Draft 12/31/96
Prepared By Heather Babcock
Tahoe Center for a Sustainable Future
GIS is the acronym for Geographic Information Systems.
A Geographic Information System (GIS) is a set of procedures and computer hardware/software for organizing, storing, retrieving, analyzing, and displaying data which includes a geographic component. Desktop GIS is an immensely powerful computer mapping system and a tool for managing information of any kind according to where it's located.
In the strictest sense, a GIS is a sophisticated computer based mapping and information retrieval system; a system capable of assembling, storing, manipulating, and displaying geographically referenced information , i.e. data identified according to their locations. Practitioners also regard the total GIS as including operating personnel and the data that go into the system.
Every Geographic Information System (GIS) consists of three primary components:
To form a true GIS, all three components must be
tightly integrated.
The purpose of a geographic data model is to represent real-world geographies in a structured, digital format that permits users to create, edit, manipulate, display, analyze, and output data. Most types of information or data includes some kind of geographic component which allow for a graphical representation of that information.
If what you do involves managing information, and that information can be linked to geographic locations, then GIS can help you organize that information so that you can make new discoveries and get more out of the information you have. Geographic information systems let you visualize your information in new ways that reveal relationships, patterns, and trends not visible with other popular systems. No matter how many or how complex the questions, GIS gives you the tools for understanding and analyzing the information so you can get the answers you need.
Because features on maps are organized according to relative position or location, maps are particularly good for showing the relationships between feature locations. These relationships, called spatial relationships, are important because understanding them helps us solve problems.
To represent real-world objects, maps use three basic shapes-points, lines and areas-and symbols to help identify features and provide information about them. On paper maps, each symbol; color, pattern, picture, or label gives you information about the features on the map. But, the amount of information you can get from a paper map is limited to what is shown. With GIS, you are not limited to the amount of information you can get about what you see on the map. GIS stores all the information about map features in a GIS database and links the features on the map to the information about them. This means that you can access all the information about a feature (usually by simply clicking on it).
The information that a GIS stores about map features is referred to as attribute information, or attributes. The attributes of a river, for, might include its name, length, average depth, rate of flow, water quality, how many dams are on it, and how many bridges cross it. GIS formats attributes in rows and columns, and stores them as tables. Each column stores a different attribute and each row relates to a single feature. The link between features and attributes is a two-way relationship, changing an attribute in the table automatically results in a change on the map. This simple link between features and attributes makes GIS a truly dynamic system far superior to paper maps!
GIS links sets of features and their attributes and manages them together in units called themes or layers. Themes are made up of features with a set of common attributes. For example, all the roads have lanes, pavement type, and a route or street name. On the other hand, the rivers and streams have a different set of characteristics in common. All the themes or layers for a geographic area taken together make up a GIS database. You can use the themes or layers in a GIS database to analyze multiple situations and solve multiple problems. The design of a GIS database is strong because it's flexible. You can add new themes to a GIS database or delete old ones; you can separate themes to create more themes, or combine themes if they have a common characteristics. What you want to do with a GIS database, and what information you need, will determine the best design for you.
Think how far we've come from the paper maps of yesterday. No longer are maps an outdated snapshot of a rapidly changing world. GIS reflects the world as it changes, so your maps are as current as your information.
"Our greatest responsibility is to be good ancestors."
-Jonas Salk
As we look ahead to the 21st Century, we have learned there are some mistakes we can't afford to make. In just one example, we face growing environmental threats and tightening public budgets. Today's issues are complex and require better tools for better decision-making. Using advanced technology tools, we find we can actually save money by not making costly errors. This is why GIS is important!
An electronic Geographic Information System allows complete organization and retrieval of information based on geographic location. The analysis potential provides an opportunity to look at data with complex computer models using spatial and analytical displays. This enables quick and easy development of alternative solutions to complex problems. It also uses the power of computer graphics to prepare maps displaying this information.
