Intergovernmental Information Systems Advisory Council


NAIS Implementation Guide Glossary

absolute accuracy
See 'positional accuracy'.

applications development
The creation of highly specific computer software tools tailored for a particular purpose or related set of purposes. This often is done to reduce the amount of operator learning required in order to accomplish a specific set of tasks with a computer. Scripting or macro languages are often used to create these applications.

board digitizing
The hand guided electronic tracing of hard copy maps on a digitizing board.

computer literacy
Familiarity with the basic functions and methods of computer use and the major operating systems and types of software. This includes a general understanding of how to communicate with computers and learn to use new computer software.

coordinate shift
A process used to preserve double precision accuracy in a GIS dataset when working with it in a single precision GIS processing environment.

coordinate geometry (COGO)
This refers to a data conversion process in which a digital map is constructed from written descriptions, such as legal descriptions of land parcel boundaries. These descriptions often contain information about line length and direction, and point locations, all relative to the locations of certain key features such as geodetic control points, survey monuments, previously COGOed survey lines, etc.

Coordinate systems
Imaginary grids superimposed on the earth's surface that can be used to reference the exact or absolute location of a feature on the earth.

data conversion
Converting spatial data into a GIS digital format. This can take the form of board digitizing or scanning and vectorizing hard copy maps, COGO from written descriptions such as legal descriptions of land parcel boundaries, or conversion from one digital format to another.

data form
The form in which the data exists. This can include hard copy (paper, mylar, etc.), digital (ASCII, binary, bit map, etc.).

data format
The particular logical format of the data. Usually software specific. See 'digital format'.

data model
The particular way a GIS software handles data. "1. The result of the conceptual design process. A generalized, user-defined view of the data related to applications." "2. A formal method of describing the behavior of the real-world entities. A fully developed data model specifies entity classes, relationships between entities, integrity rules and operations on the entities." [source: ESRI]

A collection of related data maintained in a computer readable electronic format. This collection will have a tabular form, that is, organized into rows and columns in which each column (also called a field or item) represents a kind of information and each row (also called a record) represents an instance of information. See 'tabular data' and 'GIS dataset'.

digital form
Data maintained in a computer readable electronic form.

digital formats
A specific logical format in which digital data is maintained. This is often software specific.

digital map
A map maintained in a computer readable electronic format.

digital maps
Maps maintained in a computer readable electronic format that can be displayed to a computer screen and plotted on paper with the appropriate software.

digital elevation model (DEM)
A regular pattern of ground elevation observations maintained in digital form. These can be used to generate contour lines and Triangulated Irregular Networks (TINs) which look like wire frame representations of the terrain. They are also used in the process of orthogonalizing air photos.

digital orthophotography
Orthophotography maintained in a digital form.

field survey
Establishing and recording real world coordinates of physical features where they exist in the world. Checking and recording the accuracy of map feature depiction by visiting the real world features represented on the map.

geodetic control
Locating map features correctly in relation to their actual locations on the surface of the earth. Features on digital maps used in a GIS should represent, as faithfully as possible, the true geographic locations of places on the earth and the true spatial relationships between these places on the earth. One important way to aid this endeavor is to register key points in the digital map to the real world coordinates of those points on the ground. These real world coordinates should be in one or another of the recognized projections, units, and datums. The map is then said to be "in control." Doing this will greatly aid the internal consistency and accuracy of the digital map and support the use of this digital map with any others that are similarly in control.

This is not the same as having your base map tied to the Public Land Survey system (PLSS) of townships and sections. The PLSS is important in land ownership descriptions, but its section corners may or may not have geodetic control coordinates (coordinates representing a position on the earth in an established coordinate system) established for them.

geodeticly controlled GIS parcel base map
A parcel base map created and maintained in reference to a real world coordinate system and the known coordinates of certain points on the earth that are also represented on the map and to which other features on the map are referenced.

geographic information system (GIS)
"An organized collection of computer hardware, software, geographic data, and personnel designed to efficiently capture, store, update, manipulate, analyze, and display all forms of geographically referenced information." [source: ESRI]

geographic literacy
Familiarity with the basic concepts and practices of geography and cartography. Ability to read and understand maps and standard mapping conventions. General understanding of the concepts of map projection, scale, map error, and related issues.

GIS analysis
Analysis of GIS datasets based upon common values contained in a database or common location on the earth. This often results in the creation of new GIS datasets containing the results of analyses. Common components of GIS analysis include database query and selection, database linking, buffering, topological overlay, clipping, dissolving, etc. This work is mainly done by GIS processing software.

