Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

agonic line - The line along which magnetic declination is zero.

Arc/Info - An ESRI GIS package for building and editing GIS data, as well as analyzing and publishing.

ArcView - An ESRI GIS package primarily for visualization and analysis.

azimuth angle - The directional bearing on a 360-degree circle measured clockwise from north.

bathymetric map - A map portraying water body depths. Contour lines and a color scheme ranging from dark blue for deep water to light blue for shallow areas are commonly used to portray elevations.

bearing - A system for designating the directions of lines by means of angle and quadrant letters. The bearing angle of a line is the acute horizontal angle between a reference meridian and the line.

benchmark - A surveying term for a control point whose height relative to a datum is known.

cadastral map - A map defining land ownership and boundaries. More information on Cadastral Maps.

cartography - The art and science of expressing graphically the natural and social features of the earth.

CD-ROM - A laser disk digital file storage medium.

color infrared - A type of color camera film that is most often used to map or evaluate growing vegetation. It is sensitive to green and red light as well as near infrared, but is insensitive to blue.

Content Standard for Digital Geospatial Metadata (CSDGM) - A standard adopted by the FGDC to systemize the collection and subsequent search of metadata.

coordinates - Numbers that identify imaginary points on a reference system. Coordinates describe position in two or three dimensions.

coordinate system - A reference device used to determine distance and direction in relative terms with respect to their location. The geographical coordinate system uses latitude and longitude to locate points on the earth; most other coordinate systems use regularly spaced grids which are placed on artificially derived planes.

Continuously Operating Reference Stations (CORS) - Federally-maintained community GPS base stations that provide carrier phase and code range measurements for real-time and post-processing differential correction.

Cyan, Magenta, Yellow, Black (CMYK) - The four ink colors used in process printing.

densification - The establishment of geodetic control points for a better control network. More information on Geodetic Network Densification.

Digital Elevation Model (DEM) - A raster file containing elevations at regularly-spaced surface coordinates over an area. Specialized computer software can interpret this data and create a three-dimensional rendering of the surface, or analyze the terrain in various ways. More information on DEMs.

Digital Line Graph (DLG) - A vector file containing points, lines, and/or areas that represent roads, water bodies, topographic contour lines, public lands, and railroads. Each data layer comes in a separate file. The DLG format was created by the USGS.

digital orthophoto (DOP) - An image derived by geometrically correcting a scanned aerial photograph so that it is positionally accurate like a planimetric map. More information on Orthophotography.

Digital Orthophoto Quarter Quadrangle (DOQQ) - A format type of digital orthophoto that covers 3.75 x 3.75 minutes of latitude and longitude (one quarter of a 7.5-minute quadrangle).

digitizing - The process of converting information shown on "paper" maps into digital form for computer processing either by scanning or by manually capturing point and line features using specialized computer hardware.

Disk Operating System (DOS) - The name of a number of operating systems which include facilities for storing files on disk.

Digital Raster Graphic (DRG) - A scanned topographic map produced by the USGS. More information on DRGs and topographic maps.

Digital Terrain Model (DTM) - A term that describes a variety of digital formats for displaying or analyzing elevation information. A DEM is one of these formats.

dots per square inch (DPI) - A measure of image resolution.

electromagnetic energy - Energy rays that are either reflected or emitted as radiation by objects. X-rays, ultraviolet light, visible light, near-infrared light, and heat (thermal) radiation are different wavelengths of electromagnetic energy.

electromagnetic spectrum - The continuum of electromagnetic energy from long wavelengths such as radio or radar, to medium wavelengths like visible light, to short wavelengths like X-rays.

ellipsoid - A smooth mathematical surface that can be used to approximate the shape of the earth. It is an ellipse, with axes approximate to the dimensions of the earth, rotated around the polar axis.

fiducial marks - Marks placed on aerial camera film during the exposure. Either four or eight fiducial marks are placed along the edges of the film, and the imaginary lines that connect opposite fiducials intersect at the principal point of a photograph. This is important for recreating the orientation of the film in relationship to the ground.

