Geospatial Analysis Vocabulary

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Adjacency

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The sharing of a common side or boundary by two or more polygons (AGI). Note that adjacency may also apply to features that lie either side of a common boundary where these features are not necessarily polygons

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Arc

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Commonly used to refer to a straight line segment connecting two nodes or vertices of a polyline or polygon. Arcs may include segments or circles, spline functions or other forms of smooth curve. In connection with graphs and networks, arcs may be directed or undirected, and may have other attributes (e.g. cost, capacity etc.)

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Artifact

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A result (observation or set of observations) that appears to show something unusual (e.g. a spike in the surface of a 3D plot) but which is of no significance. Artifacts may be generated by the way in which data have been collected, defined or re-computed (e.g. resolution changing), or as a result of a computational operation (e.g. rounding error or substantive software error). Linear artifacts are sometimes referred to as “ghost lines”

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Aspect

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A data item associated with an individual object (record) in a spatial database. Attributes may be explicit, in which case they are typically stored as one or more fields in tables linked to a set of objects, or they may be implicit (sometimes referred to as intrinsic), being either stored but hidden or computed as and when required (e.g. polyline length, polygon centroid). Raster/grid datasets typically have a single explicit attribute (a value) associated with each cell, rather than an attribute table containing as many records as there are cells in the grid

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Azimuth

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The horizontal direction of a vector, measured clockwise in degrees of rotation from the positive Y-axis, for example, degrees on a compass

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Azimuthal Projection

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A type of map projection constructed as if a plane were to be placed at a tangent to the Earth's surface and the area to be mapped were projected onto the plane. All points on this projection keep their true compass bearing

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Autocorrelation (Spatial)

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The degree of relationship that exists between two or more (spatial) variables, such that when one changes, the other(s) also change. This change can either be in the same direction, which is a positive autocorrelation, or in the opposite direction, which is a negative autocorrelation

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Cartogram

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A cartogram is a form of map in which some variable such as Population Size or Gross National Product typically is substituted for land area. The geometry or space of the map is distorted in order to convey the information of this alternate variable. Cartograms use a variety of approaches to map distortion, including the use of continuous and discrete regions. The term cartogram (or linear cartogram) is also used on occasion to refer to maps that distort distance for particular display purposes, such as the London Underground map

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Choropleth

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A thematic map [i.e. a map showing a theme, such as soil types or rainfall levels] portraying properties of a surface using area symbols such as shading [or color]. Area symbols on a choropleth map usually represent categorized classes of the mapped phenomenon

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Conflation

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A term used to describe the process of combining (merging) information from two data sources into a single source, reconciling disparities where possible (e.g. by rubber-sheeting — see below). The term is distinct from concatenation which refers to combinations of data sources (e.g. by overlaying one upon another) but retaining access to their distinct components

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Contiguity

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The topological identification of adjacent polygons by recording the left and right polygons of each arc. Contiguity is not concerned with the exact locations of polygons, only their relative positions. Contiguity data can be stored in a table, matrix or simply as [i.e. in] a list, that can be cross-referenced to the relevant co-ordinate data if required

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Curve

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A one-dimensional geometric object stored as a sequence of points, with the subtype of curve specifying the form of interpolation between points. A curve is simple if it does not pass through the same point twice (OGC). A LineString (or polyline — see below) is a subtype of a curve

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Datum

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Strictly speaking, the singular of data. In GIS the word datum usually relates to a reference level (surface) applying on a nationally or internationally defined basis from which elevation is to be calculated. In the context of terrestrial geodesy datum is usually defined by a model of the Earth or section of the Earth, such as WGS84 (see below). The term is also used for horizontal referencing of measurements

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DEM

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Digital elevation model (a DEM is a particular kind of DTM, see below)

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DTM

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Digital terrain model

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EDM

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Electronic distance measurement

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EDA, ESDA

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Exploratory data analysis/Exploratory spatial data analysis

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Ellipsoid/Spheroid

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An ellipse rotated about its minor axis determines a spheroid (sphere-like object), also known as an ellipsoid of revolution (see also, WGS84)

