 | Glossary for Students R-Y |
Raster data model: System of tessellating rectangular cells in which individual cells are used as building blocks to create images of point, line, area, network and surface entities.
Rasterization: The process of converting data from vector to raster format.
Reclassification: The process of reclassifying values in a map layer to produce a new map layer.
Rectangular co-ordinates: See Cartesian co-ordinates''.
Regional Research Laboratories (RRL): A network of research laboratories set up by the Economic and Social Research Council to pump-prime GIS research in the UK.
Relational database: A computer database employing an ordered set of attribute values or records known as tuples grouped into two-dimensional tables called relations.
Relations: Two-dimensional tables of data in a relational database.
Relief shadows: Areas calculated to be in the shadow of hills using a DTM. The amount of shadow varies according to the relative position of the Sun as determined by the time of day and date.
Remote sensing: The science of observation without touching. Often used to refer to Earth observation from satellite platforms using electromagnetic sensors.
Resolution: The size of the smallest recording unit or the smallest feature that can be mapped and measured.
Rich picture: A graphical method for expressing the scope of a problem.
Root definition: A clear statement of purpose. In GIS this may be a statement of why the GIS is required and the purpose it is to serve.
Route tracing: Application of network analysis to trace the route of flows through a network from origin to destination. This is particularly useful where flows are unidirectional (as in hydrological networks).
Routed line cartograms: Maps in which routes are plotted as generalized lines showing the sequence of stops and connections rather than the actual route taken. An example is the map of the London Underground system.
Rubber sheeting: The method used to adjust the location of features in a digital map layer in a non-uniform or non-linear manner.
Run-length encoding: The method of reducing the volume of raster data on a row-by-row basis by storing a single value where there are a number of cells of a given type in a group, rather than storing a value for each individual cell.
Satellite image: Graphical image (usually in digital form) taken of the Earth''s surface using electromagnetic sensors on board an orbiting satellite or spacecraft.
Scale: The size relationship or ratio between the map document and the area of the Earth''s surface that it represents.
Scale analogue model: A model that is a scaled down and generalized replica of reality, such as a topographic map or aerial photograph.
Scale-related generalization: Cartographic simplification of mapped features, the level of which is directly related to the scale of the map being drawn.
Scanner: Raster input device similar to an office photocopier used to convert maps and other analogue data into digital data using the raster data model.
Secondary data: Data collected by another individual or organization for another primary purpose. Many data sources used in GIS, including maps, aerial photographs, census data and meteorological records, are secondary.
Shortest path problem: A classic problem in network analysis which involves identifying the shortest and/or quickest route through a network, taking link and turn impedances as well as distance into account.
Sieve mapping: The consecutive overlay of various maps to find a set of feasible areas that satisfy a given set of criteria.
Simple raster: Simplest raster data structure in which the value of each pixel is stored as a separate value in a matrix.
Sliver polygons: Small, often long and narrow, polygons resulting from the overlay of polygons with a common but separately digitized boundary. Errors in the digitizing process mean two versions of the common boundary are slightly different.
Slope: The steepness or gradient of a unit of terrain, usually measured as an angle in degrees or as a percentage.
Soft systems analysis: A general-purpose methodology for investigating unstructured management problems.
Spaghetti model: Simplest form of vector data structure representing the geographic image as a series of independent ( x , y ) co-ordinate strings.
Spatial: Anything pertaining to the concepts of space, place and location.
Spatial analysis: See Geographical analysis''.
Spatial autocorrelation: Tobler''s Law of Geography (1976), which states that points closer together in space are more likely to have similar characteristics than those that are further apart.
Spatial data: Data that have some form of spatial or geographical reference that enables them to be located in two- or three-dimensional space.
Spatial data model: A method by which geographical entities are represented in a computer. Two main methods exist: raster and vector data models.
Spatial data retrieval: The process of selectively retrieving items from a GIS database on the basis of spatial location.
Spatial data structure: Approach used to provide the information that the computer requires to reconstruct the spatial data model in digital form. There are many different data structures in use in GIS.
Spatial decision support system (SDSS): A decision support system (DSS) with a strong spatial component and incorporating spatial data models and spatial analysis to assist the user in arriving at a solution.
Spatial entities: Discrete geographical features (points, lines and areas) represented in a digital data structure.
Spatial interaction models: Models that are used to help understand and predict the location of activities and the movement of materials, people and information.
Spatial interpolation: The procedure of estimating the values of properties at unsampled sites within an area covered by existing observations (Waters, 1989).
Spatial model: A model of the real world, incorporating spatial data and relationships, used to aid understanding of spatial form and process.
Spatial process models: Models that simulate real-world processes which have a significant spatial component. These are used to help evaluate and understand complex spatial systems.
