How maps are used in GIS

Maps play a special role in GIS:

• They portray logical collections of geographic information as map layers.
• They are at the heart of how GIS is used.
• They provide an effective metaphor for modeling and organizing geographic information as a series of thematic layers.

In addition, interactive GIS maps provide the focal point for using geographic information and bringing that information to life. GIS maps are the way that GIS content is shared among professional GIS users and with everyone online.

This topic provides an important context for the role that maps play in delivering GIS to many new users.

Maps are important, and almost everyone understands and appreciates good maps.

And mapping encompasses a lot—from traditional printed maps and imagery to new media maps that are used on computers, across the Web, and on mobile devices.

A new kind of map is a GIS map, and each GIS map is more than a static map presentation. It is an interactive window into all geographic information and descriptive data, and into rich spatial analysis models created by GIS professionals.

GIS maps are:

• How you communicate and share GIS
• How GIS content is compiled and maintained
• How geographic information is designed and organized using thematic layers
• How you derive new information using geoprocessing and, subsequently, how you visualize, summarize, analyze, compare, and interpret analytic results
• How you share geographic information for use on the Web

Examples of map use

For communication and understanding

Maps are used to communicate and convey overwhelmingly large amounts of information in an organized way. Humans, as spatial thinkers, are able to view a map, associate map locations with real-world phenomena, and perceive and interpret critical information from the sea of content that is contained within each map display.

Maps convey large amounts of information yet still manage to communicate that information effectively and clearly. You can begin to understand and gain insights by using maps.

For finding patterns

Maps are used to discover and investigate patterns such as the characteristics of a population across a city or the movement of antelope between winter and summer habitats. In GIS, many maps can be dynamic and generate reports and views about multiple features and changes across time frames.

Maps visually convey patterns. This map shows the age distribution of populations across parts of Southern California. Darker colors represent areas with older populations. You can click a feature on the map to show the age distribution for the selected block group. In this way, you are using the map as a window into richer sets of geographic and tabular information.

This is a key point. GIS maps provide interactive reports of the information behind the map—not solely lists of attributes but also charts, reports, photos, and virtually any relevant content (for example, a link to a Web site). Defining how features are reported and what you access through a map feature is one of the key specifications that you design and capture when you create a GIS map.

You can also define and capture map interaction properties for time-aware layers as part of your GIS map definition. For example, here is a dynamic map that shows animal movements from GPS tracking devices. You can use the time slider tool to control the display of animal locations on various days. Clicking forward moves to the next day's observations.

Here are four snapshots from a dynamic online map of pronghorn antelope locations from four particular days (out of 300). These dots represent daily movement of pronghorn over a 10-month time span from October 2002 through August 2003. The Time Slider tool in ArcMap is used to put this dynamic map into motion.

For deriving new information using analysis

GIS maps combine powerful visualization with a strong analytic and modeling framework. Analytic models in a GIS are used to generate model results that can be added to your map display as new derived map layers.

Just like you can use each map layer as a window into rich information about features, you can use the map as a window into rich analytic results. You essentially use your GIS map to access analysis models and display their results as a new map layer, which can have the same types of feature reporting, visualization, and animation capabilities that are described above.

A "heat map" showing criminal activity. The hotter colors represent higher crime rates. Image courtesy of the Philadelphia Police Department (http://www.phillypolice.com/).

This map shows predictions for malaria outbreaks in Africa. Darker colors represent a higher projected density of malaria cases. Image courtesy of Adaptation Atlas (http://www.adaptationatlas.org/).

This map illustrates three routes used to optimize travel time between stops for three vehicles in a fleet. Organizations that use network analysis to optimize their vehicle routes typically save 20 percent or more on their annual delivery costs.

Spatial analysis is one of the more interesting and remarkable aspects of GIS. Using spatial analysis, GIS users can combine information from many independent sources and derive an entirely new set of information (results)—applying a large, rich, and sophisticated set of spatial operators. GIS professionals use Geoprocessing to "program their own ideas" in order to derive these analytical results. In turn, these results are applied to a wide variety of problems.

