### The Ultimate GIS Dictionary: Your Complete Guide to GIS

GIS is more than just *“maps and data”*. Instead, it’s multi-disciplinary.

It impacts various sectors and uses different skill sets.

That’s why we’ve put together this list of GIS definitions to give you 20/20 vision.

From A to Z, sharpen your GIS knowledge with these **GIS dictionary** definitions and meanings.

### Jump to Letter

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

#### A

**Active Sensors**: *[Remote sensing]* Active sensors illuminates their target and measure the reflected backscatter that returns back to the sensor.

**Adjacency**: *[geometry]* Adjacency occurs when two objects share the same boundary and are next to or adjoining with a common side or vertex.

**Advanced Very High Resolution Radiometer (AVHRR)**: *[Remote sensing]* AVHRR collects infrared, visible, and thermal images with an approximate 1-kilometer spatial resolution cell size.

**Affine transformation**: *[geometry]* An affine transformation scales, rotates, skews, or translates points, polylines, and polygons preserving points, straight lines, and planes.

**Arc**: *[data structure]* Arcs are lines or polygon boundaries, represented as a series of vertices or coordinate points.

**ArcCatalog**: *[software]* ArcCatalog is an application in the ArcGIS suite for managing geographic data – similar to windows file explorer.

**ArcGIS**: *[software]* ArcGIS is a GIS software package produced by the Environmental Systems Research Institute (Esri) which allows you to collect, store, manage, visualize, export, analyze and map geographic data.

**ArcGIS Pro**: *[software]* ArcGIS Pro is Esri’s latest GIS software with a ribbon-based user interface, project files and 64-bit processing.

**ArcGlobe**: *[software]* ArcGlobe is a 3D visualization and analysis environment as part of the Esri ArcGIS suite (3D analyst), specializing in global datasets and larger study areas.

**ArcScene**: *[software]* ArcScene is a 3D feature and raster viewer part of the Esri ArcGIS suite of applications (3D analyst) specializing in small study area scenes.

**American Standard Code for Information Interchange (ASCII)**: *[file format]* ASCII uses a set of numbers between 0 and 255 for information storage and processing.

**Aspect**: *[GIS processing]* Aspect is the slope direction on a terrain surface measured clockwise starting north as 0° to 360° north again with flat areas given a value of -1 or 0 degree.

**Atmospheric window**: *[remote sensing]* An atmospheric window are wavelengths at which electromagnetic radiation (sunlight) from the sun will penetrate the Earth’s atmosphere overall constricting these spectrum bands from reaching the Earth.

**Attribute table**: *[data structure]* An attribute table stores non-spatial information in columns and rows about geographic data – similar to spreadsheets.

**Azimuth**: *[surveying]* An azimuth is an angle between 0° and 360° measured clockwise from north based on true north – Magnetic azimuths are based on magnetic north.

#### B

**Base station**: *[surveying]* A base station is a precisely surveyed location used as a fixed Global Navigation Satellite System (GNSS) recording station, typically used in differential correction.

**Basemap**: *[cartography]* A basemap is a background, non-editable, georeferenced image that gives a point of reference on a map, providing aesthetic appeal such as aerial imagery, topography, terrain, and street layers.

**Bearing**: *[surveying]* A bearing is a direction expressed as a geographic angle measured from a baseline used in surveying and navigation.

**Benchmark**: *[surveying]* Benchmarks are precisely surveyed points usually marked with brass or metal disk in the ground also genetically called survey marks, geodetic marks, and control stations.

**Bilinear interpolation**: *[mathematics]* Bilinear interpolation is a technique for calculating values of a grid location by taking the weighted average of the four neighboring cells in an image to generate new values.

**Boolean Algebra**: *[mathematics]* Boolean algebra are conditions used to select features with a set of algebraic conditions including AND, OR, or NOT.

**Buffer**: *[GIS processing]* The buffer tool is a proximity function that creates a polygon at a set distance surrounding a selected feature or features.

**Bundle adjustment**: *[geometry]* Bundle adjustments remove geometric distortions given a set of images of three-dimensional points at different viewpoints, minimizing the error between observed and predicted image points during reprojection.