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Feature

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Frequently used within GIS referring to point, line (including polyline and mathematical functions defining arcs), polygon and sometimes text (annotation) objects (see also, vector)

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Geodemographics

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The analysis of people by where they live, in particular by type of neighborhood. Such localized classifications have been shown to be powerful discriminators of consumer behavior and related social and behavioral patterns

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Gradient

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Used in spatial analysis with reference to surfaces (scalar fields). Gradient is a vector field comprised of the aspect (direction of maximum slope) and slope computed in this direction (magnitude of rise over run) at each point of the surface. The magnitude of the gradient (the slope or inclination) is sometimes itself referred to as the gradient (see also, Slope and Aspect)

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Heuristic

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A term derived from the same Greek root as Eureka, heuristic refers to procedures for finding solutions to problems that may be difficult or impossible to solve by direct means. In the context of optimization heuristic algorithms are systematic procedures that seek a good or near optimal solution to a well-defined problem, but not one that is necessarily optimal. They are often based on some form of intelligent trial and error or search procedure

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iid

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An abbreviation for “independently and identically distributed”. Used in statistical analysis in connection with the distribution of errors or residuals

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Invariance

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In the context of GIS invariance refers to properties of features that remain unchanged under one or more (spatial) transformations

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Kernel

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terally, the core or central part of an item. Often used in computer science to refer to the central part of an operating system, the term kernel in geospatial analysis refers to methods (e.g. density modeling, local grid analysis) that involve calculations using a well-defined local neighborhood (block of cells, radially symmetric function)

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Map algebra

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A range of actions applied to the grid cells of one or more maps (or images) often involving filtering and/or algebraic operations. These techniques involve processing one or more raster layers according to simple rules resulting in a new map layer, for example replacing each cell value with some combination of its neighbors’ values, or computing the sum or difference of specific attribute values for each grid cell in two matching raster datasets

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Resampling

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Procedures for (automatically) adjusting one or more raster datasets to ensure that the grid resolutions of all sets match when carrying out combination operations. Resampling is often performed to match the coarsest resolution of a set of input rasters. Increasing resolution rather than decreasing requires an interpolation procedure such as bicubic spline.

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Rubber sheeting

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A procedure to adjust the co-ordinates all of the data points in a dataset to allow a more accurate match between known locations and a few data points within the dataset. Rubber sheeting … preserves the interconnectivity or topology, between points and objects through stretching, shrinking or re-orienting their interconnecting lines (AGI). Rubber-sheeting techniques are widely used in the production of Cartograms (op. cit.)

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Slope

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The amount of rise of a surface (change in elevation) divided by the distance over which this rise is computed (the run), along a straight line transect in a specified direction.

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Spheroid

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A flattened (oblate) form of a sphere, or ellipse of revolution. The most widely used model of the Earth is that of a spheroid, although the detailed form is slightly different from a true spheroid

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Tesseral/Tessellation

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gridded representation of a plane surface into disjoint polygons. These polygons are normally either square (raster), triangular (TIN — see below), or hexagonal

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TIN

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Triangulated irregular network. A form of the tesseral model based on triangles.

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Transformation

2. Affine

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Affine transformation: When a map is digitized, the X and Y coordinates are initially held in digitizer measurements. To make these X,Y pairs useful they must be converted to a real world coordinate system.

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Viewshed

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Regions of visibility observable from one or more observation points.

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[a,b]

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A closed interval of the Real line, for example [0,1] means the set of all values between 0 and 1, including 0 and 1

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(a,b)

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An open interval of the Real line, for example (0,1) means the set of all values between 0 and 1, NOT including 0 and 1. This should not be confused with the notation for coordinate pairs, (x,y), or its use within bivariate functions such as f(x,y), or in connection with graph edges (see below) — the meaning should be clear from the context

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(x,y,z)

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A (spatial) data triple, usually representing a pair of coordinates in two dimensions, plus a third coordinate (usually height or depth) or an attribute value, such as soil type or household income