Spatial query: Action of questioning a GIS database on the basis of spatial location. Spatial queries include What is here?'' and Where is...?''
Spatial referencing: The method used to fix the location of geographical features or entities on the Earth''s surface.
Splines: Mathematical method of smoothing linear features, often used to interpolate between points digitized along a curved feature.
SPOT: A French remote sensing satellite (Systme Pour l''Observation de la Terre) yielding high-resolution (10 m) data.
Spot height: A single height value on a topographic map, usually representing the location and height of a prominent feature between contour lines.
Standardization: The process of transforming data onto a common scale of measurement. This is used in MCE to ensure comparability between criteria measured in different units and different scales of measurement.
Stereoscopic aerial photography: The use of overlapping vertical aerial photographs to determine height based on the parallax effect gained from viewing the same object from slightly different angles.
Stochastic model: A model that recognizes that there could be a range of possible outcomes for a given set of inputs, and expresses the likelihood of each one happening as a probability.
Stream mode digitizing: The method of manually digitizing lines whereby the digitizer automatically records a co-ordinate at a set time or distance interval.
Structured Query Language (SQL): The computer language developed to facilitate the querying of relational databases. Also known as Standard Query Language.
Structured systems analysis and design methodology (SSADM): A method used for the design of GIS projects.
Surface: The entity type used to describe the continuous variation in space of a third dimension, such as terrain.
Surface drape: The draping of an image on top of a 3D view of a terrain model for the purpose of landscape rendering or visualization.
Surface significant points: Points in a TIN model that cannot be closely interpolated from the height values of neighbouring points.
SYMAP: Synagraphic Mapping. An early commercial computer cartography package.
SYMVU: Early 3D computer mapping package developed from SYMAP.
System life cycle: An IT software development and methodology that uses a linear approach to project management.
Temporal data: Data that can be linked to a certain time or period between two moments in time.
Terrain model: Surface model of terrain. See digital terrain model (DTM)'', digital elevation model (DEM)'' and triangulated irregular network (TIN)''.
Thematic data: Data that relate to a specific theme or subject.
Thematic maps: Maps pertaining to one particular theme or subject.
Theodolite: A survey instrument for measuring horizontal and vertical angles using a rotating and tilting telescope.
Thiessen polygons: An exact method of interpolation that assumes the values of unsampled locations are equal to the value of the nearest sampled point to produce a mesh of irregular convex polygons.
Tight coupling: A method of linking models to GIS in which the link between the GIS and the model is hidden from the user by an application interface and GIS and model share the same database.
Topographic maps: Maps whose primary purpose is to indicate the general lie of the land. These maps generally show terrain, basic land use, transport networks, administrative boundaries, settlements and other man-made features.
Topology: The geometric relationship between objects located in space. Adjacency, containment and connectivity can describe this.
Total station: A theodolite or EDM combined with data logger and automated mapping soft ware.
Transformation: The process of converting data from one co-ordinate system to another.
Travelling salesperson problem: A problem in network analysis where the best route between a series of locations to be visited in one journey must be identified.
Trend surface analysis: A routine that interpolates a complex surface or series of data points to produce a much-simplified surface showing the overall trend in the data.
Triangulated irregular network (TIN): An irregular set of height observations in a vector data model. Lines connect these to produce an irregular mesh of tri-angles. The faces represent the terrain surface and the vertices represent the terrain features.
Tuples: Individual records (rows) in a relational database.
UNION overlay: Polygon-on-polygon overlay corresponding to the Boolean OR overlay. The output map will contain a composite of all the polygons in both of the input map layers.
Universal soil loss equation (USLE): Method of predicting soil loss over large areas based on relationships between soil loss and determining factors measured empirically using small erosion plots.
Vector data model: A spatial data model using two-dimensional Cartesian ( x , y ) co-ordinates to store the shape of spatial entities.
Vectorization: The process of converting data from raster to vector format.
Viewshed: A polygon map resulting from a visibility analysis showing all the locations visible from a specified viewpoint.
Virtual reality (VR): The production of realistic-looking computer-generated worlds using advanced computer graphics and simulation models.
Virtual Reality Modelling Language (VRML): A computer language developed to assist VR development on the World Wide Web.
Visibility analysis: An analysis of visible features on a terrain surface using a DTM.
Voronoi polygons: See Thiessen polygons''.
Weights: Priorities or preferences attached to criteria in MCE. Usually specified by the user in order to indicate the relative importance of each criterion.
Weird polygons: See Sliver polygons''.
World Wide Web (the Web): Interconnected global network of computers and the software interface used to access and exchange digital information and multimedia.
ZIP code: See Postal code''.