To get status reports

On the Web, maps can be used to communicate status and keep teams up-to-date on events. GIS information is dynamic and, for many layers, is updated on a frequent basis. Dynamic maps are an effective way for everyone to see a common picture of the latest information.

This status map from the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) shows humanitarian response efforts for the Haitian earthquake in January 2010. image courtesy of OCHA (http://ochaonline.un.org/).

A very common application for GIS is the use of operational dashboards that present data feeds and status for a particular set of users. The information layers in dashboards are targeted to a specific audience and their operational needs, enabling them to work more effectively and responsively.

Many Web maps act as operational dashboards that communicate status information across work teams to keep everyone informed and up-to-date. This is an operational dashboard for a water utility. Up-to-the-minute information coming directly from the field, from the operations center, and from the call center is displayed within this operational dashboard.

To compile geographic information

Maps are used to compile and edit features and other data, which are managed and maintained in geodatabases. The best GIS maps for editing present the specific types of features that you want to add to your maps along with the relevant editing tools and attribute properties.

ArcGIS enables users to define and share these editing properties as part of a layer design.

In this map, a feature palette of land-use types is used to lay out land-use zones. You can sketch in (outline) proposed zones for visualizing and analyzing various land-use alternatives.

Mobile GIS maps can be used for collecting data in the field and for receiving and viewing status reports on a mobile map.

To communicate ideas, concepts, plans, and designs

Maps help to communicate ideas, plans, and design alternatives. Effective layer display, combined with interactive feature reporting, provide an important mechanism to visualize, communicate, and understand various alternatives.

Here are a series of 2D and 3D maps that are used to develop and present design alternatives and some of the analysis used as inputs into the design decisions.

To openly share geographic knowledge

As illustrated by these map examples, maps are both effective and efficient for visualizing geographic knowledge. Great maps are how GIS users communicate and how geographic information and knowledge are shared.

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).

Georeferencing and coordinate systems

Georeferencing is about using map coordinates to assign a spatial location to map features. All the elements in a map layer have a specific geographic location and extent that enables them to be located on or near the earth's surface. The ability to accurately locate geographic features is critical in both mapping and GIS.

Describing the correct location and shape of features requires a coordinate framework for defining real-world locations. A geographic coordinate system is used to assign geographic locations to objects. A global coordinate system of latitude-longitude is one such framework. Another is a planar or Cartesian coordinate system derived from the global framework.

Maps represent locations on the earth's surface using grids, graticules, and tic marks labeled with various ground locations—both in measures of latitude-longitude and in projected coordinate systems such as UTM meters. The geographic elements contained in various map layers are drawn in a specific order (one on top of another) for the given map extent.

GIS datasets contain coordinate locations within a global or Cartesian coordinate system to record geographic locations and shapes. In this way, multiple GIS data layers can be overlaid onto the earth's surface.

Latitude and longitude

One method for describing the position of a geographic location on the earth's surface is using spherical measures of latitude and longitude. They are measures of the angles (in degrees) from the center of the earth to a point on the earth's surface. This type of coordinate reference system is often referred to as a geographic coordinate system.

Longitude measures angles in an east–west direction. Longitude measures are traditionally based on the prime meridian, which is an imaginary line running from the North Pole through Greenwich, England, to the South Pole. This angle is longitude 0. West of the prime meridian is typically recorded as negative longitude, and east is recorded as positive. For example, the location of Los Angeles, California, is roughly plus 33 degrees, 56 minutes latitude and minus 118 degrees, 24 minutes longitude.

Although longitude and latitude can locate exact positions on the surface of the globe, they do not provide uniform units of measure for length and distance. Only along the equator does the distance represented by one degree of longitude approximate the distance represented by one degree of latitude. This is because the equator is the only parallel as large as a meridian. (Circles with the same radius as the spherical earth are called great circles. The equator and all meridians are great circles.)

Above and below the equator, the circles defining the parallels of latitude get gradually smaller until they become a single point at the North and South Poles where the meridians converge. As the meridians converge toward the poles, the distance represented by one degree of longitude decreases to zero. On the Clarke 1866 spheroid, one degree of longitude at the equator equals 111.321 kilometers, while at 60° latitude, it is only 55.802 kilometers. Since degrees of latitude and longitude don't have a standard length, you can't measure distances or areas accurately or display the data easily on a flat map or computer screen. Using many (but not all) GIS analysis and mapping applications often requires a more stable, planar coordinate framework, which is provided by projected coordinate systems. Alternatively, some of the algorithms used for spatial operators take into account the geometric behavior of spherical (geographic) coordinate systems.

