Definitions of some commonly used GIS (Geographic Information System) terms.

This page: Q | R | S

Q

quadrangle (quad)

See topographic map.

quadtree

A spatial index which recursively decomposes a data set (e.g., image) into square cells of different sizes until each cell has a homogeneous value. Quadtrees are often used for storing raster data. See also spatial indexing.

query

See map query.

R

raster

A cellular data structure composed of rows and columns for storing images. Groups of cells with the same value represent features. See also grid.

RAT

See route attribute table.

RDBMS

Relational database management system. A database management system with the ability to access data organized in tabular files that can be related to each other by a common field (item). An RDBMS has the capability to recombine the data items from different files, providing powerful tools for data usage. See also relate.

real numbers

Decimal numbers (e.g., 3.1417, 0.25, 1.8992, 6.0).

record

1. In an attribute table, a single ‘row’ of thematic descriptors. In SQL terms, a record is analogous to a tuple.

2. A logical unit of data in a file. For example, there is one record in the ARC file for each arc in a coverage.

rectification

The process by which an image or grid is converted from image coordinates to real-world coordinates. Rectification typically involves rotation and scaling of grid cells, and thus requires resampling of values.

referential integrity

The capability to ensure that changes to one table that affect other tables are transmitted to those other tables. For example, a table will not be given a foreign key value that does not exist as a primary key in another table.

region

A coverage feature class used to represent a spatial feature as one or more polygons. Many regions can be defined in a single coverage. Regions have attributes (PAT) that describe the geographic feature they represent.

registered table

A DBMS table which is part of an ArcStorm database is said to be ‘registered’ with the database.

relate

An operation that establishes a temporary connection between corresponding records in two tables using an item common to both (i.e., relate key). Each record in one table is connected to those records in the other table that share the same value for the common item. Compare with relational join.

relate key

The common set of columns used to relate two attribute tables. See also relate, primary key and foreign key.

relation

See table.

relational database

A method of structuring data as collections of tables that are logically associated to each other by shared attributes. Any data element can be found in a relation by knowing the name of the table, the attribute (column) name, and the value of the primary key. See also relate, relate key, and relational join.

relational join

The operation of relating and physically merging two attribute tables using their common item.

remote sensing

Acquiring information about an object without contacting it physically. Methods include aerial photography, radar, and satellite imaging.

resampling

The process of reducing image data set size by representing a group of pixels with a single pixel. Thus, pixel count is lowered, individual pixel size is increased, and overall image geographic extent is retained. Resampled images are “coarse” and have less information than the images from which they are taken. Conversely, this process can also be executed in the reverse. In ArcInfo, the GRID function RESAMPLE supports resampling of raster data using Cubic Convolution, Bilinear Interpolation, Nearest Neighbor Assignment, and custom “Nearest Data” assignment methods.

resolution

1. Resolution is the accuracy at which a given map scale can depict the location and shape of geographic features. The larger the map scale, the higher the possible resolution. As map scale decreases, resolution diminishes and feature boundaries must be smoothed, simplified, or not shown at all. For example, small areas may have to be represented as points.

2. Distance between sample points in a lattice.

3. Size of the smallest feature that can be represented in a surface.

4. The number of points in x and y in a grid or lattice (e.g., the resolution of a U.S. Geological Survey one-degree DEM is 1201 x 1201 mesh points).

restore

To return a database to a previous state by undoing all changes made since the given time. ArcStorm provides a restore mechanism.

RMS error

Root mean square error. A measure calculated when registering a map to a digitizer, indicating the discrepancy between known point locations and their digitized locations. The lower the RMS error, the more accurate the digitizing or transformation. See also tic.

roll back

To cancel any changes to a database made during the current transaction. Compare with commit.

route

A feature class in ArcInfo that is part of the route-system data model used to represent linear features. Routes are based on an arc coverage and are defined as an ordered set of sections. Because sections represent the portion of an arc used in a route, routes do not have to begin or end at nodes. The route attribute table (RAT) stores route attributes. See also route-system and route measure.

route attribute table (RAT)

Route attribute table. An RAT stores route attributes. There is one RAT for each route-system in a coverage. In addition to user-defined attributes, an RAT contains a sequence number and feature identifier for each route. See also route-system, section, SEC, and feature attribute table.

route measure

A location along a route, defined as a measure from a start point, where measures include distance, time, milepost, address range. Measures are useful for locating events along a route. There may be more than one start or end within a route, in which case, the measures may not be unique within a route-system. For example, there are many places that are within a one-minute response time from a fire station.

route system

A collection of routes representing separate instances of a common linear entity, for example, all school bus routes in a city. A single line coverage can contain many route-systems, differentiated by name. For example, a road coverage can contain a bus route-system, a highway route system and a pizza delivery route-system. Both an RAT and an SEC exist for each route system. See also event.

row

1. A record in an attribute table. The horizontal dimension of a table composed of a set of columns containing one data item each.