#### C

**Carrier-phase GPS**: *[surveying]* Carrier-phase GPS signals are relatively slow but are very accurate (within 10cm or better) to determine position on the ground.

**Cartesian coordinate**: *[surveying]* A Cartesian coordinate system specifies each point uniquely with a pair of numerical coordinates or tripless for three-dimensional coordinate spaces.

**Cartography**: *[GIS dictionary]* Cartography is the study, aesthetics and science of representing real-world entities on maps by communicating spatial information.

**Cartometry**: *[GIS dictionary]* Cartometry is a division of cartography concerned with depicting objects with a high level of spatial accuracy such as measuring the perimeter of coasts, areas of countries, volumes, slopes, and densities.

**Centroid**: *[geometry]* A centroid (or geometric center) is a central point of an area feature that represents the average position of all the points in the shape.

**Clip**: *[geometry]* A clip is an overlay tool that involves clipping an input layer to the extent of a defined feature boundary.

**Code-Phase GPS**: *[GPS]* The coarse acquisition (C/A) code or code-phase GPS, which is available to the general public, is a GPS signal that delivers rapid, low accuracy position estimates in tens of meters.

**Coordinate Geometry (COGO)**: *[editing]* COGO involves the entry of spatial coordinate data points, usually obtained from field survey equipment.

**Computer Aided Design/Drafting (CAD)**: *[GIS dictionary]* CAD is primarily used by engineers and architects for the purpose of producing two and three-dimensional drawings.

**Conformal Projection**: *[map projection]* A conformal map projection preserves the correct shapes of small areas with the scale being the same in all directions and greatly distorted areas.

**Conic Projection**: *[map projection]* A conic projection uses a cone to develop its surface on a plane with meridians converging at a single point.

**Connectivity**: *[geometry]* Connectivity is the representation of the connectedness of linear features when arcs share a common node.

**Continuous raster**: *[data structure]* Continuous rasters are grid cells with gradually changing data such as Digital Elevation Models (DEM), temperature data or distance from features.

**Contour Line**: *[data structure]* A contour line is a constant value for mapping any variable such as elevation, temperature, isopleth or isoline maps.

**Control point**: *[coordinate system]* Control points are locations known to have a high degree of accuracy that are used to convert digitized coordinates from paper maps and georeferencing to standard map projection coordinates.

**Coordinate Transformation**: *[coordinate system]* A coordinate transformation is the conversion from a non-projected coordinate system to a coordinate system using a series of mathematical equations.

**Coordinates**: *[coordinate system]* Coordinates are pairs (X, Y) or triplets (X, Y, Z) of values that are used to represent points and features in a two and three-dimensional space.

**Cubic Convolution Interpolation**: *[mathematics]* Cubic convolution interpolation averages the 16 nearest cells, which is generally used for continuous surfaces where much noise exists.

**Cylindrical Projection**: *[map projection]* A cylindrical projection uses a cylinder to develop a plane surface such as Mercator and Plate Carree projections.

#### D

**Data Model**: *[data structure]* The two main GIS data models are rasters (pixels with a specific cell size) and vectors (points, lines and polygons).

**Database Management System (DBMS)**: *[data structure]* A DBMS is a collection of tools that allows the entry, storage, input, output and organization of data, serving as an interface between users and a database.

**Database Schema**: *[data structure]* A database schema is supported in relational database management systems (RDBMS) and acts as blueprints for how database entries will be constructed.

**Developable Surface**: *[map projection]* A developable surface (cylinders, cones, planes, etc) is the geometric shape that a map projection can be mathematically built on.

**Differential Global Navigation Satellite System (GNSS)**: *[surveying]* Differential GNSS is a highly accurate (within centimeters) conventional surveying technique that uses a known location from a receiver to determine an unknown position.

**Digital Elevation Model (DEM)**: *[GIS processing]* A Digital Elevation Model (DEM) is a bare earth elevation model representing the surface of the Earth without features like houses, bridges, and trees.

**Digital Line Graph (DLG)**: *[file format]* DLG is vector data format developed and distributed by the United States Geological Survey (USGS) depicting geographic features like terrain, hydrography, transportation and man-made features.

