Spatial Information Clearinghouse - Glossary

Absorption - the process by which radiant energy is absorbed and converted into other forms of energy

Aliasing - Unwanted visual effects caused by insufficient sampling resolution or inadequate filtering to completely define an object; most commonly seen as a jagged or stepped edge along object boundaries or along lines.

Base Station - carefully surveyed receiver that collects satellite data then calculates error vectors for correcting field data

Cartography - The organisation and communication of geographically related information in either graphic or digital form. It can include all stages from data acquisition to presentation and use.

Coordinate System - A recognised reference system for the unique location of a point in space. The Cartesian coordinate system and the system of latitude and longitude of the earth are examples of coordinate systems based upon Euclidean geometry.  A coordinate system is usually defined by a map projection, a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the X and Y directions to locate X,Y positions of point, line, and area features. A more complete explanation is available in the Standards Section.

Datum - Any point, line or surface used as a reference for a measurement of another quantity.  A model of the earth used for Geodetic calculations. A more complete explanation is available in the Standards Section.

Differential Correction - differential correction is the method used, via base station, to calculate error vectors that coincide with field data to correct the field data error

Digital Image Processing (DIP)– the use of the computer on raster image files, to carry out such operations as image enhancement, image compression, image analysis, mapping, georeferencing, etc.

Digitizing - A method of data capture that involves the conversion of data in hard copy or raster form, such as maps and aerial photographs, into a digital vector format. This is usually done by a human operator using on a digitizing tablet or sitting at a computer screen with a mouse, drawing over top of a raster image. Methods of automated digitizing and semi-automated digitizing also exist.

Electromagnetic Spectrum - the ordered array of known electromagnetic energies, extending from the shortest cosmic ray, through gamma rays, x-rays, ultraviolet radiation, and including microwave and all other wavelengths of radio energy.

Ellipsoid - Athree-dimensional ellipse which is used to represent the shape of the surface of the earth. A more complete explanation is available in the Standards Section.

Enhanced Thematic Mapper Plus (ETM+) - the primary sensor system aboard Landsat 7

Features - A set of points, lines or polygons in a spatial database that represent a real-world entity. The terms feature and object are often used synonymously.

Geodesy - The science of measuring the shape and size of the earth, together with the determination of the exact position of particular points on its surface by taking the earth's curvature into account.

Geographic Information System - A computer system for capturing, storing, checking, integrating, manipulating, analyzing and displaying data related to positions on the earth's surface. Typically, a Geographical Information System (or Spatial Information System) is used for handling maps of one kind or another. These might be represented as several different layers where each layer holds data about a particular kind of feature. Each feature is linked to a position on the graphical image of a map.  Layers of data are organised to be studied and to perform statistical analysis.

Geoid - An imaginary shape for the earth defined by mean sea level and its imagined continuation under the continents at the same level of gravitational potential. A more complete explanation is available in the Standards Section.

Georeferencing – the process of taking an image and assigning it geographic coordinates

Global Positioning System - A satellite-based navigational system allowing the determination of any point on the earth's surfacewith a high degree of accuracy given a suitable GPS receiver. The network of satellites is owned by the US Department of Defense. Error in the accuracy of GPS derived positions can be introduced through the nature of local conditions. These errors can be greatly reduced using a technique known as differential GPS.

Metadata - Data about data and usage aspects of it. This information will often include some of the following: What it is about, Where it is to be found, Who one needs to ask to get it, How much it costs, Who can access it, In what format it is available, What is the quality of the data for a specified purpose, What spatial location does it cover and over what time period, When and where the data were collected and by whom and what purposes the data have been used for, by whom and what related data sets are available, etc. A more complete explanation is available in the Standards Section.

Network analysis - A data processing method using topologically linked data such as street maps or river networks with the purpose of determining routes between geographic locations, and other analyses requiring the consideration of path and direction. Such analyses might include finding the most efficient travel route, generating directions, or defining service coverage based on travel time.

Orthophotography - A modified copy of a perspective photograph of the earth's surface with distortions due to tilt and relief removed.

Photogrammetry - the art or science of obtaining reliable measurements by means of photography.

Pixel - A contraction of the words picture element. Pixel refers to the smallest unit of information available in an image or raster map that can be independently assigned attributes such as colour and intensity.

Position Dilution of Precision (PDOP) - regulates amount of interference the rover unit accepts and still records data; the lower, the better.

Projection - A method of representing the earth's three-dimensional surface as a flat two-dimensional surface. This normally involves a mathematical model that transforms the locations of features on the earth's surface to locations on a two-dimensional surface. Because the earth is three-dimensional, some method must be used to depict the map in two dimensions. Therefore, such representations distort some parameter of the earth's surface, be it distance, area, shape, or direction.  There are a variety of map projections, but all are generally of three basic types; these are the Azimuthal, conical and cylindrical projections. For example, the Transverse Mercator Projection is a variant of the cylindrical projection. A more complete explanation is available in the Standards Section.

