Geographic Information Systems: A Glossary Charles R. Blinn, Lloyd P. Queen and Les W. Maki

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• scale. The ratio or fraction between the size of an object on a map and its size in the real world. A scale represented as 1:15,840 means that one unit of distance on the map represents 15,840 of the same units of distance on the earth.

• scale bar. A map component that graphically depicts the map scale.
• scanning. Process of using an electronic input device (a scanner) to convert analog information from maps, photographs, or overlays into a digital format usable by a computer.
• scroll. To adjust the display window so that the user can view seamless windows across a display.

• sliver. A gap or overlap that is generated by combining two or more coverages that are not coincident or perfectly conflated.

• slope. The rate of rise or fall of a quantity against horizontal distance. It may be expressed as a ratio, decimal, fraction, percentage, or the tangent of the angle of inclination. Contrast with aspect.
• small scale. A map scale that covers a relatively large area and has generalized labels. A large area of the earth's surface on one page is a small-scale map (i.e., a 1:1,000,000 map where 1 map unit equals 1,000,000 ground units is small scale compared with a 1:500 map). Contrast with large scale.
• smoothing. The reduction of the local variability of data and, when applied to a spatially distributed variable, results in a reduction of local variance. Smoothing, applied to a line, results in a reduction in the sharpness of angles between line segments.
• snap. The automatic intersecting of disjoint lines or nodes that arise when map data are being digitized or scanned.
• soft copy. A temporary image of a map or diagram on the screen of a computer display.
• spaghetti digitizing. Refers to the digitizing of map features without any initial regard to the sequence or identification of line/point intersections.
• spatial. Refers to phenomena distributed in space and therefore having physical dimensions and geography.
• spatial analysis. Analytical techniques associated with the study of the location of geographical entities together with their spatial dimensions.
• spatial data. Data pertaining to the location, shape, and relationships among geographical features. These can be classified and stored as point, line, area, polygon, grid cell, or object.
• spatial data transfer standard (SDTS). Federal information processing standard in the United States for exchanging digital cartographic files and associated spatial and attribute data between GIS systems. The purpose of the standard is to ensure that no data will be lost during transfer, that the fidelity of the data and data relationships will be preserved.
• spatial database. A collection of spatial information related by common facts or themes.
• spatial decision support system (SDSS). A customized computer-based information system that utilizes decision rules and models and incorporates spatial data.
• spatial resolution. A measure of the ability of an imaging system, such as LANDSAT, to separate the images of closely adjacent objects. It is also the smallest area identified as a separate mapping unit.
• SPOT. The generic name for a series of natural resource scanning satellites launched by France.
• standards. See data standardization.
• state plane coordinates. A system of x,y geographic coordinates defined individually for each state. Locations are based on the distance from a unique origin for each state or portion of a state.
• stereo pair. Two photographs having sufficient perspective overlap to record parallax of detail to make possible stereoscopic examination of an object or an area common to both photographs. A three-dimensional perspective is provided.
• stereoplotter. A machine which, when loaded with a pair of overlapping aerial photographs, can be used to determine precise location and altitudes of objects that appear in the photographs.
• tabular data. Data (usually attribute) organized into logical tables. Tables contain items and records or rows and columns.
• terrain analysis. Analytical techniques that quantify terrain parameters (slope, aspect) or the effect of terrain on a particular operation.
• tesselation. The process of splitting an area into small, manageable units or subareas. Subareas may consist of tiles or rasters.
• thematic map. A map related to a topic, theme, or subject of discourse. Also called topical, geographic, special purpose, distribution, parametric, or planimetric maps. Thematic maps emphasize a single topic such as vegetation, geology, or land ownership. Contrast with base map.
• theme. The overall topic of a map layer in which the spatial variation of a single phenomenon is illustrated (e.g., a vegetation theme map might illustrate vegetative areas such as hardwoods, conifers, and sage brush).
• thinning. A process whereby a line is generalized through a series of rules that reduce the number of data points while maintaining the basic shape of the feature. See generalize.
• tics. Geographic control points representing known or identifiable locations on the earth's surface (e.g., longitude, latitude). Also called ground control points.
• 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. TIGER files contain street address ranges along lines and census tract and block boundaries.

• tile. A spatial unit by which data are organized, subdivided, and stored. A tile can be a regular, geometric shape (e.g., corresponding to USGS quadrangle map sheets) or an irregular area, such as state boundaries or watersheds. It is used to subdivide large areas into small, manageable units or subareas.
• tool. A computer program provided within a GIS to allow the user to perform a specific set of operations on map and attribute data. Examples of spatial analysis tools include overlay, window, proximity and network analysis, and map algebra.
• topography. The shape of the surface of the earth in a given area.
• topology. The spatial relationships between connecting or adjacent coverage features (e.g., points, lines, and polygons). It provides a way in which geographic features are linked together.
• topological relationships. How data elements relate to each other within the database. Changing one element affects other elements.
• topological structuring. Organizing data topologically so that the relationships and reference linkages are specified.
• transform. The process of changing the scale, projection, or orientation of a mapped image. Sometimes refers to the conversion of data from one GIS system to another.

• triangulated irregular network (TIN). A data structure that describes a three-dimensional surface as a series of irregularly shaped triangles. Often used in connection with terrain modeling where terrain characteristics are determined from sets of irregularly distributed points.
• universal transverse mercator (UTM) grid. A system of plane coordinates based upon 60 north-south trending zones, each 6 degrees of longitude wide, that circle the globe. Used to derive geographic coordinates, normally in meters, east and north of an origin that are defined uniquely for each zone.
• user interface. Method by which the human operator communicates with the various database, system, and applications modules.
• user requirements analysis. A strategic planning approach for implementing a GIS. Provides a comprehensive assessment of the analytical capabilities and products required by potential system users. Sometimes called needs analysis.

• vector data. Data comprised of x-y coordinate representations of locations on the earth that take the form of single points, strings of points (lines or arcs), or closed lines (polygons).

• vector/raster conversion. To convert data from vector or point formats to raster format with grid cell size, position, and orientation selected by the user. It is also known as a vector-to-raster conversion, or rasterization.
• vectorization. See raster/vector conversion.
• vertex (plural vertices). Intermediate points along a line curve, or arc. They represent the critical points of inflection along the arcs, thereby reflecting its shape.
• viewshed modeling. See intervisibility functions.
• warping. See rubber sheeting.
• weighting. Process of systematically increasing the value of a particular data element or elements so as to give that element more significance in the analysis or calculations.
• window. A user-defined area that is used to view a subset of the original map.
• workstation. A computer that consists of a graphic terminal, central processor, digitizer, graphics tablet (optional), and a mouse (optional). It may also be a stand-alone central processing unit (CPU) and its peripheral devices. It is often linked to other computers through a network.
• z coordinate. The attribute coordinate in a data triplet is often used for representing height or elevation in a three-dimensional x,y,z coordinate system.
• zoom. Used in phrases "zoom in," meaning to enlarge the scale of a display to see more detail, and "zoom out," meaning to decrease the scale to see a more overall view. See pan and scroll.

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