Symbology is defined in geographic information systems (GIS) as the set of conventions, rules, or encoding systems that define how geographic information is represented with symbols on a map. A characteristic of a map feature may influence the size, color, and shape of the symbol used.
Generically, a symbol is something such as an object, picture, written word, sound, or particular mark that represents something else by association, resemblance, or convention. For example, Roman numerals are symbols for quantitative values and personal names are symbols representing individuals. On a map, a red cross is a commonly understood symbol to indicate the location of a hospital, and crossed sabres may indicate the site of a battlefield. 
The use of symbology is fundamental to mapping, cartography, and GIS because it allows the information to be displayed more effectively. Through symbology, maps are better able to portray the geographic world and help the reader gain an understanding of what is being communicated. In cartography, an organized collection of symbols forms a map legend. This provides an explanation for the symbology used in the map. In a legend, the meanings of the different symbols, colors, lines, etc. are explained by text that is placed next to pictures of the symbols used. The reader is then able to associate the proper symbol with the intended meaning for the map.
Maps typically include symbols that represent features such as roads, buildings, streams, and vegetation. Features are shown as points, lines, or polygons, depending on their size and extent. Many features are shown by lines that may be straight, curved, solid, dashed, dotted, or as a combination of any of the previously listed. The colors of the lines usually indicate similar classes of information: topographic contours; lakes, streams, irrigation ditches, and other hydrographic features; land grids and important roads; and other roads and trails, railroads, boundaries, and other cultural features.
Symbology comes in many different forms on maps. Graduated colors, Graduated symbols, Dot Density, Proportional Symbols, bar and pie charts, and custom symbols are all types of symbology that are common elements in maps.
Military symbology has a very large and detailed library of symbols for use in military cartography, Intelligence, and Engineering. Each branch of America's armed forces use the same symbols making it easy to identify units that are friendly, neutral, or enemy. Also military symbols are used to show equipment, installations, military or civilian, and military operations on and around the battle space. All these are a large part of military symbology. The army gets its symbology from NATO. NATO develops and approves the symbols that the armed services use. The Allied Procedural Publication (APP) are NATO's standards for military map-making symbology. An example of military symbols are APP-6A.
Emergency Mapping Symbology
Emergency Mapping Symbology are specialized sets of symbols used by various organizations when planning for or responding to emergencies. These emergencies can be naturally caused (tsunami, earthquake, tornado, etc.) or human caused (rioting, terrorism, hijacking, etc.). Currently there is no international standard for emergency mapping symbology which has meant that various nations have created their own national symbology set. Recognized and standardized symbol sets help create a Common Operating Picture (COP) for varied organizations that have been brought together during a crisis or emergency. Symbols that are easy to identify with and easy to distribute are seen as key elements in creating maps that can be used to reduce fatalities, injuries or loss of property.
United States HSWG - FGDC
In September of 2005 the Department of Homeland Security released version 2.20 symbol set which was later incorporated into an ANSI Standard (ANSI INCITS 415-2006). This Symbology Standard was developed by the Federal Geographic Data Committee's Homeland Security Working Group (FGDC HSWG). From this group, a Symbology Subgroup with representatives from Federal, State, and local government worked to develop this Symbology standard and its usage. More information can be found at http://www.fgdc.gov/HSWG/. The standard is currently working its way through standards processes of ANSI and the Department of Homeland Security. 
The Emergency Mapping Symbology was funded by GeoConnections, a Canadian government program that is national in scope. EMS is designed to support emergency management applications across Canada, including the national Multi-Agency Situation Awareness Systems (MASAS). EMS is part of efforts to create a Common Operational Picture both with emergency management agencies and with the public at large. It targets Web applications, but can also be used with traditional desktop applications.
The symbols are tailored to Canadian requirements for situational awareness reporting, first response, natural hazards assessment, public alerting, and critical infrastructure protection. The symbols are already being used by federal and provincial agencies, and are a valuable resource for Canada's Multi-Agency Situational Awareness System initiative, enabling emergency management organizations to share geospatial incident, event and alert information within the context of a national common operational picture. Although the dataset is designed to meet needs in Canada, very few of the symbols are specific to Canada. Most symbols are applicable globally. As discussed elsewhere, the work has been heavily influenced by similar efforts carried out in other countries. What distinguishes the EMS symbol set is its design, which follows a set of design principles that make it particularly suitable for Web applications by both trained and untrained personnel alike.
The development of EMS was strongly influenced by three primary sources:
HSWG - Homeland Security Working Group (ANSI INCITIS 415-2006 and the related mil spec: MIL-STD 2525C)
CAP-CP - Canadian Profile of the Common Alerting Protocol
NIDM - Canadian National Infrastructure Data Model (which was heavily influenced by the US-Canada Cross-Border Infrastructure Plan)
Thirty-seven organizations contributed to the development of the symbology. These included federal agencies in Canada and the United States, provincial and municipal emergency management organizations, emergency management software vendors, consulting companies, and universities. Further information can be obtained at (http://emsymbology.org).
- Wade, T. and Sommer, S. eds. A to Z GIS: An illustrated dictionary of geographic information systems, ESRI Press, 2006
- Symbol, Wikipedia contributors, Wikipedia, The Free Encyclopedia, Accessed May 4, 2010
- Topographic Map Symbols. USGS Publications. 28 April 2005.
- Homeland Security Working Group (HSWG)
- "Emergency Mapping Symbology". http://emsymbology.org.
- ESRI ArcGIS Desktop Help - An Overview of the Symbology Toolset]
- Military symbology, Military ClipArt Library. Retrieved May 4, 2010, from U.S Army Fort Sill website
- USGS - Topographic Map Symbols]
- The Atlas of Canada - Map Symbology
- Symbols.net - Map Symbols
- The Unofficial ArcInfo/ArcView Symbol Page
- WikiTravel - Common Map Symbols
- Cartographic representations