DISCLAIMER

National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center staff obtained the majority of the data contained on this CD-ROM from a variety of local, state, and federal sources. Center staff converted these spatial data into a common data format and geographic referencing system for use in this project. Because the Center was not the originator of many of these databases, we are unable to guarantee the accuracy of their geographic features or attributes. Please see the metadata records for each database for complete information on the source, limitations, and proper use.

CONSTRAINTS OF SPATIAL DATA

Map Projections

Maps are flat representations of earth's curved surface. Locations on earth's three-dimensional surface are referenced using the geographic coordinate system (latitude and longitude). Mathematically transforming these three-dimensional coordinates onto a two-dimensional surface (paper or digital map) is called "projection". This process unavoidably distorts at least one the following properties: area, distance, shape, and direction. Because there are an infinite number of map projections, a single "best" projection for any given application does not exist. The error caused by projecting data has been calculated and minimized where possible. Although every effort has been made to ensure that all data sets on this CD-ROM are as accurate and correct as possible, project staff has no control over several external sources of error (cartographic interpretation, drafting errors, data conversions, precision, datum information, etc.).
 

Scale

Map scale specifies the amount of reduction between the real world and the graphic representation on a map. It is usually expressed graphically, as a fraction (1/20,000), a ratio (1:20,000), or equivalence (1mm = 20m). Since map scale is most often used to describe paper map products, it is often assumed that the scale is fixed, and cannot change. However, a map in a Geographic Information System (GIS) can be shrunk or enlarged on the screen by zooming in or out. This implies that geographic data in a GIS does not really have a true "map scale". 

When scale is used to describe digital data, it is often referring to the scale of the source data or the scale at which the digital data looks "right". As a result, this display scale influences the amount of detail that can be shown. Digital data viewed at inappropriate display scales within a GIS can be misleading. 

Map Resolution and Accuracy

Map resolution refers to the accuracy of the location and shape of a map feature shown at a given scale. In general, as map scale increases (e.g. 1:100,000 to 1:50,000 to 1:20,000), so do map resolution and accuracy. However, accuracy is also affected by the quality of source data used to map a feature.

Features on large-scale maps more closely represent the real world because the amount of reduction (from real world to map) is less. As the level of detail of a paper map increases for a given area of earth, the size of the paper map required to cover the same area also increases. Similarly, as digital map resolutions become more detailed and accurate, file sizes increase because more information is now represented for the same area. Producing high-resolution digital data for a large area of the earth results in very large files. Larger file sizes result in larger amounts of data to organize and manage. A fundamental challenge for a regional GIS is to provide data for a very large area at a scale detailed enough to enable resource managers to make sound decisions, yet not so detailed that the amount of data becomes too large to manage.

Multiple Data Sources and Problems with Undocumented Data

Because data for this CD-ROM were acquired from multiple sources in a variety of formats with varying accuracy standards and processing techniques (refer to the metadata records for specific information on data), some inconsistencies were encountered. Hence, users should be aware that data deficiencies and, in some cases, gaps exist.  If you have questions about specific data layers available from this site, or spatial data in general, please send an e-mail to csc.clearinghouse@noaa.gov.