**Digital Orthophotos Quadrangle (DOQ)**: *[file format]* DOQ is a geometrically-corrected photograph produced by the USGS with a scale of 1:40,000 and spatial resolution of about 1-meter pixels.

**Digital Raster Graphics (DRG)**: *[file format]* DRG is a digital version map of USGS topographic maps including imagery (NAIP), roads, place names, hydrography, elevation contours, and boundaries.

**Digital Surface Model (DSM)**: *[file format]* A digital surface model is an elevation model that includes the top of buildings, tree canopy, powerlines and other features above the bare earth.

**Digital Terrain Model (DTM)**: *[file format]* DTM is a bare earth representation of the Earth’s surface that is augmented by natural features such as ridges and breaklines.

**Digitize**: *[editing]* When you digitize in a GIS, you are creating geographic computer-compatible stored data with lines, points, and polygons in a spatial database.

**Dissolve tool**: *[GIS processing]* The dissolve tool is a common GIS processing task where boundaries are merged with neighboring boundaries based on common attribute values.

#### E

**Easting**: *[coordinate system]* Eastings refer to the x-axis (eastward) points approximately parallel to lines of equal latitude commonly used in the Universal Transverse Mercator map projection.

**Electromagnetic (EM) Spectrum**: *[remote sensing]* The EM spectrum refers to the range of energy wavelengths or frequencies from x-rays, ultraviolet, visible, infrared, and microwave to radio waves.

**Ellipsoid**: *[surveying]* Reference ellipsoids are mathematical models of the shape of the Earth with the major axis along the equatorial radius, primarily used as a surface to specify point coordinates such as latitude (north/south), longitude (east/west) and elevation (height).

**Ellipsoid Height**: *[surveying]* The ellipsoid height is the height measured from an ellipsoidal surface to a point on the surface of the Earth.

**Endlap**: *[remote sensing]* Endlap is the coverage overlap in aerial photographs from end to end between flight lines.

**Enhanced Thematic Mapper (ETM+)**: *[remote sensing]* ETM+ is a sensor equipped on Landsat-7 which produces images of Earth in 8 spectral bands (blue, green, red, NIR, MIR, panchromatic, and thermal band).

**Entity**: *[cartography]* Entities represent a real-world point, line, or polygon features with a geographic location such as fire hydrants, hospitals, state boundaries, roads, rivers, lakes, etc.

**Equal Interval Classification**: *[cartography]* The equal interval classification method divides attribute values into equal size ranges such as 0-20, 21-40, 41-60, 61-80, and 81-100.

**Earth Resources Data Analysis System (ERDAS) Imagine**: *[software]* ERDAS Imagine is a leading remote sensing software owned by Hexagon Geospatial.

**Esri (Environmental Systems Research Institute)**: *[software]* Esri is the largest global supplier and developer of GIS software based in Redlands, California.

#### F

**Facet**: *[data structure]* A facet is a triangular face in a Triangular Irregular Network (TIN). TINs are three-dimensional coordinates depicting elevation surfaces.

**False northing**: *[coordinate system]* A false northing is a number added in a map projection in the y direction to avoid negative coordinate locations within the specified area of that map projection.

**Feature**: *[data structure]* A feature is a cartographic point, line, or polygon object with a spatial location in the real-world landscape that can be used in a GIS for storage, visualization, and analysis.

**Field (Attribute Table)**: *[data structure]* An attribute field (or item) are characteristics used to describe each feature in a geographic data set usually viewed as columns in a table.

**Federal Information Processing Standard (FIPS)**: *[organization]* FIPS is a federal unique identifier code used to define political or physical features in the United States.

**Flow direction**: *[GIS processing]* Flow direction calculates the direction water will flow using the slope from neighboring cells.

**Focal Operation**: *[mathematics]* The focal operation is a spatial function that computes an output value of each cell using neighborhood values such as convolution, kernel, and moving windows.

#### G

**GDAL**: *[GIS software]* Geospatial Data Abstraction Library (GDAL) is a C++ library for reading and writing raster geospatial data formats, implementing common GIS operations (unions, intersections, joins, clipping, etc.) with command line utilities.

**Generalization Tool**: *[GIS processing]* The generalization tool is an editing process that simplifies the shapes by reducing the number of vertices in a line or polygon.

