Coastal Services Center

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C-CAP Data FAQs

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Note that glossary terms on this page will be followed by a icon, which is linked to a definition of the term.

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What land cover classes does C-CAP document?

Each C-CAP land cover data base has up to 22 standard classes. These classes are briefly described below. Detailed information on C-CAP classes can be found at NOAA C-CAP: Guidance for Regional Implementation.

High Intensity Developed – Urban land cover with greater than 75 percent impervious surface
Low Intensity Developed – Urban land cover with greater than 25 percent and less than 75 percent impervious surface
Cultivated land – Active agriculture, orchards, and vineyards
Grassland – Both managed and unmanaged grasslands
Deciduous Forest – Hardwood forest with a pronounced seasonal dormancy period
Evergreen Forest – Forest without a pronounced seasonal dormancy period
Mixed Forest – Forest not dominated by either deciduous or evergreen species
Scrub/Shrub – Woody vegetation less than 20 feet tall
Palustrine Forest – Freshwater wetland forest
Palustrine Scrub/Shrub – Freshwater wetland scrub/shrub
Palustrine Emergent – Freshwater wetland-rooted emergent species (marsh, lilies, etc.)
Estuarine Forest – Saltwater wetland forest greater than 20 feet (mangrove)
Estuarine Scrub/Shrub – Saltwater wetland scrub/shrub (mangrove)
Estuarine Emergent – Saltwater wetland emergent species (Spartina marsh, juncus grass, etc.)
Unconsolidated Shore – Tidal flats, shoals, and intertidal areas
Bare Land – Bare exposed rock, sand, and soil
Water – Open water
Palustrine Aquatic Bed – Floating vegetation and algal communities
Estuarine Aquatic Bed – Marine algal communities
Tundra – Permafrost and pariglacial conditions and communities
Snow/Ice – Perennial snow and ice

For the technical descriptions of the 22 standard C-CAP classes see the "Classification Scheme."

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What is the difference between land use and land cover?

Land cover is the natural landscape recorded as surface components: forest, water, wetlands, urban, etc. Land cover can be documented by analyzing spectral signatures of satellite and aerial imagery.

Land use is the documentation of human uses of the landscape: residential, commercial, agricultural, etc. Land use can be inferred but not explicitly derived from satellite and aerial imagery. There is no spectral basis for land use determination in satellite imagery.

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What formats are C-CAP data distributed in?

Currently, C-CAP data sets are available in ERDAS^® Imagine format (.img). More data formats may be available at a later date. All C-CAP data will be downloadable from the Web. See the "Online Data Access" section of this Web site for more information.

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What map projection and datum are C-CAP data distributed in?

C-CAP data, provided on the Web, are distributed in Universal Transverse Mercator (UTM), North American Datum 1983 (NAD83), meters. The reason for the use of a Mercator-based projection is that raster data are not well represented in Geographic Latitude/Longitude (GLL) projection. GLL is a non-Cartesian coordinate system, meaning that the pixel size will change over space, becoming more extreme as you approach the poles. Since C-CAP images cover approximately 100 miles x 100 miles (more than 1 x 1 degree lat/long), the pixel size would have to change within one project area, causing distortion. This situation causes some serious problems for image processing. For this reason, a Mercator projection is used to maintain a constant pixel size.

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How do I use C-CAP data in ArcView?

Through surveys, the Center has found that a majority of its customers use ESRI^® ArcView; as a result, the Center facilitates data use through ESRI products. To this end, the Center has developed the C-CAP Data Handler extension for ArcView. Users will be able to download data from this Web site and use the Data Handler to import and manipulate the data. This extension requires ESRI Spatial Analyst^® extension and provides a menu-driven set of tools to accomplish the following:

• import C-CAP data and convert to GRID
• apply the standard C-CAP legend
• clip grid to a shapefile or buffer
• generate statistical change table
• analyze groups of related pixels
• compare land cover changes in specific classes

A user manual for the Data Handler extension will also be developed. These tools are available for download. For more information about the C-CAP Data Handler, see the "ArcView Extensions" Web page.