File Transfer Protocol (FTP) - Method of transferring digital files over the Internet.

Flood Insurance Rate Map (FIRM) - A standard map series produced by FEMA.

focal length - The distance between the film and the lens of the camera, when the lens is focused on a distant object.

geodetic control - A network of carefully measured points that can be used as a reference for other surveys or to establish or measure accuracy in map making. Horizontal control and vertical control networks are typically separate because the former is based on an ellipsoid model of the earth, while the latter is based on a geoid model. More information on Geodetic Control.

Geodetic Reference System (GRS) - A commonly used equipotential ellipsoid.

geoid - A undulating surface of constant gravitational force that approximates the shape of the earth.

geographic encoding and referencing - Assigning a geographic reference to an entity.

Geographic Information System (GIS) - A tool for analyzing and querying spatial data. Its function is facilitated by computer hardware, GIS software, and the technical expertise of the people involved.

Geographic Names Information System (GNIS) - Service that allows searches of the NGNDB catalog of named features on USGS maps. More information on Geographic Names.

Global Land Information System (GLIS) - The USGS' online store for geographic data and other products.

GLObal NAvigation Satellite System (GLONASS) - Space navigation system established in order to locate the Soviet Union's civil aircraft and its merchant and fishing vessels. Russian equivalent of U.S. GPS/ Navstar.

Global Positioning System (GPS) - A group of 24 satellites maintained by the U.S. Department of Defense which broadcast signals that allow anyone with a GPS receiver to determine their position on the earth. More information on GPS.

Graphics Interlaced Format (GIF) - An 8-bit (256 color) graphic file format, commonly used for web graphics.

gravity anomaly - The difference between the observed gravity value reduced to sea level, and the theoretical gravity obtained from the gravity formula. Maps of gravity anomaly over an area may reveal geologic patterns. More information on Gravity Anomaly Maps.

High Accuracy Reference Network (HARN) - A horizontal geodetic control network of highly reliable positional coordinates that serve as control for other surveys. Points are measured and adjusted using GPS. The Wisconsin HARN was originally called the Wisconsin High Precision Geodetic Network (WHPGN).

hydrography - The process of charting or mapping water features and characteristics, based on specific measurements at a point or over a distance or area.

hydrology - The science dealing with the properties, distribution, and circulation of water on and below the earth's surface and in the atmosphere. The emphasis in hydrology is on mechanisms by which water acts as part of the overall environment.

Ice Age Trail - A National Scenic Trail crossing Wisconsin and tracing the terrain features left from the last period of glaciation. The Wisconsin DNR manages the trail corridor and offers an interactive map of the Ice Age Trail.

image processing - Manual and digital techniques used to improve image geometry and appearance, to identify factors, and to extract selected information.

Imagine - A digital image processing software package, by ERDAS.

infrared radiation - Electromagnetic energy with wavelengths of .8 to 5 microns. These rays are not visible to the human eye, but infrared radiation reflected off objects can be recorded as an image on special camera film or by digital instruments. Infrared reflectance data is very useful for detecting the health of vegetation.

International Great Lakes Datum (IGLD) - A vertical reference system developed to standardize elevation measurements around the Great Lakes. More information on IGLD.

isogal - A line on a map or chart along which there is a constant gravity value.

Joint Photographic Experts Group (JPEG) - A compressed image format used on the WWW.

Land Information Offices (LIO) - To facilitate its participation in the WLIB's program, each county designates a person to be its Land Information Office contact.

Land Information System (LIS) - A GIS specially designed for use with land information. Land rights, ownership, boundaries, utility, landcover, and zoning data are common layers and attributes in a Land Information System.

Landnet - A GIS data set produced by the Wisconsin DNR representing the PLSS at 1:24,000 scale.

LANDSAT - A series of satellites managed as part of NASA's Earth Observing System that acquire imagery of the Earth from space. The images are used in the areas of global change, agriculture, geology, forestry, regional planning, education, and national security. More information about LANDSAT.