There are many opportunities to use a GIS system. For example, any activity that effects the environment, and thus requires an EIS or environmental assessment, requires complex analysis of a wide variety of natural resource information. These analyses also require graphic displays and tabular summaries of analysis to communicate with the interested public. When information can be easily retrieved by its geographic location, a meaningful analysis is more readily accomplished. These kinds of environmental assessment activities require a thorough analysis, display, and understanding of environmental and natural resource characteristics in their geographic setting. Analysis, display, and understanding now limited by manual processes is enhanced and speeded up by computer based GIS capabilities.
GIS is a useful tool to help…
GIS technology is used by anyone who needs to interpret or display large quantities of data on a spatial or geographic basis.
Desktop GIS is best used by organizations that…
GIS software (off the shelf software) is used by more than 60,000 users worldwide, in thousands of applications, from locating new store sites and finding the most efficient routes for a fleet of delivery trucks, to managing the assets and operations of entire cities and utility companies. Desktop GIS can help users reduce operational costs, be more efficient with resources, and make better decisions in such applications as forest management, local government, oil and gas exploration, utilities mapping, land resource management, and many more. See Table 1.1
Frequent user's of GIS include:
| Category | Examples of Use | Regional Examples |
| Government |
|
(ie. TCPUD, IVGID, STPUD)
|
| Business |
|
|
| Science |
|
|
| Education |
|
|
Needs more detail (Lynn??)
Current geographic data is decentralized in many organizations
within the Tahoe Basin. The data is not well coordinated between
organizations, it is subject to some duplication, and is managed
manually in most cases and with the aid of automation in some
cases. This data is costly to collect and maintain, it is maintained
to different standards in different areas (with widely differing
needs for archiving historical versions), and has a strong potential
for effective use, given careful management and exploitation.
Data collection and maintenance costs exceed equipment costs by
some factor; and hardware and software systems to manage and store
the data are emerging rapidly. Data accuracy standards are very
important, as are procedures to verify compliance. A small amount
of these data are common to a large number of maps. There are
a few key maps used by many departments/organizatons and a large
number of specialized maps. Support for a comprehensive strategy
to deal with geographic data is increasing.
By sharing datasets regionally, we can all off-set the cost of implementing a Geographic Information System that will work for each of us. Currently, a large amount of our information resources are stored on paper maps and overlays, in files and functional databases. Incompatible formats and inconsistent standards make it difficult to provide consistent and useful information about local resource issues. With shared GIS-datasets the basic data is stored electronically after it is collected and can be easily organized so that everyone can easily access it. By developing regional GIS capability in the Tahoe Region, everyone can use the same set of information and the data can be updated by those closest to the action as changes occur.
Most GIS programs are supported on several leading UNIX platforms including:
Some Desktop GIS programs are now available for workstations or personal computers. (see below)
GIS software currently in use includes both public domain and commercial packages. The most frequently used public domain sofware includes:
The most frequently used commercial software includes:
Basic principles of GIS software:
The operational tools of GIS include map entry and edit, map display and manipulation, area/district calculation, query on geographic location, query on attribute characteristics//, query on spatial relationships, tabular data reporting, terrain analysis, polygon analysis, network analysis, and routing analysis.
GIS supports the retrieval of data by location, geographic conditions, or attribute selection criteria. It can display selected data as a map with appropriate features highlighted or colored, or it can display a tabular report. In addition to simply retrieving data, GIS performs analysis displaying spatial intelligence (for example, relationships between objects or "adjacency", connections between objects or "connectivity"). Using this intelligence, the GIS can perform polygon overlaying and analysis, network route and flow analysis, facility service area analysis, proximity analysis, and other functions.
One particular "off the shelf" GIS software package is a generic software product with extensive functionality. This particular example contains hundreds of advanced functions for processing geographic information including
Building a Geographic Information System (GIS) is a matter of constructing graphic and non-graphic data bases, developing or obtaining information processing capabilities, installing the appropriate computer hardware and software, and then implementing the organizational, procedural, and staffing changes needed to successfully operate and use the system. The process for determining these factors is called Needs Assessment. Needs Assessment is a process which determines the answers to many difficult questions, including:
Lynn to add detail?
GIS exists throughout the Sierra Nevada Region including the Northern Sierra Nevada GIS (NSNGIS) and the Tahoe Environmental GIS (TEGIS) in the Lake Tahoe Basin. The next section illustrates an outline for the Tahoe Region as an example.
