal change.
  Barren Mostly non-agricultural areas free from vegetation, such as sand, sand and gravel operations, bare exposed rock, mines, and quarries. Also includes some urban areas where the composition of construction materials spectrally resembles more natural materials. Also includes some bare soil agricultural fields.
  Utility Rights-of-way (Forest) Includes utility rights-of-way. This category was manually digitized on-screen from rights-of-way visible in the Landsat satellite imagery. The class was digitized within the deciduous and coniferous categories only.

Change To Category Descriptions

  Class Description
No Change Classes
  Developed before 1985 Areas (pixels) developed before the 1985 satellite image was captured.
  Turf & Grass Areas (pixels) that were turf and grass before the 1985 satellite image was captured.
  Water Any pixel that was water in at least one of the four land cover maps.
  Undeveloped Pixels still undeveloped* on the 2002 Land Cover Map.
Change Classes Change from Undeveloped to . . .
  Developed 1985-1990 Areas (pixels) undeveloped* in 1985 and developed in 1990.
  Turf & Grass 1985-1990 Areas (pixels) undeveloped* in 1985 and turf and grass in 1990.
  Developed 1990-1995 Areas (pixels) undeveloped* in 1990 and developed in 1995.
  Turf & Grass 1990-1995 Areas (pixels) undeveloped* in 1990 and turf and grass in 1995.
  Developed 1995-2002 Areas (pixels) undeveloped* in 1995 and developed in 2002.
  Turf & Grass 1995-2002 Areas (pixels) undeveloped* in 1995 and turf and grass in 2002.
  Developed 2002-2006 Areas (pixels) undeveloped* in 2002 and developed in 2006.
  Turf & Grass 2002-2006 Areas (pixels) undeveloped* in 2002 and turf and grass in 2006.
*Here, undeveloped includes deciduous forest, coniferous forest, non-forested wetlands, forested wetlands, tidal wetlands, barren, and utility rights-of-way.

Change From Category Descriptions

  Class Description
No Change Classes
  Developed before 1985 Areas (pixels) developed before the 1985 satellite image was captured.
  Turf & Grass Areas (pixels) that were turf and grass before the 1985 satellite image was captured.
  Water Any pixel that was water in at least one of the four land cover maps.
  Undeveloped/Other Pixels still undeveloped* on the 2002 Land Cover Map.
Change Classes Change from Undeveloped to . . .
  Agricultural Field to Developed Areas (pixels) that were Agricultural Field in 1985 and developed in 2006.
  Agricultural Field to Turf & Grass Areas (pixels) Agricultural Field in 1985 and turf and grass in 2006.
  Forest to Developed Areas (pixels) Forest* in 1985 and developed in 2006.
  Forest to Turf & Grass Areas (pixels) Forest* in 1985 and turf and grass in 2006.
  Other Classes to Developed Areas (pixels) belonging to other undeveloped classes** in 1985 and developed in 2006.
  Other Classes to Turf & Grass Areas (pixels) belonging to other undeveloped classes** in 1985 and turf and grass in 2006.
* Forest includes deciduous forest, coniferous forest, forested wetlands and utility rights-of-way. * Other undeveloped classes includes other grasses, non-forested wetland, tidal wetlands and barren.