**Geocentric**: *[geodesy]* Geocentric is a measuring system that uses the center of the Earth as its origin, such as the WGS 1984 datum.

**Geocoding**: *[GIS processing]* Geocoding is the process of assigning geographic coordinates to places based on street address, town/city, province/state and country.

**Geodesy**: *[GIS dictionary]* Geodesy (or Geodetics) is a branch of applied mathematics and earth science of locating and assigning three-dimensional points on Earth by measuring the shape of the Earth.

**Geofencing**: *[GIS dictionary]* A virtual geographic boundary designed to give real-time alerts and increase awareness.

**Geographic Information Systems (GIS)**: *[GIS dictionary]* GIS is a computer-based tool that analyzes, stores, manipulates and visualizes geographic information on a map, good for finding spatial patterns, relationships and trends.

**Geoid**: *[geodesy]* A geoid is a measurement-based equipotential surface model of the shape of the Earth primarily used as a basis of assigning terrain height.

**Geomedia**: *[software]* GeoMedia is a GIS software package produced by Hexagon Geospatial (previously Intergraph) used to analyze, store and visualize geographic data.

**Global Mapper**: *[software]* Global Mapper is a commercial GIS software product for spatial analysis and visualization.

**Global Operation**: *[mathematics]* A global operation is a process or function that is performed on each output cell using all of the cells of the input raster.

**Gnomonic Projection**: *[map projection]* A Gnomonic projection uses the center of the spheroid as the projection center.

**GRASS GIS**: *[software]* GRASS GIS (Geographic Resource Analysis Support System) is an open source GIS software package, highly used in academic institutions with over 400 modules for GIS analysis.

**Graticule**: *[coordinate system]* Graticules are lines of latitude or longitude on a digital or hard copy map and assist in showing the geographic locations of map features.

**Greenwich Meridian (Prime Meridian)**: *[coordinate system]* The Greenwich meridian passes through the Royal Observatory in Greenwich, England, and is defined as a 0° line of longitude.

**Geodetic Reference System 1980 (GRS 80)**: *[geodesy]* GRS80 is a geodetic reference system consisting of a global reference ellipsoid and a gravity field model (geoid).

#### H

**Horizontal Datum**: *[geodesy]* A horizontal datum provides a frame of reference as a basis for placing locations at a specific latitude and longitude points on the spheroid.

**Hydrography**: *[GIS dictionary]* Hydrography is a term describing the geographic representation of water features such as streams, rivers, and lakes.

**Hypsography**: *[ GIS dictionary ]* Hypsography is the geographic representation of features on a map related to elevation, altitude, and height above sea level from a reference surface. (Hypso is Greek for height).

#### I

**Idrisi**: *[software]* Idrisi is a GIS/remote sensing software package developed by Clark Laboratories in 1987, widely used in the education system.

**Inverse Distance Weighted (IDW) Interpolation**: *[mathematics]* IDW interpolation is a technique used to estimate values of unsampled locations based on the value and distance of known, sampled locations.

**IKONOS**: *[remote sensing]* IKONOS is a high-resolution commercial imaging satellite that provides 1-meter panchromatic and 3-meter multispectral (blue, green, red, near-infrared) imagery.

**Infrared**: *[remote sensing]* An infrared image represents reflectance grid cells that are recorded in the near-infrared wavelengths, typically 0.7 to 1.1 micrometers.

**Interpolation**: *[mathematics]* Interpolation is the estimation of unsampled locations based on known location sampled values such as elevation, temperature, and other predicted variables.

**Intersect Tool (Overlay)**: *[GIS processing]* The intersect tool uses two inputs and restricts the output geometry similar to a clip by preserving the attributes in both input layers.

#### K

**Kriging**: *[GIS processing]* Kriging is a statistical interpolation technique that uses known location values to interpolate values at unknown locations and also estimates standard error and uncertainty.

#### L

**Land Information System (LIS)**: *[GIS dictionary definition]* LIS was a name originally applied for GIS systems specifically developed for property ownership and boundary records management.

**Landsat**: *[remote sensing]* Landsat consists of 8 satellite missions spanning more than 4 decades with space-borne remote sensing capabilities for scanning land resources.