GIS dataset
A collection of related digital files containing a particular set of GIS data. The terms GIS dataset and GIS database are sometimes used interchangeably. In this document we use GIS database to refer to the part of the GIS dataset that contains user editable GIS feature attribute data. For our purposes, the GIS dataset contains GIS database(s) and other data files that contain information supporting the drawing of digital maps that can not be directly accessed by the user.

GIS design
The overall design of the GIS, including hardware and software configurations, staffing, data layers, database design, data access, and data security procedures. This term is also used to indicate the design of GIS datasets or databases.

GIS implementation
The process of implementing a GIS plan. This includes investing in GIS technology, obtaining technical labor, data, training, and other processes necessary to make a GIS functional and useful.

GIS links
Identifiers common to both a digital map and a tabular database that enable a GIS to maintain the relationship between the digital map feature and the appropriate row in the database.

GIS processing hardware/software
The system on which the digital maps are made and maintained and on which higher level topological analysis is done. "GIS viewing" hardware/software refers to the systems on which more casual users query the data and make simple thematic maps.

GIS processing
Capturing spatial data into a GIS digital format, preparing such data for use by the GIS, conducting GIS analysis that results in the altering of spatial data sets or the creation of new ones.

GIS query
Asking questions of the GIS dataset. For example: "Where is Main Street?" "What parcels are zoned Commercial?" "Who are the owners of parcels that pay more than $1500 per year in property tax?"

GIS viewing & mapping software
Software that supports the viewing and querying of GIS datasets and databases, the conduct of simple spatial analyses that do not change the underlying data, and the creation of maps from the GIS datasets. Examples of such software include MapInfo, ArcView, VistaMap, etc. Each of these softwares works primarily or exclusively with data in its manufacturer's proprietary format. These softwares are not suitable for GIS processing.

Global Positioning System (GPS)
A system of satellites and ground equipment that support the determination of absolute position on the earth. GPS surveying involves the capture of signals broadcast by a 24 satellite constellation operated by the Department of Defense. By capturing signals from multiple satellites simultaneously, users can precisely determine their positions on the earth. GPS techniques usually involve placement of one receiver over a geodetic control point whose position has previously been determined (like a HARN station), with other receivers collecting data at monument locations whose geodetic locations are unknown. Data from all receivers are then simultaneously processed to determine the unknown station locations.

hard copy maps
Maps on material such as paper or mylar, as opposed to 'soft copy' which refers to maps in an electronic (digital) form.

latitude/longitude (Lat/Long)
The fundamental geographic coordinate system, consisting of parallel lines of latitude circling the globe in an east-west direction and north-south lines of longitude (meridians) that converge at the poles. These are the lines you see on a globe dividing up the earth into sections. The distance covered by one degree of longitude varies by latitude because of the convergence of the meridians as they approach the poles. Therefore, latitude/longitude is a non-orthogonal system. When plotted on a two dimensional (flat) map, latitude & longitude lines are not perpendicular to each other. Since Lat/Long can correctly represent the exact location of a place on the earth without the use of projection, it is an excellent "lowest common standard" for the exchange of spatial data, but is not suitable for the mapping of this data due to the major distortions that would result from the use of such a non-orthogonal coordinate system. In general, maps made by plotting the Lat/Long coordinates of features have a characteristic "squashed" look due to progressive width distortion in the areas farther from the equator.

map layers
Spatial features of a particular type maintained as separate GIS datasets that can be combined in a digital map as if they were different maps laid over each other on a light table. For example, roads could be one layer, water features another, parcel lines another, etc.

Data about data. This should at least include dataset names with descriptions of what they contain, coordinate system & datum, theoretical accuracy, source document names with dates & scales, data conversion methods, update history, lists of field names & what they contain, and keys to any codes used in these fields (for example: R1 = single family residence).

orthogonal system
A coordinate system in which the X (Easting), and the Y (Northing) axes are scaled the same and consistently in all parts of the map, and the curvature of the earth is ignored.

An air photo is said to be orthogonalized when it is first rectified, then processed to remove errors introduced by factors such as elevation variations in the terrain, camera lens parallax, and the curvature of the earth. An orthogonalized air photo will have consistent scaling of X and Y axes in all areas subject to the tolerances of the process used.

parcel-base map
A map showing land parcels that is used as the fundamental data layer to which other data layers are referenced.

parcel-based GIS
A GIS with data organized with a land parcel base map as the fundamental data layer to which other data layers are referenced.

parcel identification numbers (PINs)
Numbers that are used to identify land parcels and link them to other information maintained about the parcels and their owners. It is most useful if they are unique to a particular parcel or taxable land entity.