Sponsored by the Tahoe Center for a Sustainable Future
Drafted August 19, 1996
Objectives
Determine the feasibility of integration of
Geographic Information Systems in the Tahoe Basin. This
structure should aid in regional planning, in environmental review,
in economic development, and in public education. The focus would
be on sharing of existing GIS information across agencies and
GIS platforms, and on improved integration in the future.
Upgrade GIS functionality.
Determine needs for improved GIS capabilities in the Basin; with
special reference to multiagency action, and in dispersed
datasets.
Improve access. Develop
a prototype system structure to make existing (and future) GIS
information more widely available to the professional planning
and management community and to the general public.
Tasks
Phase I. Feasibility
and Initial Prototype
Project Definition and Setup. Meet with
and confirm agency participation, assigned staff and establish
task priorities.
Identify existing GIS layers. Identify
existing GIS layers and supporting data available for distribution
(this task is partially done). Prepare a matrix showing data
layers, coverage, resolution/scale, format.
Computing. Identify
and characterize the source organizations involved in terms of
computer capabilities, technology, staff, and plans (partially
done). Ascertain the degree to which each source agency can
prepare information for clean handoff, or if additional effort
is needed.
Release. Identify
fire walls, and/or administrative actions which would authorize
formal release of information.
Technical status.
Characterize the specific technical situation at source agencies
for hardware, software, and database situation for each source
organization. Identify plans for future computing/GIS hardware
and software. Technical experts need to confer on exact specifications
and means of access or transfer.
Metadata. Review
and comment on metadata at each source organization and for each
dataset. Assist parties in adopting metadata standards. We will
use as referents the CERES metadata standards (by Quinn Hart)
at http://ceresweb.ucdavis.edu/standards/index.html and the BCDC/FGDC
Supplement (Gardels) at http://www.regis.berkeley.edu/bcdc/fgdcproposal
Determine the degree to which source organizations have completed, or can complete, metadata for their holdings. We will compile a shortform portion of the metadata if needed to implement the prototype.
Evaluate existing GIS layers and metadata,
with regard to interoperability. Ideally,
this would be done by FTP of files to UCB so that the evaluation
can be done there. To the extent that this is not possible, a
review will have to be made at the source agency's workstations.
Post. Create a web
page with gif images of files which have been made available,
with descriptions of each layer and notes on availability. Link
to source agencies FTP sites if these have been established.
Access needs. Identify
most important needs for access (partially done). Relate data
availability to upcoming issues and needs, such as the 1996 Threshold
Review, and the Realtime Monitoring Program, with special
reference to layers and tools needed by both technical and public
users.
User characteristics. Evaluate
userside capabilities (partially done). Classify user groups
by needs, and technical characteristics.
User capabilities. Evaluate
userside capabilities for each of two alternatives.
a) Webbased system. Target users for
a Webbased system would have the following or better:
highresolution color monitor, 28.8 modem, and the equivalent
of Netscape 2.0. Check for prevalence of Windows 95, Windows
NT.
b) Net of local GIS sites. Target users
would have desktop GIS capabilities, such as AutoCad, ArcCad,
ArcView, MapInfo, or similar software and supporting hardware
and net access.
Mount prototype. Develop
and mount a user interface as a stand-alone product which will
illustrate the potential for the expanded prototype which is described
below in Phase II.
Phase I. Report. Post
a report on-line on the Web, summarizing findings from above,
and recommend next steps (e.g. take stock at this point).
Assumptions. To be as efficient as possible,
this proposal is based on expanded bandwidth being available at
TRPA and each participating agency which allows file transfer
protocols (FTP) to be made by TRPA and other agencies directly
to U.C. Berkeley. These agencies will also run Netscape v. 3.0b4
with Java and Frames.
Phase II. Evaluation
and Prototype Expansion
Integration. Consider development of
engines for translation and rectification to permit joint use
of GIS layers at different projections and levels of resolution.
This can be done largely through the use of tools being developed
for the California Coastal Commission and BCDC, in a project
underway at UCB. These operations can be performed at a central
site, or through the establishment of protocols to be followed
by the contributing source agencies. It should be understood
that at present, there is as yet no good example of a truly distributed
network of systems operating seamlessly across disparate platforms
and software. However, it is feasible to think of crossaccess
to and from remote systems, or rectification and integration at
a central site.