**Latitude**: *[coordinate system]* Latitudes are spherical coordinates of Earth locations that vary in north-south directions ranging from 0° at the Equator to 90° (North or South) at the poles.

**Light Detection and Ranging (LiDAR)**: *[remote sensing]* LiDAR uses laser pulse measurements to identify heights, depths, and other properties of features on the Earth’s surface.

**Local Operator**: *[GIS processing]* A local operation is a spatial operation where the output extent is limited to the same location as the input such as arithmetic, statistical, relational, trigonometric, exponential, and logarithmic operations.

**Locator Map**: *[cartography]* Locator maps show the extent of the primary map in relation to a larger extent area giving context to the map reader to gain a better understanding of the overall area.

**Longitude**: *[coordinate system]* Longitudes are coordinate on Earth in east-west directions ranging from 0° to +180° east and −180° west.

#### M

**Magnetic North Pole**: *[geodesy]* The magnetic north is a point in Northern Canada where the northern lines of attraction enter the Earth and which compass needles orient in direction.

**Manifold GIS**: *[software]* Manifold is a GIS software package used for mapping with a focus on speed in analysis and visualization.

**Map Algebra**: *[mathematics]* Map algebra is the combination of spatial data using mathematical, statistical, and trigonometric operations to generate new raster outputs.

**Map Legend**: *[cartography]* A map legend is a visual graphic of the symbology used in a map that tells the map reader what polygons, lines, points, or grid cells represent.

**Map Projection**: *[cartography]* A map projection is a systematic rendering of features that transforms a 3D ellipsoid or spheroid of Earth into a 2D map surface with some distortion.

**MapInfo**: *[software]* MapInfo by Precisely (founded in 1986) is a GIS location intelligence suite that allows users to manage, view, and create geospatial data.

**Meridian**: *[coordinate system]* Meridians are lines of longitude that vary in east-west directions ranging from 0° to +180° east and −180° west.

**Metadata**: *[data structure]* Metadata is data that describes data such as the date, abstract, coordinate system, attribute information, origin, and accuracy.

**Minimum Mapping Unit**: *[remote sensing]* A minimum mapping unit is the smallest resolution area when interpreting remotely-seemed satellite or aerial imagery.

**Moderate Resolution Imaging Sensor (MODIS)**: *[remote sensing]* MODIS is a high spectral resolution and moderate spatial resolution (250-1000 meters) imaging scanner.

**Moving Window**: *[mathematics]* A moving window is a rectangular arrangement of cells that applies an operation to each cell in a raster dataset while shifting in a position entirely.

**Multispectral Imagery**: *[remote sensing]* A multispectral image had several channels consisting of several spectral bands of wavelengths such as red, green, blue, and NIR.

#### N

**Nadir**: *[remote sensing]* Nadir is the point directly below the aircraft which is usually near the center of the aerial image.

**National Land Cover Dataset (NLCD)**: *[organization]* The NLCD is a Landsat Thematic Mapper (TM) based classification of land cover in the United States.

**National Oceanic and Atmospheric Administration (NOAA)**: *[organization]* NOAA is the US government agency that oversees the development of national datums and several weather and ocean satellites.

**National Wetlands Inventory (NWI)**: *[organization]* The NWI is a dataset compiled by the US Fish and Wildlife Services that describes the type and extent of wetlands in North America.

**Nearest Neighbor Interpolation**: *[mathematics]* Nearest neighbor interpolation involves taking the output value from the nearest input layer cell center, commonly used for discrete data like land covers.

**Neat Line**: *[cartography]* A neat line is a solid border cartographic element that surrounds all the data, legend, scale, and other features helping the map readers focus on the inner details in a map.

**Neighborhood Operation**: *[mathematics]* A neighborhood operation is a spatial function where the output location, area, and extent come from areas larger than and adjacent to the input cells.

**Network**: *[data structure]* A network in GIS is a connected set of line features used to model for and demand through real-world networks such as rivers and roads.

**Node**: *[data structure]* A node is an important point along a line feature where two lines intersect.

**Nominal Scale**: *[cartography]* A nominal scale is a type of measurement that indicates the difference between classes or categories of data.