Land Use/Land Cover (LU/LC) - A categorization scheme developed by the USGS to standardize the interpretation and classification of aerial photographs. The scheme describes criteria for specific land use and land cover categories at various scales.

magnetic declination - The angle between magnetic north and geographical north at a point on the Earth. It is expressed in degrees east or west. More information on Magnetic Declination Maps.

Managed Forest Law - A Wisconsin law under which landowners can receive tax benefits in exchange for practicing sustainable management of their forested land and opening the land for public uses such as hunting, fishing, cross-country skiing, sight-seeing, and hiking. The program is administered by the Wisconsin DNR.

map compilation - A map making process that involves the accurate transfer of map unit boundaries, symbols, drainage, and cultural features from field sheets to photo base map overlays.

map separate - Positive or negative film copies of any color layers of a map. More information on Topographic Maps.

metadata - Documentation covering the who, what, when, why, where, and how associated with a unit of information. In other words, data about data.

micron - A unit of length equal to one millionth of a meter, or one thousandth of a millimeter. One thousandth of an inch is equivalent to 25 microns.

monument - A permanent structure used to mark the position of a surveyed point. Common locations for monuments include property or PLSS corners, vertical control points (benchmarks) and horizontal control points (sometimes called triangulation stations). More information on Monuments.

multipath - Corruption of the direct GPS signal by one or more signals reflected off the local surroundings. Reflections affect both code and carrier based measurements in a GPS receiver.

nadir - The line extending perpendicularly from an aerial camera to a point on the ground. If the camera and airplane are perfectly level (without pitch or tilt), the nadir point will be the same as the principal point.

North American Datum of 1927 (NAD 27) - A geodetic reference for horizontal positions, established in 1927 and based on the Clark ellipsoid of 1866.

North American Datum of 1983 (NAD 83) - A geodetic reference for horizontal positions, established in 1983 and based on the GRS 80 ellipsoid.

North American Vertical Datum of 1988 (NAVD 88) - A geodetic reference for elevations, created by the NGS to replace the NGVD 29. This new datum was published in 1991, and incorporates vertical datums used on the Great Lakes. It is adjusted based on field work prior to 1929 as well as surveys as recent as 1988.

National Geodetic Vertical Datum of 1929 (NGVD 29) - A geodetic reference for elevations, completed and adjusted in 1929. These elevations were used to define the mean sea level datum. This datum was replaced by NAVD 88.

orthophoto - An aerial photo that has been corrected to eliminate the effects of camera tilt and relief displacement. The ground geometry is recreated as it would appear from directly above each and every point. Digital orthophotos (DOPs) can be created by scanning the original photograph and applying a process called differential rectification to each pixel in the image. In creating digital orthophotos, it is also possible to remove the effects of tangential displacement. More on Orthophotography.

panchromatic - Sensitive to all or most of the visible electromagnetic spectrum, between 0.4 and 0.7 microns.

parallax - The apparent shift of an object as it is observed from different locations. For example, having two eyes allows your brain to consider the parallax and gives you the benefit of improved depth perception. Similarly in photogrammetry, the parallax for a point that appears in overlapping photographs is used to determine its elevation.

photogrammetry - The science of deriving the physical dimensions of objects from measurements on aerial photographs. More information on Photogrammetry.

planimetric - A type of map that only shows horizontal features on a plane surface, without elevation contours to depict the terrain.

plat book - A bound set of plat maps, typically for an entire county with one map per PLSS township per page. More information on Plat Books.

plat map - A map of land ownership parcels for an area.