User interface. Propose a user interface that will allow experts and the public to access, query, browse, and perform basic GIS functions on the distributed data layers. There are at least two alternatives to be considered:
a) Local GIS group, with Net Browsing.
Desktop GIS located at several sites. This alternative would
use widely available commercial GIS software. Site licenses or
software would have to be purchased if not donated. Each user
site would have to be equipped with software. In this case,
source organizations would prepare files for shipment to client's
computers, using common formats. Training in use of software
and support would be a good idea. If this approach is selected,
a supplementary web page for browsing images of layers could be
prepared.
b) Webbased GIS. Tahoe GIS files
would be accessed and converted if needed, for use over the Web..
An example of a working webbased system under development
for the CA Delta Protection Commission can be seen at http://www.regis.berkeley.edu/dpc2.html.
Users need have only a web browser such as Netscape to perform
GIS operations on a central system.
c) Webbased on anticipated software developments.
A number of vendors are working on JAVA based GIS web systems,
and several may come online within the study period.
Expand Prototype. Expand
the user interface selected above in prototype format. Demonstrate
use of the prototype, with an eye to housing such functionality
longterm in the Basin. If this system requires extensive
file conversion, only a few data layers may be mounted. The prototype
at minimum would include four of the TEGIS layers and two USFS
data layers, and at least one layer from outside the region.
Central server. Consider
needs for central server functions and evaluate pros and cons
of the handoff of certain functions to CERES or to key agencies
in the Tahoe Basin. Consider and develop options for central
server and related functions in the Basin.
Technology and hardware transfer. Place
as much functionality as possible in the Tahoe Basin itself.
Phase II. Report. Post
a report online as a Web page, with documentation, including
suggestions for further action.
Phasing
Phase I. This initial
investigative work would move sample data sets to Berkeley, where
virtually all analytical GIS evaluation would be done. Work would
rely on very short inputs of time from highlytrained staff
at Berkeley. A fourgigabyte hard drive would be purchased
and set up with software and sample data at UCB. This drive would
become part of the computer capabilities moved to the Tahoe Basin
in Phase II.
TCSF staff and UCB staff would conduct joint interviews
in the Basin. Webbased GIS would be pushed as far as possible,
new developments evaluated, and compared to the nonweb system
described above.
Phase II. A highend
PC would be purchased, and combined with the drive described above.
This would be linked via the anticipated Pac Bell ISDN connection,
or alternative higher bandwidth connection if available. Depending
on the alternative selected, this capability would serve as the
local Web development engine, and/or a GIS server to the sitebased
GIS computers. It would be left with TCSF or a key agency in
the Basin, as TRPA and the other agencies agree is appropriate.
TCSF will make a proposal for TRPA and the agencies to review.
Schedule
Phase I. To be completed six (6) months from receipt of funds.
Phase II. To be completed eight (8) additional months from receipt of funds.
Metadata is data about data. This information is similar to a card-catalog entry for a book, containing source information such as author, date, etc.
At least four categories of standards apply to GIS - application standards, data standards, information technology standards, and education and training (professional) standards. Data standards, including data transfer, quality, and metadata, or data documentation are being worked on in detail for the Tahoe Regional GIS. The CERES program is working on adapting the Federal Geodetic Data Standards (FGDC) for all data standards by using a database program written with Microsoft Access. This system is still in progress.
Overview - visit http://www.usgs.gov/fgdc-catalog/preface.html for detailed information regarding the FGDC.
On October 19, 1990, the Executive Office of the President, Office of Management and Budget (OMB) revised Circular A-16,"Coordination of Surveying, Mapping, and Related Spatial Data Activities." A major objective of A-16 is the development of a national spatial data infrastructure with the involvement of Federal, State, and local governments, and the private sector. This national information resource, linked by criteria and standards, will enable sharing and efficient transfer of spatial data between producers and users.