2. A horizontal group of cells in a grid, or pixels in an image.

RPC

A Remote Procedure Call (RPC) is a communication mechanism which allows one UNIX process to communicate with another UNIX process. These communicating processes can be on different computers over a network. ArcStorm servers and clients use RPCs to communicate with each other.

rubber sheeting

A procedure to adjust coverage features in a nonuniform manner. Links representing from- and to-locations are used to define the adjustment.

run-length encoding

A data compression technique for storing raster or gridded data. Run-length encoding stores data by row. If two or more adjacent cells in a row have the same value, the ‘run’ is recorded, as opposed to recording an individual value for each cell. The more adjacent columns having the same value, the greater the compression.

S

satellite image

A picture of the earth taken from an earth-orbital satellite. Satellite images may be produced photographically or by on-board scanners (e.g., MSS).

scale

See map scale.

scale bar

A map element that shows the map scale graphically.

scanning

The process of capturing data in raster format with a device called a scanner. Some scanners also use software to convert raster data to vector data.

scratch file

A temporary file holding intermediate data during an operation, such as when calculating arc intersections, or building feature topology.

SDTS/TVP

Spatial Data Transfer Standard/Topological Vector Profile. A United States Federal standard designed to support the transfer of different types of geographic and cartographic spatial data. This standard specifies a structure and content for spatially referenced data in order to facilitate data transfer between dissimilar spatial database systems. TVP addresses a wide variety of vector data types, models, and structures, as well as associated attribute data. Also known as Federal Information Processing Standard (FIPS) 173.

SEC

Section table for the section feature class in a coverage. The SEC holds attributes about sections. In addition to user-defined attributes, the SEC contains information on both the route number and arc number to which the section belongs, the starting and ending positions expressed as percentages of the arc length, starting and ending positions expressed as measures along the route, an internal sequence number and a section feature identifier. See also feature attribute table.

section

A feature class in ArcInfo that is a component of the route system data model used to implement routes. They form the infrastructure of route-systems much like arcs form the infrastructure of polygons. Sections are the arcs, or portions of arcs, used to define each route. See also SEC.

section line

A line on a surface defining the position of a profile.

selection coverage

A coverage whose area overlaps that of a map library. It identifies the area to extract data from, or insert data into, a map library.

setuid

A process which can set its effective user to super-user (root). That is, although any user can run the process, the process can then execute operations which require root privileges. One example of a setuid process is the ArcStorm wservice process.

shade symbol

A pattern used to shade polygons in ArcInfo. Shade symbol patterns include crosshatch, repeating, and solid fill.

SIF

Standard interchange format, a spatial data exchange format. A standard or neutral format used to move graphics files between computer systems.

single precision

Refers to a level of coordinate accuracy based on the number of significant digits that can be stored for each coordinate. Single-precision numbers store up to 7 significant digits for each coordinate, retaining a precision of 5 meters in an extent of 1,000,000 meters. ArcInfo data sets can be stored as either single- or double-precision coordinates. See also double precision.

sliver polygon

A small areal feature commonly occurring along the borders of polygons following the topological overlay of two or more coverages.

slope

A measure of change in surface value over distance, expressed in degrees or as a percentage. For example, a rise of 2 meters over a distance of 100 meters describes a 2% slope with an angle of 1.15. Mathematically, slope is referred to as the first derivative of the surface.

SNA

Systems network architecture. Networking protocol popular in IBM environments.

snapping

The process of moving a feature to coincide exactly with coordinates of another feature within a specified snapping distance, or tolerance.

soundex

A phonetic spelling (up to six characters) of a street name, used for address matching. Each of the 26 letters in the English alphabet are replaced with a letter in the soundex equivalent:

English: 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

Soundex: A B C D A B C H A C C L M M A B C

R C D A B W C A C

Where possible, geocoding uses a soundex equivalent of street names for faster processing. During geocoding, initial candidate street names are found using soundex, then real names are compared and verified. See also geocode.

spatial analysis

The process of modeling, examining, and interpreting model results. Spatial analysis is useful for evaluating suitability and capability, for estimating and predicting, and for interpreting and understanding. There are four traditional types of spatial analysis: topological overlay and contiguity analysis, surface analysis, linear analysis, and raster analysis.

