How maps convey geographic information

Fundamental GIS concepts are closely related to maps and their contents. In fact, map concepts form the basis for understanding GIS more fully. This topic explores some fundamental map concepts and describes how they are applied and used within GIS.

Maps

A map is a representation of spatial or geographic information as a series of thematic layers of information for an area of interest. A printed map also includes additional map elements laid out and organized on a page. The map frame provides the geographic view of information while other elements—for example, a symbol legend, scale bar, north arrow, descriptive text, and a map title—around the map collar help you to understand, read, and interpret the map's contents.

People also work with computer maps—interactive images on computer screens with tools that allow you to interrogate and interact with the map's underlying geographic information.

Common to all maps is the set of thematic layers that represent the real-world features.

Layers

Geographic entities are presented as a series of map layers that cover a given map extent—for example, you can view map layers such as roads, rivers, place-names, buildings, political boundaries, surface elevation, and satellite imagery.

Geographic elements are portrayed in maps through this series of map layers.

Map layers are thematic representations of geographic information, such as transportation, water, and elevation. Within each map layer, symbols, colors, and text are used to portray important information that describes each of the individual geographic elements. Map layers help convey information using the following:

• Discrete features such as collections of points, lines, and polygons
• Map symbols, colors, and labels that help to describe the objects in the map
• Aerial photography or satellite imagery that covers the map extent
• Continuous surfaces, such as elevation, which can be represented in a number of ways—for example, as a collection of contour lines and elevation points or as shaded relief

Map layout and composition

Along with the map frame, a map presents other information using an integrated series of map elements laid out on a page. Common map elements include a north arrow, a scale bar, a symbol legend, and other graphic elements. These elements aid in map reading and interpretation by defining the meaning of each map symbol and often by providing messages and insight into the map's contents.

This information enables each map to communicate more, simply by portraying large amounts of information in a systematic, intuitive way. This in turn helps each map reader visualize and understand interesting facts critical to their work.

Spatial relationships in a map

Maps help convey geographic relationships that can be interpreted and analyzed by map readers. Relationships based on location are referred to as spatial relationships. Here are some examples:

• Which geographic features connect to others (For example, Water Street connects with 18th Ave.)
• Which geographic features are adjacent (contiguous) to others (For example, the city park is adjacent to the university.)
• Which geographic features are contained within an area (For example, the building footprints are contained within the parcel boundary.)
• Which geographic features overlap (For example, the railway crosses the freeway.)
• Which geographic features are near others (proximity) (For example, the courthouse is near the State Capitol.)
• The feature geometry is equal to another feature (For example, the city park is equal to the historic site polygon.)
• The difference in elevation of geographic features (For example, the State Capitol is uphill from the water.)
• The feature is along another feature (For example, the bus route follows along the street network.)

Within a map, such relationships are not explicitly represented. Instead, as the map reader, you interpret relationships and derive information from the relative position and shape of the map elements, such as the streets, contours, buildings, lakes, railways, and other features. In a GIS, such relationships can be modeled by applying rich data types and behaviors (for example, topologies and networks) and by applying a comprehensive set of spatial operators to the geographic objects (such as buffer and polygon overlay).