Circular A-16 established the Federal Geographic Data Committee (FGDC) to promote the coordinated development, use, sharing, and dissemination of geographic data. The committee oversees and provides policy guidance for agency efforts to coordinate geographic data activities. Federal agencies were assigned the responsibilities of leading coordination activities for categories of data. Agency responsibilities include
providing government-wide leadership in developing data standards, assisting information and data exchange, and coordinating data collection.
The Manual of Federal Geographic Data Products describes Federal geographic data products that are national in scope and commonly distributed to the public. Geographic data products include maps, digital data, aerial photography and multi-spectral imagery, earth science, and other geographically - referenced data sets. Data products are described in a standardized format and grouped by producing agency. A cross-reference matrix is provided to help readers find products by data type.
For more information about the FGDC, its activities,
the Manual, or to be added to the newsletter mailing list, please
visit http://www.usgs.gov/fgdc-catalog/preface.html or
contact:
Federal Geographic Data Committee Secretariat
U.S. Geological Survey
590 National Center
Reston, VA 20192 USA
(703) 648-4533
The California Environmental Resource Evaluation System (CERES) has been working on a system for cataloging the metadata for geospatial data (as well as other types of data) that incorporates the FGDC standards. Visit the web page at http://ceresweb.ucdavis.edu/metadata/access/ to learn about use of the CERES-sponsored Access Metadata Database System:
Metadata Quick Start -
Here are the steps that you need to do in order to start documenting
your geospatial data using the CERES Metadata program:
Metadata Catalogs - The CERES Metadata database is based on the notion of sharable catalogs of metadata being used in a distributed computing environment. To achieve this, the CERES Metadata database uses a specific naming scheme for catalog entries that allows for local freedom of naming, without corrupting the distribution wide name space. The scheme is simple, each id in the database, for identification of all entries, consists of the naming authority prepended to the id, separated with double colons AUTH::Entry Identifier. This allows for local database administrators to create Catalogs, and seemlessly enter them into the CERES system. It also allows for simple incorporation of other exisiting Catalogs.
The following Catalogs are available (they are directory structures in zip format. To incorporate into your database, download and expand. Then start you ACCESS database and from the General Functions form, specify the directory and use the All Tables In button to import.)
Users Guide - The users guide for this database is available at http://ceresweb.ucdavis.edu/metadata/access/users_guide/html/manual.html. We are currently working on this guide, and this is very crude version.
| Organization or Name | Web Site URL |
| ABRODY's Geography, GIS and Map Sites | http://www.bayserve.net/~abrody/gis.html |
| Dictionary of Acroyms in GIS, Cartography, and Remote Sensing | http://www.lib.berkeley.edu/EART/abbrev.html |
| Environmental Systems Research Institute, Inc. (ESRI) | http://www.esri.com/ |
| Essential Information's Geographic Information Systems Project | http://www.essential.org/gis/ |
| Federal Geographic Data
Committee |
http://fgdc.er.usgs.gov/gdc/html/fgdc.html |
| Friends of the Earth | http://www.foe.co.uk |
| GBR: Geographic/GIS resources | http://www.gdt1.com/geolinks.html |
| Geographic Information Referral Page | file://waisqvarsa.er.usgs.gov/wais/html/geog.html |
| Geography Information on the Internet |
http://www.geog.buffalo.edu/GIAL/netgeog.html |
| GIS - Geographic Information Systems | http://www.libertynet.org/~bgmap/gis.html |
| GIS and Environmental Science | http://www.usgs.gov/network/science/earth/gis.html |
| GIS FTP Resource List | http://www.geo.ed.ac.uk/home/gisftp.html |
| GIS Information - links to GIS information on the Internet | http://www.laum.uni-hannover.de/gis/gis-engl.html |
| GIS Net Sites | http://www.io.com/~frank/gis.html |
| GIS Resource Guide - Frequently Asked Questions | http://www.census.gov/geo/faq-index.html |
| GIS, Geospatial Data, Remote Sensing, and other useful and interesting sites | http://www.seic.okstate.edu/serv.html |
| GIS: Geographic Information Systems - Assorted Items Related to Environmental Protection | http://ice.ucdavis.edu/environmental_protection/gis.html |