**North American Datum 1927 (NAD27)**: *[geodesy]* NAD27 is the adjustment of long-baseline surveys to establish a network of standardized horizontal positions in North America using the Clarke Ellipsoid of 1866 and a fixed center at Meade’s Ranch, Kansas.

**North American Datum of 1983 (NAD83)**: *[geodesy]* NAD83 is the successor of NAD27 that creates a set of standard horizontal positions for North America based on the reference ellipsoid GRS80.

**North American Vertical Datum of 1929 (NAVD29)**: *[geodesy]* NAVD29 or National Geodetic Vertical Datum of 1929 (NGVD29) gives a vertical measurement network reference frame for which vertical elevations and depressions can be based in North America.

**North American Vertical Datum of 1988 (NAVD88)**: *[geodesy]* NAVD88 is a vertical datum that provides a reference surface in which vertical heights (altitude) and depth (depression) above and below mean sea level is measured.

**North Arrow**: *[cartography]* A north arrow provides an orientation to the map reader by showing the north direction as a graphical depiction.

**Northing**: *[coordinate system]* A northing is the axis in the approximate north-south direction (or y-value) in UTM and other standard coordinate systems.

#### O

**Object Based Image Analysis (OBIA)**: *[remote sensing]* OBIA is an image classification technique that segments images and classifies them using spectral, spatial, and relational properties and characteristics.

**Ordinal Scale**: *[coordinate system]* An ordinal scale represents the relative order of values but does not record the magnitude of differences between values.

**Orthographic Projection**: *[map projection]* An orthographic projection is a map projection that represents 3D data in 2D where all the projection lines are orthogonal to the projection plane.

**Orthometric Height**: *[map projection]* Orthometric height is a height measured from the Geoid surface to a point on the surface of the Earth.

**Overshoot**: *[editing]* An overshoot is a digitized line that extends past the intended line of connection.

#### P

**Panchromatic Image**: *[editing]* A panchromatic image records wavelengths in only one wider range band resulting in grayscale images.

**Parallax **: *[remote sensing]* Parallax measures the apparent shift in relative positions of Earth features when it is viewed in different locations.

**Passive Sensors**: *[remote sensing]* Passive sensors measure natural energy from the sun as reflected sunlight or thermal radiation such as Landsat, SPOT, and GeoEye.

**Pixels (GIS)**: *[data structure]* Pixels are the grid cells that make up raster images, which are identical in size.

**Planar Topology**: *[editing]* Planar topology requires that intersections for lines and polygons in a digital data layer are enforced and that no two lines or polygons cross.

**Polygon**: *[data structure]* A polygon is a closed, connected set of lines that defines a geographic boundary with an area and perimeter such as lakes, forests, and country boundaries.

**Positional Dilution of Precision (PDOP)**: *[data structure]* PDOP is a quantitative measurement used to represent the quality of the satellite geometry when taking GPS readings.

**Public Land Survey System (PLSS)**: *[organization]* The PLSS is a land measurement system used in the western United States to define parcel boundaries and locations.

#### Q

**QGIS**: *[software]* QGIS (formerly Quantum GIS) is a free and open source software package that allows you to create, edit, visualize, analyze and publish geospatial information.

**Quad-Trees**: *[remote sensing]* Quad-Trees are raster data structures based on successive reduction of homogeneous cells for the purpose of reducing storage requirements.

**Quantile Classification**: *[cartography]* Quantile classification is a classification method that divides classes so that the total number of features in each class is approximately the same.

**Query**: *[programming]* A query is a request or search of spatial or tabular data based on user-defined criteria, resulting in a subset of selected records.

#### R

**Random Location Sampling**: *[statistics]* Random location sampling is a statistical technique where sample locations are selected by a random process with minimal biases.

**Raster Data**: *[data structure]* A raster is a data model used in GIS which is usually regularly-size rectangular or square shaped grid cells arranged in rows in columns.

**Raster Resampling**: *[mathematics]* Raster resampling is a technique used to recalculate and assign new cell values when adjusting the cell size or orientation of a raster grid.

**Record**: *[data structure]* A record is an attribute table row that specifies an instance or spatial feature of an entity.

