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

Frequently Asked Questions


Q. What is the Connecticut Land Cover Map Series?

A. The Connecticut Land Cover Map Series (Version 2) consists of five dates of land cover data (1985, 1990, 1995, 2002 and 2006) created from satellite imagery. Each dataset includes twelve consistently interpreted land cover categories. The data were produced in a way to insure easy comparison, especially for land cover change studies. The land cover data are available in digital format for use in Geographic Information Systems and can be used to produce a wide variety of map products.


Q. Can I mix and match Version 1 and Version 2 land cover?

A. No! Each series was created to be used together. Every date of the Version 1 series is different from every date of the Version 2 series.


Q. How does the Agricultural Field class compare to other estimates of Agricultural areas, such as the NRI?

A. Our Agricultural Field category includes areas that are under agricultural uses such as crop production and/or active pasture. It is also likely to include some abandoned agricultural areas that have not undergone conversion to woody vegetation.  The USDA NRCS “Natural Resources Inventory” is similar, in that is based on Landat imagery, but since the NRI is a national survey it is created by statistical sampling, with a larger likelihood of error than CLEAR’s processing of each and every pixel.  CCLV2 and NRI data are nonetheless fairly compatible, if you make allowances for differences in how the major land cover categories are defined.  The USDA Agricultural Census data is collected via field office reports, mostly gathered via survey of farmers, and thus is not comparable to our research methods.  Also, these data include all land included on farmsteads, not only agricultural fields but also forest, developed lands, and other land uses; thus it predictably quotes larger acreage statistics for comparable time periods.


Q. How is the Land Cover Map Series different from the previously created 1990 and 1995 land use land cover data?

A. The previously developed 1990 and 1995 land use land cover data also were created at the University of Connecticut from Landsat satellite imagery (the 1995 data also incorporated SPOT imagery - a French satellite). However, the purpose of these earlier versions primarily was for modeling nutrient inputs to Long Island Sound. For both datasets efforts were made, with mixed success, to infer land use (e. g. rural residential, commercial, industrial, etc.). The 1990 land use land cover dataset, which used 25 land use land cover classes, was the first statewide land use land cover dataset created by UConn researchers. Improved technologies and a different approach were used to create the 1995 land use land cover data and the classification was expanded to 28 categories. These earlier land use land cover data are very different from the new Land Cover Map Series and should not be compared to the new data.

Back to Top


Q. How can I view the land cover maps?

A. Beyond viewing static maps on the Statewide page and Your Town and Your Watershed pages, there are three ways on the web site to view the actual maps:

  • PDF files on the Your pages for 1985, 2006 and the change maps,
  • The online interactive map,
  • Data download for a GIS.

Each way has different features outlined in the table below. Click the title to jump to a choice.

 

PDFs

Interactive Map

Download Data

Software Required

Adobe Reader*

Web Browser (internet explorer, firefox, etc.)

GIS Software

GIS Expertise Required

None

None

Some

Allows for Zooming

To some degree

Yes

Yes, in GIS software after download

Allows for Printing

Yes, although format is for 8.5 x 11 paper

Yes, prints the view allowing for smaller geographic regions

Yes, within GIS software

Data can be viewed with your own GIS data

No

If you connect to the service using a GIS

Yes

Geographic Coordinates maintained

No

Yes

Yes

Ability to turn on/off and compare maps

No. Multiple pdfs can be opened in different browsers, but not easily compared

Yes

Yes in a GIS

* Adobe reader can be downloaded for free at http://www.adobe.com/products/acrobat/readstep2.html.

Back to Top


Q. Two change maps? I'm confused. What are the two change maps showing?

The purpose of these maps is to show areas that went from one of our “undeveloped” land cover classes (Other Grasses, Agricultural Field, Deciduous Forest, Coniferous Forest, Non-forested wetland, Tidal Wetland, etc.) to the “developed footprint” classes of Developed or Turf/Grass. Turf and grass is included because, most of the time, it is closely associated with development. 