Pseudo Random Code - means by which time error is corrected between the receiver and the satellite via a code that the receiver matches with the satellite's

Radiometric Resolution - Radiometric resolution, or radiometric sensitivity refers to the number of digital levels used to express the data collected by the sensor. In general, the greater the number of levels, the greater the detail of information. The number of levels is normally expressed as the number of binary digits needed to store the value of the maximum level, for example a radiometric resolution of 1 bit would be 2 levels, 2 bit would be 4 levels and 8 bit would be 256 levels. The number of levels is often referred to as the Digital Number, or DN value.

Raster Data - An abstraction of the real world where spatial data is expressed as a matrix of cells or pixels, with spatial position implicit in the ordering of the pixels. With the raster data model, spatial data is not continuous but divided into discrete units. This makes raster data particularly suitable for certain types of spatial operations, for example overlays or area calculations. Unlike vector data however, there are no implicit topological relationships.

*Reference Ellipsoid -  A geometric model of the earth, required for accurate range and bearing calculations over long distances.  Ellipsoidal models define an ellipsoid with an equatorial radius and a polar radius. The best of these models can represent the shape of the earth over the smoothed, averaged sea-surface to within about one-hundred meters. A more complete explanation is available in the Standards Section.

Reflection - Electromagnetic Radiation neither absorbed nor transmitted is reflected.  Reflection may be diffuse, when the incident radiation is scattered upon being reflected from the surface, or specular when all or most angles of reflection equal the angle of incidence

Refraction - the bending of Electromagnetic radiation rays when they pass from one medium to another having a different index of refraction or dielectric coefficient.

Remote Sensing - the measurement or acquisition of information of some property of an object or phenomenon, by a recording device not in physical or intimate contact with the object or phenomenon under study

Resolution - A measure of the ability to detect quantities. High resolution implies a high degree of discrimination but has no implication as to accuracy. Resolution is a term that is used often within remote sensing.

Rover Unit - also termed: field unit or data logger - The mechanism used in the field that receives GPS satellite signals from outer space

Scale - The ratio of the distance measured on a map to that measured on the ground between the same two points. In Britain, most map scales are now metric and are shown, for example, as 1:50,000, which represents a scale of 1cm = 50,000 cm (or 500 metres).  Often, the difference between large and small map scales is confused. The larger the ratio, the smaller the map scale. Therefore, a map of the world, would have a very small scale, whereas a map of a town centre, will have a large scale. A more complete explanation is available in the Standards Section.

Scanning - A method of data capture whereby a hard copy image, map, or even text is converted into digital raster form by using a specialized high quality digital camera called a scanner.

Spatial Data - Any information about the location and shape of, and relationships among, geographic features. This includes remotely sensed data as well as map data.

Spatial Resolution - Spatial resolution refers to the area on the ground that an imaging system, such as a satellite sensor, can distinguish. There are many measures of spatial resolution, the most common include the Instantaneous Field of View (IFOV), and the Effective Instantaneous Field of View (EIFOV).

Spectral Resolution - The term spectral resolution refers to the width of spectral bands that a satellite imaging system can detect. Often satellite imaging systems are multi-spectral meaning that they can detect in several discrete bands, it is the width of these bands that spectral resolution refers too. The narrower the bands, the greater the spectral resolution.

Spheroid - A solid that resembles a sphere in geometry. One of the terms used to describe the shape of the earth.

Stereoscope - a binocular optical instrument for assisting the observer to view two properly oriented photographs or diagrams to obtain the mental impression of a three-dimensional model

Stereoscopic Vision - binocular vision which enables the observer to view an object simultaneously from two different to obtain the mental impression of a three-dimensional model

Tabular or Attribute Data - A trait, quality or property describing a geographical feature.  A fact describing an entity in a relational data model, equivalent to the column in a relational table.

*Temporal Resolution - The frequency of obtaining imagery of a particular area that a sensor/platform system maintains.

Thematic Map - A map depicting selected kinds of information relating to one or more specific themes. Examples are soil type, land classification, population density and rainfall maps.

Topographic Map - A map whose principal purpose is to portray the features of the earth's surface. These features might include the cultural landscape, but normally refer to the terrain and its relief.

Topology - The relative location of geographic phenomena independent of their exact position. In digital data, topological relationships such as connectivity, adjacency and relative position are usually expressed as relationships between nodes, links and polygons. For example, the topology of a line includes its from- and to-nodes, and its left and right polygons.  Topology is useful in GIS because many spatial modelling operations don not require coordinates, only topological information. For example, to find an optimal path between two points requires a list of the lines or arcs that connect to each other and the cost to traverse each line in each direction. Coordinates are only needed for drawing the path after it is calculated.

Transmission - the passage of electromagnetic energy through a material such as air, glass, or water.

Triangulation - refers to the calculation of receiver position by way of at least four satellites

Vector Data - An abstraction of the real world where positional data is represented in the form of coordinates. In vector data, the basic units of spatial information are points, lines and polygons. Each of these units is composed simply as a series of one or more coordinate points, for example, a line is a collection of related points, and a polygon is a collection of related lines.  Vector data may or may not possess topological relationships.

Most Definitions are From the AGI On-line GIS Dictionary