Glossary


GLOSSARY OF TERMS

Accuracy - In mapping, the reduction of positional and attribute errors based on information sources and data input instruments. Data precision must reflect, not exceed its accuracy.

Attribute - Descriptive characteristic or quality of a feature. Attributes may be represented by locational or non-locational descriptive information about a feature.

Base Flood Elevation (BFE) - The elevation associated with the flood having a one-percent annual chance of being equalled or exceeded in any given year. It is shown on the Flood Insurance Rate Map.

Base Map - Map of the community that depicts cultural features (roads, railroad, bridges, dams, culverts, etc.), drainage features, and the corporate limits.

Buffer Zone - An area of specified distance (radius) around a map item or items.

Compact Disk Read Only Memory (CD-ROM) - A digital medium for the storage of data in a standardized format (ISO 9660).

Coastal Barrier Resources Act (COBRA) - An act of Congress, signed into law in 1982 and amended in 1990 and later, that identifies units of land consisting of undeveloped coastal barriers and other areas located on the coast of the U.S. Flood insurance is not available for structures built after the coastal barrier was identified.

Community Identification (CID) - A unique six-digit number assigned to each community by FEMA and used for identity in computer databases; it is shown on the FIS, FIRM, and in the Q3 Flood Data files. The first two digits of the number are always the State FIPS code.

Community Rating System (CRS) - A program created by FEMA to provide new incentive for activities that reduce flood losses and support the sale of flood insurance. Any community participating in the NFIP may apply for CRS classification by demonstrating that it is implementing floodplain management and public information activities that exceed the minimum requirements of the NFIP. Once qualified, the community benefits by obtaining flood insurance premium rate credits for its residents. The credits vary by the level of activities undertaken by the community.

Database - A collection of information related by a common fact or purpose.

Data Capture - Series of operations required to encode data in a computer-readable form (digitizing).

Data Layer - Refers to data having similar characteristics being contained in the same plane or overlay (e.g., roads, rivers). Usually information contained in a data layer is related and is designed to be used with other layers.

Data Set or Data File - A named collection of logically related data records arranged in a prescribed manner. The physical set of data of one data type being referred to or being used in the context of a data processing operation.

Digital Data - Data displayed, recorded, or stored in binary notation.

Digital Flood Insurance Rate Map - (DFIRM) - The Digital Flood Insurance Rate Map (DFIRM) is comprised of all digital data required to create the hardcopy FIRM. This includes base map information, graphics, text, shading, and other geographic and graphic data required to create the final hardcopy FIRM product to FEMA standards and specifications. These data serve the purpose of map design and provides the database from which the Digital Line Graph thematic product of the flood risks can be extracted to create the DFIRM-DLG. These products are generally produced in a countywide format. DFIRMs are subjected to community review and approval and are, therefore, the official basis for implementing the regulations and requirements of the NFIP within the community.

Digital Flood Insurance Rate Map -DLG (DFIRM - DLG) - This product is created by extracting the flood risk thematic data from the DFIRM. The format of this product is the U.S. Geological Survey Digital Line Graph Level 3 Optional format, as described in the FEMA specifications for digital FIRMs. The DFIRM-DLG does not include base map information, nor does it include graphic data required to create a hardcopy FIRM. This product is intended to be the primary means of transferring flood risk data depicted by FIRMs to GISs through a public domain data exchange format. The DFIRM-DLGs are tiled to the U.S. Geological Survey 1:24,000 scale topographic map series.

Digital Line Graph (DLG) - A computer file format for mapping data that provides a topological structure to describe points, lines and polygons. A DLG may contain lists of point coordinates describing boundaries, drainage lines, and other linear or area features, which are organized by USGS quadrangle areas. These data are the digital equivalent of the linear hydrographic and cultural data on a topographic base map. The flood risk thematic layers developed by FEMA will fit the quadrangle as an overlay. The U.S. Geological Survey Digital Line Graph Level 3 Optional format has been adopted by FEMA for the purposes of the National Flood Insurance Program mapping and engineer requirements. Level 3 data files are fully topologically structured and are designed to be integrated into GISs.

Digitizing - A process of converting an analog image or map into a digital format usable by a computer.

Edge Matching - The comparison and graphic adjustment of features to obtain agreement along the edges of adjoining map sheets.

Elevation Reference Mark (ERM) - A point of vertical ground elevation reference shown on the FIRM for comparison to the BFE. ERMs are referenced to the National Geodetic Vertical Datum (NGVD) or the North American Vertical Datum (NAVD) or an alternative local datum if applicable.

Federal Emergency Management Agency (FEMA) - The agency reporting directly to the President and responsible for identifying and mitigating natural and man-made hazards.

Federal Geographic Data Committee (FGDC) - An interagency committee, established by the Office of Management and Budget, that promotes the coordinated development, use, sharing, and dissemination of geospatial data on a national basis. The FGDC is composed of representatives from 14 Cabinet level and independent Federal agencies.

Federal Information Processing Standards (FIPS) - The official source within the Federal government for information processing standards, which are developed and published by the Institute for Computer Sciences and Technology at the National Institute of Standards and Technology (NIST).

Flood Insurance Rate Map (FIRM) - A map on which the 100- and 500-year floodplains, BFEs, and risk premium zones are delineated to enable insurance agents to issue accurate flood insurance policies to homeowners in communities participating in the NFIP.