| GIS: Geographic Information Systems - Internet Links | http://ice.ucdavis.edu/Cyberspace_Jump_Station/gis.html |
| GISnet BBS MapInfo Support | http://www.csn.net/gis/mapinfo |
| GEODESY | http://bgrg.com/geodesy |
| GRASS - public domain GIS software |
http://www.regis.berkeley.edu/grass.html |
| GRASSLinks at the Australian Geodynamics Cooperative Research Centre | http://www.ned.dem.csiro.au/AGCRC/4dgm/grasslinks/ |
| Great GIS Net Sites! | Http://www.hdm.com/gis3.htm |
| Guide to On-Line and Mostly Free U. S. Geospatial and Attribute Data | http://www.cast.uark.edu/local/hunt/index.html |
| ICE MAPS - Interactive California Environmental Management, Assessment and Planning System | http://ice.ucdavis.edu:8080/ice_maps/ |
| JAVA-GIS Transfer via Internet | http://java.sun.org |
| On-line Resources for Earth Scientists | http://gisnet.com/gis/ores/gis/hyper.html |
| Open GIS Consortium Inc. | http://www.ogis.org |
| Open GIS Foundation | http://www.regis.berkeley.edu/ogis/ogis.html |
| Proceedings: Conference on Law and Information Policy for Spational Databases | http://www.spatial.maine.edu/tempe/tempe94.html |
| REGIS Environmental Planning at Berkeley | http://www.regis.berkeley.edu |
| REGIS Home Page | http://www.regis.berkeley.edu/ |
| US Geological Survey | http://info.er.usgs.gov/ |
| Web GIS "Toy or Tool?" Resource List | http://www.gisnet.com/gis/webgis.html |
| Xerox PARC Map Viewer Views of the world | http://pubweb.parc.xerox.com/map |
GIS has generated its own jargon-rich language, so
to aid you, below is a comprehensive guide to the jargon of GIS.
Address Matching
A geocoding process which matches the street address of property to its location. This usually involves the matching of two database files, one containing the addresses of interest, the other a list of addresses and their coordinates. Address matching is central to many applications in direct marketing.
AM/FM
Automated Mapping/Facilities Management. This is a specific application of GIS to the management and production of maps of plant such as cables, pipes, valves etc. It is currently the most widely used application of GIS, and particularly relevant to local authorities and utilities.
ASCII
American Standard Code for Information Interchange. A standard set of codes which represent alphanumeric characters stored as a single byte value. For example, using the ASCII code, a byte containing the value 69 would represent the letter E. Because of its simple nature, ASCII text is one of the best ways of transferring information between different programs and platforms.
Attribute
An item of text, a numeric value or an image that
is a characteristic of a particular spatial entity.
Buffer
A zone of user-specified distance around a point,
line or area. The generation of buffers to establish the proximity
of features is one of the most common forms of GIS analysis. For
example, it may be used to find all areas of industry less than
5km from a reservoir.
CAD
Computer Aided Design. Software programs for the design, drafting and presentation of graphics. Originally designed for manufacturing drawing, now also widely used for mapping.
Cadastre
A data set containing information related to land ownership and rights. This usually takes the form of maps and descriptions of uniquely identifiable land parcels. For each parcel, legal information such as ownership, easements and mortgages are recorded.
Cell
The basic element within a grid or raster data set.
Centroid
The center point of a polygon, often used to attach attribute information to an area such as a census ward. The centroid may be mathematically derived (such as the center of gravity) or may be user defined. It must always be placed inside the polygon.
COGO
CO-ordinate GeOmetry. Algorithms for handling basic two and three dimensional vector entities built into all surveying, mapping and GIS software.
Coordinate
Numbers representing the position of a point relative to an origin. Cartesian coordinates express the location in two or three dimensions as the perpendicular distances from two or three orthogonal axes.
Data Model
A generalized, user-defined view of data representing the real world.
DEM
Digital Elevation Model (or Terrain Model). A data model used to represent a topographic surface, often based on a grid with a height value for each cell, or on a set of irregular triangles (see TIN).
Digitizing
Conversion or encoding of existing maps from an analogue form (paper) into digital information, usually in the form of Cartesian coordinates. This may be via a digitizing table or tablet with a hand-held cursor, or via a scanner.