U.S. Public Land Survey System (PLSS) - The basis for the rectangular grid of 6x6 mile townships, subdivided down to 1/4 sections (each nominally 160 acres) over most of Wisconsin. The system was inaugurated by the Northwest Ordinance of 1785 and designed to facilitate the transfer of federal lands to private citizens. More information on PLSS.

parts per million (ppm) - An amount of error proportional to the distance measured. For example, an error of +/- 3 millimeters for a measurement of 30 meters means that the measurement is good only to 100 ppm.

principal meridian - A longitudinal basis for mapping grid systems. One group of commonly referenced principal meridians are part of the U.S. PLSS, although such north-south trending lines as marked on the ground with monuments do not exactly follow a line of constant longitude. Wisconsin uses the 4th principal meridian.

principal point - The point at which lines drawn between opposing fiducials on an aerial photograph intersect. The center point of an aerial photograph.

raster - A grid structure holding data or image values.

relief displacement - A characteristic on an aerial photo where features at higher elevations are displaced away from the center of the photo. Relief displacement aids in the calculation of heights of features on an air photo, but also results in the image not having a uniform scale like a planimetric map, unless it is converted to an orthophoto.

shaded relief - A cartographic technique that provides an apparent three-dimensional rendering of the terrain, on maps and charts, by the use of graded shadows that would be cast by high ground if light were shining from the northwest.

softcopy photogrammetry - The use of scanned orthophotos and computer software, rather than traditional mechanical techniques, to make photogrammetric measurements.

soil horizons - Distinct soil layers identifiable by the material deposited there. A progression of layers occurs from years of natural separating and sifting of soil particles, and from weathering and other chemical processes.

State Plane Coordinate System (SPC) - A commonly used mapping coordinate system that in Wisconsin has 3 zones: north, central, and south. More information on Wisconsin Coordinate Systems.

System pour Observation de la Terre (SPOT) - A French satellite system that collects digital images for mapping or analysis.

stereoscopic - The mental impression of a three-dimensional model which results from viewing an object at two different perspectives.

surficial geology - Geology relating to surface layers, such as soil, exposed bedrock, or glacial deposits.

tangential distortion - Linear displacement of image points in a direction normal to radial lines from the center of the field.

Tagged Interlaced File Format (TIFF) - A 24-bit (16 million colors) graphics file format.

Topologically Integrated Geographic Encoding and Referencing (TIGER) - The U.S. Census Bureau's spatial database. More information on Census Maps.

topology - A branch of mathematics that, when applied to geographic features, refers to spatial relationships such as adjacency and connectivity.

thermal radiation - Electromagnetic energy with wavelengths of 5 to 80 microns. Thermal radiation is of longer wavelengths than near infrared radiation and is emitted by terrestrial objects like trees, human beings, heated buildings, or power plants.

topography - The shape of a surface, typically the earth's land surface. Topography is often depicted by contour lines.

ultraviolet radiation - Electromagnetic energy with wavelengths of .001 to .5 microns. These rays are of shorter wavelengths than visible light. The sun emits strong ultraviolet radiation.

U.S. Coast and Geodetic Survey (USCGS) - The former name of the NGS.

Universal Transverse Mercator (UTM) - A map projection that is the basis for the UTM coordinate system.

vector - A value with a direction vector data structure, which is common way for CAD or GIS software to encode geographic features as points, lines, or areas.

vertical exaggeration - The effect of multiplying a scalar number to the elevations at all points in an elevation model in order to exaggerate the landscape's relief.

visible radiation - Electromagnetic energy with wavelengths of .5 to .8 microns. These rays are visible to most people, and appear as violet through red on the color spectrum.

Wisconsin Initiative for Statewide Cooperation on Landscape Analysis and Data (WISCLAND) - A group of over two dozen organizations working together to build and use GIS data layers.

WISCON - Windows software that converts values amongst all commonly used coordinate systems in Wisconsin. More information on the availability of WISCON.

Wisconsin County Coordinate System - A set of 72 mapping systems each optimized for a particular county. More information on Wisconsin Coordinate Systems.

Wisconsin Transverse Mercator - A map projection and coordinate system developed by the Wisconsin DNR to avoid splitting the state of Wisconsin into two UTM zones. More information on WTM.

Windows Metafile (WMF) - A particular vector file format.