In the Your Town and Your Watershed section, we make available two types of change maps.  Both maps show the same areas, as defined above, that have changed from undeveloped to developed during the 1985 – 2006 period.  However,the different color schemes of the two maps show slightly different aspects of this new development.

Change to Developed ExampleIn the case of the Change to Developed Map, the color scheme is based on which of our four study time periods the land cover change occurred.  Thus, changes occurring from 1985 – 1990 are colored orange, 1990 to 1995 changes are in pink, and so on).  This allows the viewer to get a feel for the phasing of new development over time. 

In the case of the Change From Map, the color scheme is based on what land cover class has been converted into these developed areas.  Thus, green areas were formerly forest, brown areas agricultural fields, etc.  This allows the viewer to get a feel for which type of undeveloped areas were converted, and how much.

A more detailed description of the categories of both change maps can be found in the About the Project section.


 

Q. How do you calculate change?

A. In this kind of research, there are three ways to measure change as described in the table below. The percent change reported in the data tables the Your Town and Your Basin page is the Relative rate of change. This means that if there is a small area to begin with (little area in time 1, or 1985), then the relative rate is likely to be high (or high percentage). If there is a large area to being with (lots of area in time 1, or 1985), then the relative rate is likely to be low (a small percentage).

T stands for a time period, so T1 would be an earlier date than T2. In most data tables on this website, T1 is 1985 and T2 is 2006.

Measure of land cover change

Calculation

Significance

Absolute change

(acres T2 – acres T1)

Allows aggregation of total areal change across the same geographic areas (such as towns or basins)

Relative change
(noted as "change in percent")

(% area T2 - % area T1)

Allows comparison between areas (such as towns or basin); relates to land cover indicators

Relative rate of change
(noted as "percent change")

(area T1 - area T2) / area T1

Gives feel for how quickly land cover is changing relative to 1985 baseline, within and between the same geographic area

 


Q. Why can't I see my house on the land cover maps?

A. The land cover maps were produced by interpreting Landsat satellite images, which have a ground resolution of 30 meters or approximately 100 feet. At this resolution, the satellite sensor "sees" areas that are about a quarter of an acre in size. Within any quarter acre, there may be a number of different landscape features and what the satellite ends up "seeing" is the largest feature or the largest group of similar features. Chances are your house, especially in forested rural areas, will be "dwarfed" by the surrounding trees. This will cause the area to be classified as forest rather than developed. However, if your house is in a neighborhood where houses, driveways, sidewalks, etc. are the predominant landscape features, then it will fall into a "developed" land cover class. Thus, what gets mapped depends on what the predominant landscape features are.

Back to Top


Q. How were the land cover data created?

A. The land cover data were produced from Landsat satellite images using a computer application called image processing. Landsat images are made up of millions of small squares called pixels. Each pixel represents an area on the ground that measures 30 meters by 30 meters. For each pixel, the Landsat image records the amount of reflected energy in 6 narrow bands of the electromagnetic spectrum (red, green and blue visible light, a near-infrared and two mid-infrared bands). Because landscape features reflect light differently, we can use reflectance data to identify areas of deciduous forest, coniferous forest, water, etc. This is where the image processing software is so helpful. Rather than you or me having to analyze data for each pixel, the software can do it much faster, and with the help of image analysts, can assign pixels to land cover classes based on differences and/or similarities in reflectance values. For details about image processing methods used, visit The Project page.

Back to Top


Q. Is the land cover data correct?

A. Of course we'd like to think that the datasets are error free but the reality is that there are misclassification errors. A great deal of time was spent to avoid and correct errors. A variety of ancillary data sources such as USGS topographic maps and orthophotos, were used to aid the classification process. An accuracy assessment is under way so that we can quantitatively assess the accuracy of each land cover map. However, as of December 2008, we have not yet completed the accuracy assessment.