**Registration **: *[GIS processing]* Registration is the alignment or assignment of coordinates from a non-projected coordinate system to a coordinate system.

**Remote Sensing**: *[GIS dictionary definition]* Remote sensing is the science of obtaining information about the Earth without physically being there, such as by satellite, unmanned aerial vehicle and aircraft.

**Rubbersheeting **: *[mathematics]* Rubbersheeting is a transformation technique using polynomial or other non-linear functions to match feature geometry.

**Rotate**: *[mathematics]* Rotation is an affine transformation that changes polylines, polygons in a plane by changing the angle with a pivot location.

#### S

**Scale Bar**: *[mathematics]* A scale bar graphically shows the proportional distance on the map with units such as kilometers or meters.

**Scale**: *[mathematics]* An affine transformation changes points, polylines, polygons in a plane by scaling, rotating, skewing, or translating coordinates in two or three-dimensional spaces.

**Selection Tool**: *[GIS processing]* The selection tool identifies an object or set of objects based on user-selected criteria and properties, typically with Structured Query Language (SQL).

**Semi-Major/Minor Axis**: *[geodesy]* The semi-major axis is one-half of the major axis that is the larger of the two radial axes that define an ellipsoid.

**Semivariance**: *[statistics]* Semivariance is the variance between valued samples given an inter-sample distances known as lag.

**Server**: *[hardware]* A server is a computer component that offers data storage and organization providing subsets of GIS data in response to user requests.

**Set Algebra**: *[mathematics]* Set algebra is a method to select items in a database based on operators such as “greater than”, “less than”, “equal to” or ‘not equal to’ using command lines such as SQL.

**Shaded Relief**: *[remote sensing]* A shaded relief map displays the brightness and shadows of terrain reflection given a sun angle and direction of sunlight.

**Sidelap**: *[remote sensing]* Sidelap (or side overlap) consists of the overlapping edge areas of photographs between adjacent flight lines.

**Skeletonize**: *[GIS processing]* Skeletonizing is the process of thinning a raster line to a single pixel width, typically for conversion to vector data format.

**Skew**: *[mathematics]* Skewing distorts a feature by arranging its vertices in the x or y-direction.

**Sliver**: *[editing]* A sliver is a small, spurious gap between polygons often considered as a topology error from imprecise digitization of features.

**Slope**: *[mathematics]* Slope is the change in elevation or steepness with respect to change in location measured in degrees or percent slope.

**Snapping Environment**: *[editing]* Snapping determines the distance that newly digitized points, lines, and polygons will occupy the same location as existing features.

**Soil Survey Geographic Database (SSURGO)**: *[organization]* SSURGO is a fine-scale, county-level, soil survey database prepared by the National Resource Conservation Service.

**Spaghetti Data Model**: *[data structure]* The spaghetti data model is a simple and dated GIS model where lines may cross without intersecting or topology without attributes.

**Spatial Relationship**: *[data structure]* A spatial relationship that links features geographically with a table by a unique identifier.

**Spherical Coordinates**: *[coordinate system]* Spherical coordinates is a coordinate system based on a sphere defined by two angles of rotation in orthogonal planes such as latitudes and longitudes in a geographic coordinate system.

**Spheroid**: *[geodesy]* A spheroid is a three-dimensional mathematical model representing the shape of the Earth based on the equation of a sphere.

**Spline**: *[mathematics]* A spline is an interpolation method that generates a smooth surface by using multiple constrained polynomial functions minimizing the overall surface curvature.

**State Plane Coordinates**: *[coordinate system]* State Plane Coordinates is a standardized system in the United States based on Lambert conformal conic and transverse Mercator projections.

**State Soil Geographic (STATSGO)**: *[organization]* STATSGO is a coarse resolution digital soil dataset from detailed and derived soil survey maps.

**Stereo Pairs**: *[remote sensing]* Stereo pairs are overlapping photos taken at different positions that have parallax.

**Stereographic Projection**: *[map projection]* A stereographic projection is a mapping function that projects an entire sphere on a plane, except at the projection point.

**Structured Query Language (SQL)**: *[map projection]* SQL is a set of commands used to manage data stored in relational database management systems (RDBMS).