Flood Insurance Study (FIS) - An examination, evaluation, and determination of the flood hazards, and if appropriate, the corresponding water-surface elevations.

Geographic Information System (GIS) - System of computer hardware, software, and procedures designed to support the capture, management, manipulation, analysis, modeling, and display of spatially referenced data for solving complex planning and management problems.

Geocoding - Associating either geographic coordinates or grid cell identifiers to data, points, lines, and shapes.

Geographic Coordinates - Coordinate system in which horizontal and vertical distances on a planimetric map are represented in units of latitude and longitude rather than feet or meters.

Layer - Refers to the various "overlays" of data, each of which normally deals with one thematic topic. These overlays are registered to each other by the common coordinate system of the database. In GIS, a layer or a theme represent a specific kind of data.

Line - A level of spatial measurement referring to a one-dimensional defined object having a length and direction, and connecting at least two points.

MapInfo - A desktop mapping system that combines map graphics and a relational database for cartographic display and spatial analysis (MapInfo Corporation, Troy, NY).

Menu- A list of options on a screen display or pallet allowing an operator to select the next operations by indicating one or more choices with a pointing device.

Merge - To combine items from two or more similarly ordered sets into one set that is arranged in the same order. In a GIS, to splice separate but adjacent mapped areas into a single data set.

National Flood Insurance Program (NFIP) - The Federal regulatory program under which flood-prone areas are identified and flood insurance is made available to residents of participating communities.

Point Data - In a vector structure, data consisting of single, distinct X,Y coordinate. In a raster structure, point data is represented by single cells.

Polygon - A two-dimensional figure with three or more sides intersecting at a like number of points. In Geographic Information Systems, an area.

Projection - The mathematical transformation of three-dimensional space into a two-dimensional (flat) surface. Projection formulae are dependent on a variety of spherical models (spheroids) which distort the spatial characteristics of a location.

Quad (also USGS Quad) - A U.S. Geological Survey (USGS) topographic map; Quad stands for "Quadrangle."

Quality Assurance/Quality Control (QA/QC) - Intermediate and final review of the FIS and FIRM performed to ensure compliance with FEMA standards.

Q3 Flood Data - A digital FIRM product developed and distributed by FEMA. Q3 Flood Data are developed by scanning and vectorizing the existing hardcopy FIRM to create a raster product suitable for viewing or printing, as well as a thematic vector overlay of flood risks. Q3 Flood Data capture all FIRM data in the raster file, but vectorize only certain features.

Raster Data - The representation of spatial data as regular grid cells or tessellations. Cells may be subdivided or aggregated depending on the variety and density of features to be modeled.

Read Only Memory (ROM) - A microcircuit containing programs or data that cannot be erased. When new data or programs can replace old ones, the microcircuit is called an EROM, for erasable read only memory, or PROM, for programmable read only memory.

Record - A groups of items in a file treated as a unit. For example, all data items for a census tract can be grouped as a record and assigned to a single segment of a magnetic tape or other media file for convenient storage and retrieval.

Scale - A representative fraction of a paper map distance to ground distance. Example: 1:12,000 is the representative fraction in which one unit of measure on the map is equal to 12,000 of the same units of measure on the ground. FEMA map scales are expressed in a ratio of 1" of map distance equal to a given number of feet on the ground.

Scanner - Any device that systematically decomposes a sensed image or scene into pixels and then records some attribute of each pixel.

Scanning - Process of using an electronic input device to convert analog information such as maps, photographs, overlays, etc., into a digital format usable by a computer.

Spatial Data Transfer Standards (SDTS) - A FIPS standard (FIPS 173) developed as a mechanism for the transfer of digital spatial data between different computer systems. It specifies exchange constructs, addressing formats, structure, and content for spatially referenced vector and raster data.

Special Flood Hazard Area (SFHA) - Any area inundated by the base (1% annual chance) flood; these areas are identified on the FIRM as Zones A, AE, AH, AO, AR, A1-30, A99, V, VE, and V1-30.

Thematic Layer - A data layer containing selected information relating to a specific theme, such as soils, vegetation, land use, etc.

TIFF - Tagged Image File Format, a standard exchange format for raster or image files. Tags or identifiers are used to structure the raster data such that blocks of data may be interpreted based on tags that identify them. TIFF images may be black and white, gray scale, or color, and may be compressed using a variety of compression types.

TIGER --Topologically Integrated Geographic Encoding and Referencing File- The nationwide digital database of planimetric base map features developed by the U.S. Bureau of the Census for the 1990 Census.

Topology - A branch of non-euclidean geometry that analyzes the spatial relationships and connectivity of graphs and their components. In GIS, topology is a key element used in a number of data models, such as ARC/INFO.

US Geological Survey (USGS) - The Federal agency responsible for nationwide civilian mapping projects and standards development.

UTM Grid - The Universal Transverse Mercator grid, a system of plane coordinates based upon 60 north-south trending zones, each 6 degrees of longitude wide, that circle the globe.

Vector Data - Coordinate-based data used to represent linear geographic features. Each linear feature is represented as an ordered list of vertices.


Note: Information contained within these flooding pages has generally been excerpted from Federal Emergency Management Agency, Q3 Flood Data Specifications, Draft June 1995, Federal Emergency Management Agency, Washington, D.C.


ABAG, the Association of Bay Area Governments, is the regional planning and services agency for the nine-county San Francisco Bay Area.

This page was last updated 8/21/03 by jbp.