DXF
Digital eXchange Format A data format defined by
Autodesk originally for the transfer of data between CAD systems.
Due to its simplicity, it is now widely used in the transfer of
data between GIS, despite a number of limitations.
Gazetteer
A list of spatial entities held in computer form, such as properties or streets, which allows for rapid search and query. The gazetteer often forms the core of larger GIS-based applications such as LIS.
Geocode
The element in a database used to identify the location of a particular record, for example a zip code (postal code). The process of geocoding is similar to that of address matching, in that a data file is compared against a file of geocode and their associated coordinates.
Geodetic Datum
A set of parameters defining coordinate systems for all or parts of the earth. These datums have been refined and revised over time. NAD 27 is the North American datum for 1927, for example. ED50 is the European datum for 1950, and WGS is the World Geodetic System for 1984. Varying datums are used to produce better local ëfití of a spheroid to the actual shape of the earth - the geoid.
GPS
Global Positioning Systems. A position-finding system which uses radio receivers to pick up signals from four or more special satellites (there are 24 in orbit) and compute WGS coordinates for the receiver. Accuracy depends on the sophistication of processing and the time available for reception. Real-time navigation using GPS on aircraft and ships can be to better than 100m. Processed data from several hours of observation can provide relative positions accurate to a few centimeters.
GUI
Graphical User Interface. A method of interaction with a computer which uses pictorial buttons (icons) and command lists controlled by a mouse. It is generally regarded as simpler and easier to learn than command line interfaces, where commands have to be typed. Examples include MS WINDOWS for PCs, Open Look or MOTIF for workstations and System 7 for Macintosh.
Grid Data
A data structure composed of square cells of equal
size arranged in columns and rows.
LIS
Land Information System. A subset of the geographic information industry that is dedicated to the management, analysis and presentation of information relating to land, including ownership and legal rights. Often an automated development of the Cadastre.
Latitude-Longitude
A spatial reference system for the Earth's surface. Latitude is an angular measurement N or S of the equator, longitude is an angular measurement E or W of the meridian at Greenwich, UK.
Macro
A series of program commands or instructions which are stored in a file and can be recalled when necessary. Macros are commonly used to customize high-end GIS toolkits for individual applications.
Map Projection
A mathematical model used to convert three dimensional reality into two dimensions for representation on a map, or within a two dimensional GIS database. All map projections have particular strengths, some preserve shape, other preserve distance, area and direction. All projections have limitations, however, of which you should be aware.
Map Scale
The measure of reduction between the representation and the reality, be it a map or a spatial database. Scale is usually represented as a representative fraction of distance e.g. 1:50,000, one unit of distance on the map representing fifty thousand units in reality. The nominal scale of a spatial data set has considerable influence over the possible application of the data set, and you should always be aware of any such implications. For example, it would not be sensible to compare the shape of a road represented in a 1:625 000 scale data set with one of 1:1250. Theoretically, a dataset does not have a scale (unlike a map) but the terms Scale is usually used as a metaphor for resolution and content.
Multispectral
Remote sensing in two or more spectral bands.
Network
A model representing the interconnected elements through which some form of resource can be transmitted or will flow. In GIS this is represented as a series of nodes connected by arcs, each or which has attributes representing flow characteristics e.g. a road or pipeline network.
Node
A basic spatial entity within the vector data model
which represents the beginning or end of a segment. Also, a node
may be formed when a number of segments join. For example a node
might be represented in a road network as a highway intersection.
Operating System
A series of computer programs which control the operation
of the computer itself. Application programs such as GIS software
run under an operating system. Examples of operating systems include
UNIX, VMS, DOS and OS/2.
Peripheral
A hardware component which is connected to a computer to perform specialist functions. Common GIS peripherals include plotters, digitizing tables, and printers. When selecting GIS software it is important to ensure that it is compatible with any existing peripherals you use.
Pixel
A picture element of a raster image as displayed on a screen or raster plot.
Point
A spatial entity that represents the simplest geographical element. Represented in the vector data model as a single x,y coordinate, and in the raster as a single cell. The point may have associated attributes which describe the element it is representing; the telephone number of a public call box, for example.