**Survey Station**: *[surveying]* Survey stations are position where surveys are performed during land surveys.

**Systematic Sampling**: *[statistics]* Systematic sampling involves selecting samples using a regular and ordered sampling framework.

**Système Pour l’Observation de la Terre (SPOT) Satellites**: *[remote sensing]* SPOT satellites are a group of French high resolution optical imaging Earth observation satellite which has been in operation for nearly 35 years.

#### T

**Terrestrial Reference System**: *[geodesy]* The terrestrial reference system is a set of measured points used to define a geodetic datum.

**Thematic Layer**: *[data structure]* A thematic layer is a distinct spatial entity in a data layer that is usually delineated as points, lines, and polygons.

**Thematic Mapper (TM)**: *[remote sensing]* The Thematic Mapper is a high-resolution scanner on Landsat satellites (Landsat 4 and 5) that collects images in visible, near infrared, mid infrared, and thermal bands of the EM spectrum.

**TNTmips**: *[software]* TNTmips is a GIS software application produced by Microimages that specializes in image manipulation and vector processing.

**Topography**: *[software]* Topography is the study and mapping of Earth’s features including land surfaces, relief, natural, and constructed features.

**Topologically Integrated Geographic Encoding and Referencing (TIGER)**: *[organization]* TIGER is the central hub for United States census data with an inventory of other spatial data related to rivers, lakes, buildings, cities, political areas, and roads.

**Topology**: *[editing]* Topology is a set of rules that dictates the spatial properties of point, line, and area features such as connectivity, adjacency and contiguity.

**Traverse**: *[surveying]* A traverse is a set of survey stations spanning along a survey and are closed when returning to a starting point.

**Triangular Irregular Networks (TIN)**: *[data structure]* TINs are vector-based models depicting three-dimensional elevation surface terrains.

#### U

**Undershoot**: *[editing]* An undershoot is considered to be a digitizing error in which the new line falls short of the intended connection segment.

**Union Tool**: *[GIS processing]* The union tool spatially combines two data layers preserving the features and attributes from both layers at the same extents.

**United States Geological Survey (USGS)**: *[organization]* USGS is a United States agency responsible for Landsat satellites, nationwide map-making, and spatial data development.

**Universal Transverse Mercator (UTM)**: *[coordinate system]* UTM coordinate system is a standard set of map projections with a central meridian for each six-degree wide UTM zone.

#### V

**Variable Distance Buffer**: *[GIS processing]* Variable distance buffer uses a feature attribute from the input to create a ring around the feature that varies in distance.

**Vector Data Model**: *[data structure]* A vector data model is a common GIS feature representation of spatial information based on defining coordinates and attribute information in points, polylines and polygons.

**Vertex**: *[data structure]* A vertex is a point that specifies a position on a line for arcs, polylines polygons.

**Vertical Datum**: *[geodesy]* A vertical datum is a reference surface to which vertical heights are measured from the zero surfaces to which elevations or depths are referred.

**Vertical Dilution of Precision (VDOP)**: *[GPS]* VDOP is a quantitative measurement used to represent the quality of the height based on satellite geometry when taking GPS readings.

#### W

**Wavelength**: *[remote sensing]* An electromagnetic wave produce sinusoidal patterns with the distance between the peak and trough of a wave for successive waves.

**Web Mapping Service (WMS)**: *[Web Mapping]* A WMS is pre-established tiles on the server side for viewing only that have features pre-cached at a set of scales.

**Web Feature Service (WFS)**: *[Web Mapping]* A Web Feature Service allows you to visualize and edit features (points, lines, and polygons) over the internet.

**Wide Area Augmentation System (WAAS)**: *[GPS]* WAAS is a satellite-based signal correction that improves the estimation of GPS positions primarily by removing atmospheric distortion.

**World Geodetic System 1984**: *[geodesy]* WGS84 is a reference ellipsoid used for defining spatial locations in three dimensions.

#### Z

**Zenith**: *[mathematics]* Zenith is the angle measured directly in the z-direction (vertical direction opposite to the apparent gravitational force) on the imaginary celestial sphere at that location.

**Zonal Operation**: *[mathematics]* A zonal operation is a spatial function that computes an output value of each cell using the zone containing that cell.