spatial data

Information about the location and shape of, and relationships among, geographic features, usually stored as coordinates and topology.

spatial feature

See geographic feature.

spatial indexing

A means of accelerating coverage drawing, spatial selection, and feature identification by generating feature-based indexes for one or more feature classes of a coverage.

spatial interaction

An analytical technique that estimates the number of interactions occurring between an origin and destination locations. The number of interactions is based on the properties of the origin to produce a trip (production), the destination’s attractiveness and the impedance of the link between the two locations. The goal of spatial interaction modeling is to be able to model and predict the number of interactions occurring between populations for a particular type of activity such as retailing.

spatial modeling

Analytical procedures applied with a GIS. There are three categories of spatial modeling functions that can be applied to geographic features within a GIS: (1) geometric models, such as calculating the Euclidean distance between features, generating buffers, calculating areas and perimeters, and so on;

(2) coincidence models, such as topological overlay; and

(3) adjacency models (pathfinding, redistricting, and allocation).

All three model categories support operations on spatial data such as points, lines, polygons, tins, and grids. Functions are organized in a sequence of steps to derive the desired information for analysis. See also model and analysis.

spatial order

An index assigned to features based on their relative closeness in two-dimensional space.

spatial query

See map query.

spike

1. An overshoot line created erroneously by a scanner and its rasterizing software.

2. An anomalous data point that protrudes above or below an interpolated surface representing the distribution of the value of an attribute over an area.

spline

A mathematical curve used to smoothly represent spatial variation. A spline operation inserts vertices to create a curve in an arc. See also grain tolerance and densify.

SQL

Structured Query Language. A syntax for defining and manipulating data from a relational database. Developed by IBM in the 1970s, it has become an industry standard for query languages in most relational database management systems.

SQL/MM

An international standards effort, sponsored by ISO to extend SQL to support multimedia applications, including access and manipulation of geographic data.

SRG

Standardized raster graphic, a digital representation of a map or chart, which is captured by automatic digitization (scanning), stored on a digital storage media, and displayed on a raster screen or raster plotter; obtained by a regular scan of a paper map or chart or repromat. It consists of a raster data set of RGB intensities or colour (sic) codes.

station file

An AML file containing commands needed to establish the environment for graphic display and graphic input. Typically, station files contain commands that define the DISPLAY device, the AML &TERMINAL device, the type of DIGITIZER, if any, and the method to be used for COORDINATE input.

stops stops

Stops are locations visited in a path or tour; they may represent customers on a delivery route or cities in a highway system. Stops and stop attributes are maintained in INFO files referred to as stops files.

stop impedance

The time it takes for a stop transfer to occur. This is used to compute the impedance of a path or tour.

stop transfer

The number of things or packages transferred at a stop. This is used to find the total number of transfers on a path or tour.

string

A series of alphanumeric characters of any length enclosed by quotes.

subclass

A special feature class in a coverage which allows many features of the same class to be defined. Annotation, region, route-system, and section are types of subclasses. For example, a road coverage may have three route-systems stored as subclasses for mail delivery, street cleaning, and garbage pickup.

supply

The availability of services or goods at centers. An example of a type of supply is the availability of milk at grocery stores, cars at a car dealer, or the number of movie screens at a theater.

surface

A geographic phenomenon represented as a set of continuous data, such as elevation or air temperature over an area. A clear or sharp break in values of the phenomenon (breaklines) indicates a significant change in the structure of the phenomenon (e.g., a cliff), not a change in geographic feature. Surfaces can be represented by models built from regularly or irregularly spaced sample points on the surface. See also surface model.

surface model

Digital abstraction or approximation of a surface. Because a surface contains an infinite number of points, some subset of points must be used to represent the surface. Each model contains a formalized data structure, rules, and x,y,z point measurements that can be used to represent a surface. The TIN software package supports two data models for representing surfaces: lattice and tin.

SYBASE

A relational database management system to which ArcInfo has access through the DATABASE INTEGRATOR.

symbol

A graphic pattern used to represent a feature. For example, line symbols represent arc features; marker symbols, points; shades symbols, polygons; and text symbols, annotation. Many characteristics define symbols, including color, size, angle, and pattern. See also text symbol, marker symbol, shade symbol, and line symbol.

symbol environment

Defines the types of map symbols and their characteristics during a graphic display session in ArcInfo. There are four types of active map symbols: line, marker, shade, and text.

system tables

Tables which contain information about a database, such as the data dictionary and database transactions.

Definitions of some commonly used GIS terms:

GIS commonly used terminology abbreviations: