| 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.
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.
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.
