In the context of Cartography, Symbology, is the use of graphical techniques to represent geographic information on a map. Map symbols for geographic features include visual variables such as color, size, and shape.
Generically, a symbol is 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 individual people. On a map, a red cross is a commonly understood symbol to indicate the location of a hospital, crossed sabres may indicate the site of a battlefield, and a blue region would commonly be interpreted as a water body.  Semiotics is the scientific and philosophical study of how symbols work by establishing these connections between the representation and the represented concepts and real world features.
Maps communicate their messages through symbols--graphics that represent spatial phenomena such as objects, places, or attributes. At their most basic, map graphics include points, lines, and areal phenomena/polygons. Other forms of symbology include graduated colors or graduated symbols, dot density, proportional symbols, bar and pie charts, and custom-made symbols. Although there is no set standard on symbology, especially among thematic maps, various practices (such as using contour lines for elevations in topographic maps) are commonly followed.
The choice of symbology is a crucial part cartographic design; the cartographer must select symbols that are easy to interpret. A good symbol is easily recognizable (i.e., connected to the geographic features and concepts it represents), is aesthetically pleasing, and works in concert with other symbols (for example, as part of a clear visual hierarchy). A legend is typically used to explain the meaning of the symbols used in the map.
Common symbols represent features such as roads, buildings, streams, and vegetation. When a feature is represented by a line, the colors of the line usually indicate 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.
Types of Map Symbols
According to semiotics, map symbols are "read" by map readers making a connection between the graphic mark on the map (the sign), a general concept (the interpretant), and a particular feature of the real world (the object). Map symbols can thus be categorized by how they suggest this connection:
- Image or Iconic Symbols look like the real-world feature, although it is often in a generalized manner. For example, a tree icon to represent a forest, or green denoting vegetation.
- Functional Symbols directly represent the activity that takes place at the represented feature. For example, a picture of a skier to represent a ski resort or a tent to represent a campground.
- Conceptual Symbols directly represent a concept related to the represented feature. For example, a dollar sign to represent an ATM, or a Star of David to represent a Jewish synagogue.
- Conventional Symbols do not have any intuitive relationship, but are so commonly used that map readers eventually learn to recognize them. For example, a red line to represent a highway or a cross to represent a hospital.
- Ad Hoc Symbols do not have any intuitive or commonly accepted relationship; in fact, the symbol is often used only for a single map. For example, a polygon in a choropleth map filled with a shade of red that means "high income." These symbols require effective legends and/or labels to be correctly interpreted.
Cartographers typically have a great degree of freedom to design and implement map symbology according to their personal tastes, creativity, and innovation. However, in some applications, the need for consistency and immediate usability has driven the development of standard sets of symbols.
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).
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