Simple aerial photography contains distortion of shape, scale, and area due to several factors: curvature of the earth, differences in elevation on the ground, tilt of the aircraft, and parallax of the camera lens. Photogrammetry is the process of correcting some or all of these distortions.

planimetric data
Data located by measurement in a single (X,Y) plane. Thus, no elevation data is recorded. In practice this would include lines such as edge of pavement and building foot prints, etc., but would not include such things as contour lines, spot elevations, etc.

positional accuracy
The degree to which map features are shown in the correct position on the map in relation to their real locations on the earth.

precision in GIS software
The degree to which the software can maintain the detail of a feature's location. The data may be maintained in either "single" or "double" precision.

Single precision can store a coordinate of seven digits without rounding. Thus, a point located at X=1,234,565 feet and Y=4,701,114 feet would remain precise through many processing operations down to the nearest foot. However, if any of the coordinate pairs were more than 7 digits long, rounding would occur and 1 foot of precision would be lost. Thus 44,701,114 would be rounded to 44,701,110. In this example, location in the GIS would be recorded precisely to the nearest 10 feet.

Double precision can handle 14 digits in a coordinate without rounding. This will prevent rounding error on very large coordinate pairs.

The mathematical process required to represent the spheroid surface of the earth on a flat map. Project determines how features on the map look and what kind of distortion will be present. Different projections are appropriate for representing large, medium, and small areas on the earth's surface. When mapping a relatively small area, like a county or city, less distortion is apparent on the flat map.

Public Land Survey System (PLSS)
The survey system used in the original land survey conducted by the U.S. Government in order to transfer title of federally owned land. Its major divisions are townships, ranges, sections, government lots and divisions of sections. The PLSS is important in land ownership descriptions, but its section corners may or may not have geodetic control coordinates (coordinates representing a position on the earth in an established coordinate system) established for them.

An air photo is said to be rectified when it has been processed to remove errors introduced by factors such as airplane tilt. A rectified air photo will not have consistent scaling of X and Y axes in all parts of the photo. The most consist scaling will be in the center of the photo.

relative accuracy
The degree to which map features are shown with the correct distances and directions from each other.

Request For Proposal is the term often used to describe government or organization solicitations of proposals from vendors and contractors.

scale of the source map
The scale of a map that contains data converted to a GIS format as part of a GIS dataset. This is the only scale that matters to the relative and positional accuracy of a GIS dataset. A GIS dataset can be no more accurate than the scale of the source map will support. For instance, it is very hard to distinguish anything smaller than about 1/50th of an inch on a map (the theoretical "minimum mapping unit"). A thin pencil line, or the smallest identifiable movement of the digitizing puck, is about 1/50th of an inch. For this reason, digitizing a USGS 7.5 minute quad map can not yield a positional accuracy of better than plus or minus 40 feet. At this scale, one inch on the map equals 24,000 inches on the ground. Thus, 24,000 inches divided by 50 equals 480 inches or 40 feet on the ground. If you digitize a road center from this map, you can say with certainty that its positional accuracy is no better than 40 feet to the right or the left of where you show it in the digital map.

scanning and vectorizing
A data conversion technology that makes use of special hardware and software to make a bit map picture of a map (scanning) and have the computer trace lines on this bit map picture (vectorizing). This is an alternative to board digitizing.

schematic map
A simplified map concerned primarily with topological accuracy.

spatial information
Information having to do with a place on the earth. This can be a map feature such as a property line or information about a map feature such as the name of a property owner.

The earth is more or less round. The exact nature of the "more or less" can have significant effects on how well the absolute position of features can be represented on a map. The study of this "more or less" is called geodesy, and its products are spheroids. Spheroids are mathematical representations of the true shape of the earth, from which calculations of location can be made. These calculations are called "datums."

tabular data
Data maintained in a tabular form, that is, organized into rows and columns in which each column represents a kind of information and each row represents an instance of information. A telephone book can be thought of as tabular data, one column contains a name, another an address, and a third a phone number. Each row contains information of the appropriate type about a particular instance, in this case a telephone customer.

tiled datasets
GIS datasets that are broken up into geographic regions or tiles for ease and speed of processing and use.

topological accuracy
The correct pattern of relationships among map features.

work space
This is a term used by ESRI to denote a subdirectory containing a particular group of GIS datasets. It represents a useful technique of data management.

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