Back to Top


Q. Why not use more detailed satellite imagery to make more detailed maps?

A. There are several reasons why we chose Landsat imagery for the Land Cover Project.

  1. A Landsat image is about 185 kilometers on each side and one image covers almost the entire state. One image, acquired on a cloud free day, would provide the bulk of the data for each of the five dates.
  2. Landsat imagery is relatively inexpensive or free.
  3. 30-meter resolution Landsat imagery was first collected in 1982 making it possible to create historic land cover data for change studies.

High resolution datasets were not used for the regional analysis for these reasons:

  • Unmanageable file size.

 

Coverage
(sq. miles)

File Size

Number of scenes to cover 13,000 sq. miles

File size of images of cover 13,000 sq. miles

Processing Time

Landsat Scene

~13,000

365Mb

1

365Mb

Manageable

Quickbird* Scene

~100

326Mb

136

~43Gb

Unmanageable

*Quickbird is a high resolution satellite with 2.4 meter pixels (Landsat has 30 meter pixels)

  • Too many scenes. Images captured from high resolution satellites have much smaller footprints (area covered on the earth) then Landsat images. It would be difficult to create a seamless dataset from so many images.

  • Limited historical archive. Commercial high resolution imagery has only been available since the late 1990's. The limited archive would prohibit meeting the project objective - to derive a time series of land cover over a 20+ year period.

  • Likelihood of seasonal and temporal variability. Because so many scenes would be required to cover the study area, it is likely that images would be captured on many different days, introducing artifacts such as variations in vegetation phenology and atmospheric effects (haze).

  • Classification Methodology. Classification techniques applied to the Landsat images in this project do not work well on high resolution imagery. New techniques and software would need to be used. Currently, the software does not adequately handle extremely large datasets.

  • High Cost. High resolution satellites are operated commercially and not by the government, they are expensive, especially for the huge number of images necessary to cover the area.

  • Project Objectives. The objective of the project was to create a regional assessment. Detail available in high resolution images actually makes it more difficult to provide regional information.

A Landsat image (30 meter resolution ) of the UConn campus (left) and a Quickbird satellite image (2.4 meter resolution) of the same area.

Back to Top


Q. Why doesn't the data go back farther in time?

A. The first Landsat satellite was launched in 1972. However, it acquired imagery at 80-meter resolution, which is too coarse for this project, and in different bands of the electro-magnetic spectrum. Since we wanted to create land cover data sets that were comparable, we need to use spatially and spectrally consistent imagery. The first Landsat satellite to acquire 30-meter resolution imagery was launched in 1982.

Back to Top


Q. Does the land cover data show land use?

A. No. With Landsat imagery it is possible to consistently and accurately determine what is on the land's surface but it's not possible to determine how the land is being used. For example, at 30-meter resolution, a group of homes, a group of farm buildings or a group of small offices may reflect light similarly. It's possible to classify the group as developed land but it's difficult, if not impossible, to determine reliably how the buildings are being used.

Back to Top


Q. Does the land cover show all increases in developed land?

A. The land cover data certainly show an increase in developed land between any two time periods. However, it is likely that isolated or small development, especially within forested areas, may be missed due to the spatial resolution of the Landsat imagery. Therefore, we consider this to be a conservative measure of development. It's also worth mentioning that for the developed land cover class, the land cover data only show the actual developed potion of any particular site. In other words, if a 100 acre parcel is developed as a business park, only that portion that includes buildings, parking lots, roads etc. will be depicted as developed. Any vacant portion of such a site would be classified based on the predominant land cover - forest, non-forested wetland, turf & grass, etc.

Back to Top


Q. Where can I get a copy of the land cover data?

A. Any of the land cover data can be downloaded from the download portion of this website as Imagine raster files. All the data are in Connecticut State Plane Coordinates, NAD 83 and are in units of feet. To help CLEAR researchers understand how the land cover data are being used and to notify users via e-mail of new derived data products, we are requesting that you register prior to downloading the data. A simple easy-to-complete on-line form is provided for this purpose.

More questions? Email us.

Back to Top