Polygon
A representation of an enclosed region defined by
an arc or a series of arcs that make up its boundary. Polygons
may have attributes describing the region they represent, such
as the population of a census ward.
Quadtree
A data structure that subdivides any given space
into four quadrants and continues to subdivide each quadrant in
a similar way until they are uniform or the basic resolution of
the data is reached. It is mostly used to compress raster data.
Raster
A data structure composed of a grid of cells. Groups of cells represent geographical features; the value in the cell represents the attribute of the feature.
Relational Database
A database which structures data in the form of tables. Each table contains information relevant to a particular feature, and is linked to other tables by a common value. For example, two attribute tables could be linked to a spatial data table via a geocode, such as the zip code (postal code).
Remote Sensing
The science of acquiring information about the earth using instruments which are remote to the earth's surface, usually from aircraft or satellites. Instruments may use visible light, infrared or radar to obtain data. Remote sensing offers the ability to observe and collect data for large areas relatively quickly, and is an important source of data for GIS.
Resolution
The resolution of a digital dataset expresses the size of the smallest object which can be depicted. The term is most commonly associated with the raster data model where the resolution of a raster or grid is equal to the size of the cell in the real world. For example, the resolution of a remotely-sensed image may be 10m (each cell representing 10mx10m on the ground). Increased resolution leads to larger storage requirements, increased processing and higher costs for a given area.
Rubber Sheeting
A process which adjusts the relative positions of features within a data set in a non-linear, or non-uniform way. It is used to transform the coordinates of maps with different scales, orientation or coordinate systems.
Run-length Encoding
A data compression technique which encodes a digital
data stream in terms of the number of successive digital data
elements of the same value, rather than repeating every data value.
Scanning
A data capture technique which digitizes information from paper or film hard copy into digital raster data. The process is rapid, but the resulting raster data set only has color, gray scale or black and white attributes associated with it, and may not have the intelligence necessary for GIS analysis. In effect, the result of scanning is a raster image of the original source material. Segment One of the basic spatial entities, and a basis for spatial models. Formed from a set of ordered coordinates (vertices) that represents the shape of a geographic object. An arc begins and ends in a node.
Spatial Analysis
Spatial analysis is the process of applying analytical techniques to geographically-referenced data sets to extract or generate new geographical information. Spatial analysis may be used to model complex geographical interactions, and is useful for investigating site suitability and predicting future events. Although the overall analytical technique may be complex, it is usually a combination of simple techniques applied in the appropriate order.
SQL
Structured Query Language. A language developed by
IBM in the 1970s for defining and manipulating relational databases.
It has since become the industry standard, and is often used to
enable GIS toolkits to access the data held in existing corporate
databases.
Thematic Map
A map which communicates a single theme or subject. For example, a population density map and political boundary map are both thematic maps. This contrasts with a topographical map which is a general purpose map containing landscape features such as rivers, roads, landmarks and elevation.
TIN
Triangulated Irregular Network. A method of creating a surface from point data in the vector data model. The TIN is created from an arbitrary distribution of points joined to form triangles. Each point has an x and y coordinate and one or more attributes (e.g. height). Attribute values for a point anywhere in the model can then be interpolated .
Topology
The relationships in spatial terms between connected
or adjacent geographical objects. Topology is used to apply intelligence
to data held in the vector data model. For example, topological
information stored for an arc might include the polygon to its
left and right, and the nodes to which it is connected.
Vector Data
A data model based on the representation of geographical object by Cartesian coordinates, commonly used to represent linear features. Each feature is represented by a series of coordinates which define its shape, and which can have linked information. More sophisticated vector data models include topology.
Geographic Information Systems: Information about
Land for People, United States Department
of Agriculture, Forest Service
Getting to Know Desktop GIS,
Environmental Systems Research Institute, Inc., ISBN 1-879102-42-0,
1995
GIS Evaluation Report,
United States Department of Agriculture, Forest Service, Computer
Sciences and Telecommunications, Washington, D.C., March 1992
The ESRI Product Family, An Overview, Environmental Systems Research Institute, Inc. ESRI White Paper Series, April 1995