The Center recognizes that some users may not have access to ESRI's Spatial Analyst extension. These users can still view the data in ArcView, although they will not be able to see a legend. To view data in ArcView, select File, Extension, Imagine Support and then add your image to the view. In order to enhance functionality to non-Spatial Analyst users, the Center is developing a C-CAP Legend Handler extension. The Legend Handler will be available for download in fall 2001. For more information about the Legend Handler, see the "ArcView Extensions" Web page.

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Why does the data only cover a portion of my state?

The C-CAP program maps coastal areas that fall within Estuarine Drainage Area (EDA) boundaries, which are consistent with the USGS Hydrologic Unit Codes (HUC) . Each 10,000 square mile satellite image that falls within an EDA is evaluated, statistically analyzed, and then heavily field verified.

For more information on Hydrologic Unit Codes see the USGS Water Resources Web site. For more information on EDAs, see the NOAA Ocean Resource Conservation and Assessment's CAF Web site.

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What does accuracy mean in relation to the data?

Accuracy is a binary measure of correct or incorrect for a given observation. However, we can make statistical statements that something is accurate X percent of the time. It is common in remote sensing to apply this concept to derived products. To ensure the accuracy of the land cover data, Center staff conduct field verification of the data. In areas where the landscape is changing from one category to another, such as when a field gradually becomes scrub/shrub and eventually a forest, this process can be difficult.

For the purpose of land cover mapping, a minimum mapping unit is defined and the data are evaluated on the dominant vegetative cover for that area and compared to the digital map. An area is assessed as "accurate," or correct, if the dominant land cover for the area was correctly identified from the satellite image. Landsat data have a 30-meter pixel size, which means that the predominant land cover within that 30 meters (.25 acres) will be represented by one pixel. However, the mapping unit for C-CAP data is one acre, or 4 Landsat Thematic Mapper (TM) pixels. This reflects the need for 4 pixels to identify any given feature reliably.

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What is the difference in accuracy between a single pixel and a clump of pixels (i.e., how accurate are the data)?

The concept of accuracy in digital satellite image mapping is not as simple as it first appears. Since a pixel is representative of an area of ground, usually a square, the light reflected from the entire area is averaged to get one pixel value though it may contain many different materials. When the entire area is covered in a single material or vegetation cover, identification of the material is fairly straightforward. However, when a pixel falls on two or more materials, this causes problems for identification. These areas are called "mixed pixels."

For this reason, accuracy assessment is usually performed on homogeneous groups, or neighborhoods of pixels, or clumps. By identifying the clumps and field checking them, we can get a measure, statistical and empirical, of the accuracy and "fitness for use" of the data. This means that a given accuracy figure corresponds to the data's ability to represent homogeneous, or spectrally pure, cover types. The mixed pixels are identified by which material is dominant and how similar it is to other materials. This is one of the largest sources of error in land cover mapping.

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I found an error: Does this mean that all the data for this region are incorrect?

No, it does not mean all the data are incorrect. C-CAP data are developed from Landsat Thematic Mapper (TM) digital satellite imagery. Landsat imagery have a base scale of 1:100,000 for mapping applications. Maps of 1:100,000 scale and smaller, or larger areas, are used for regional analyses, but are not appropriate for identifying individual species, or for permitting-type applications. They are more suited to land cover and change analysis on a regional scale, and zoning and planning applications. They should not be evaluated at the single pixel level, which is below the minimum mapping unit of 4 pixels. The data are appropriate for capturing regional trends and changes even if single pixels are wrong.

These data should be used just as any other 1:100,000 map is used. Road maps at this scale are highly generalized, and if you tried to drive your car on the delineated roads you would quickly find yourself in a ditch or tree. This does not mean that the data are not useful, but error is inherent in remotely sensed land cover. Users should understand all data assumptions prior to conducting their analyses.

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How can I report errors?

Some amount of error is inherent in mapping land cover from satellite imagery. The expected 85 percent accuracy also means 15 percent inaccuracy. However, every logical effort is made to ensure the data are accurate. If you find problems, you can report them to the Center's Clearinghouse. Unless the errors are critical to the "fitness for use" of the product, the identified errors will be tagged to the metadata records and the edits will be made on the next change detection analysis (CDA). Requests to fix the errors prior to the next CDA can be made at the Center's Clearinghouse and will be evaluated on a case by case basis by the Coastal Remote Sensing Program management.

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