By Emily Wilson
Originally published by the UConn Center for Land Use Education and Research
With funding from CT DEEP, CLEAR has acquired and made available on CT ECO a new statewide, high-resolution, impervious cover data layer. While acquired to support new stormwater regulations, the layer can be used for other purposes as well.
What is it?
Statewide, 1 foot resolution raster (pixel) data where each pixel is one of three classes (buildings, roads and other impervious).
Why do we have it?
The 2017 Municipal Separate Storm Sewer System (MS4) General Permit regulation requires certain towns and institutions to calculate directly connected impervious area. To assist communities in meeting this requirement, CT DEEP funded the acquisition of the a statewide impervious layer (based on 2012 imagery) that may be useful in calculating directly connected impervious area and tracking disconnects of impervious cover.
How was it created?
A company called Quantum Spatial did the work. They used 2012 statewide aerial imagery which has 1 foot pixels and classified it which means identifying all pixels in the imagery that represent buildings, roads and other impervious. The rest of the pixels were excluded as they were not impervious land cover. In some places, towns and/or regional governments contributed detailed GIS data that was incorporated into the layer.
How do you get it?
The CT ECO website has a whole section devoted to the Connecticut MS4 Supporting Layers which includes the impervious surface data.
Connect. GIS users can connect to the map services of impervious surface. Three flavors are available.
The original raster data is available as map services in two different projections. One is in Web Mercator Auxiliary Sphere (service called Impervious_2012) which is best for online mapping and web viewers. The other is in Connecticut State Plane NAD83 Feet which is better for desktop GIS mapping when other layers are also in the Connecticut State Plane coordinate system (service called Impervious_2012_StatePlane). The smoothed vector version (see formats section below) is also a map service in Connecticut State Plane NAD83 feet (service called Impervious2012_simplified_vector_StatePlane).
Download. GIS users can download the files. Formats available described below.
What are the different formats?
The impervious surface data is available in several different flavors that all originated from the same base.
Raster. The raster format is the original. Download by town (extended area*).
Vector Original. The vector format was created by taking the raster layer and converting it to polygons instead of pixels. Polygons have area. Here, the polygon edges are still jagged because they originated from pixels. The outlines are shown as an example. Download by town in a geodatabase contain a clip of just the town boundary and one of the town extended area*.
Vector Smoothed. Through a fortunate turn of events, there is also a smoothed version of the vector. Here, the jagged vectors have been smoothed through geoprocessing methods. Download statewide layers for buildings, roads and other impervious. Each is in a separate file.
* Extended area refers to a rectangular area larger than the town (detailed explanation here).