Map projections using Cartesian coordinates

A projected coordinate system is any coordinate system designed for a flat surface, such as a printed map or a computer screen.

Both 2D and 3D Cartesian coordinate systems provide the mechanism for describing the geographic location and shape of features using x- and y-values (and as you will read later, by using columns and rows in rasters).

The Cartesian coordinate system uses two axes: one horizontal (x), representing east–west, and one vertical (y), representing north–south. The point at which the axes intersect is called the origin. Locations of geographic objects are defined relative to the origin, using the notation (x,y), where x refers to the distance along the horizontal axis and y refers to the distance along the vertical axis. The origin is defined as (0,0).

In the illustration below, the notation (4,3) records a point that is four units over in x and three units up in y from the origin.

3D coordinate systems

Increasingly, projected coordinate systems also use a z-value to measure elevation above or below mean sea level.

In the illustration below, the notation (2,3,4) records a point that is two units over in x and three units in y from the origin and whose elevation is four units above the earth's surface (such as 4 meters above mean sea level).

Properties and distortion in map projections

Since the earth is spherical, a challenge faced by cartographers and GIS professionals is how to represent the real world using a flat or planar coordinate system. To understand the dilemma, consider how you would flatten half of a basketball; it can't be done without distorting its shape or creating areas of discontinuity. The process of flattening the earth is called projection, hence the term map projection.

A projected coordinate system is defined on a flat, two-dimensional surface. Projected coordinates can be defined for 2D (x,y) or 3D (x,y,z) in which the x,y measurements represent the location on the earth's surface and z would represent height above or below mean sea level.

Unlike a geographic coordinate system, a projected coordinate system has constant lengths, angles, and areas across the two dimensions. However, all map projections representing the earth's surface as a flat map create distortions in some aspect of distance, area, shape, or direction.

Users cope with these limitations by using map projections that fit their intended uses, their specific geographic location, and extent. GIS software also can transform information between coordinate systems to support integration of datasets held in differing coordinate systems and to support a number of critical workflows.

Many map projections are designed for specific purposes. One map projection might be used for preserving shape while another might be used for preserving the area (conformal versus equal-area projections).

These properties—the map projection along with spheroid and datum—become important parameters in the definition of the coordinate system for each GIS dataset and each map. By recording detailed descriptions of these properties for each GIS dataset, computers can reproject and transform the geographic locations of dataset elements on the fly into any appropriate coordinate system. As a result, it's possible to integrate and combine information from multiple GIS layers regardless of their coordinate systems. This is a fundamental GIS capability. Accurate location forms the basis for almost all GIS operations.

Key aspects of GIS

A GIS utilizes a layer-based geographic information model for characterizing and describing our world.

ArcGIS models geographic information as a logical set of layers or themes. For example, a GIS can contain data layers for the following:

• Streets represented as centerlines
• Land-use areas that represent vegetation, residential areas, business zones, and so forth
• Administrative areas
• Water bodies and rivers
• Parcel polygons representing landownership
• A surface used to represent elevation and terrain
• An aerial photo or satellite image for an area of interest

Geographic information layers such as those described here are represented using a few common GIS data structures:

• Feature classes: Each feature class is a logical collection of features of a common type (such as the four feature types shown here).

• Raster datasets: Rasters are cell-based datasets used to hold imagery, digital elevation models, and other thematic data.

• Attributes and descriptive information: These are traditional tabular information used to describe features and categories about the geographic objects within each dataset.

Like map layers, GIS datasets are geographically referenced so that they overlay one another and can be located on the earth's surface.

See Overview of geographic information elements for more information about modeling and representing geographic information.

A GIS uses maps to visualize and work with geographic information.

Each GIS includes a set of intelligent, interactive maps and other views (such as 3D globes) that show features and feature relationships on the earth's surface. Various map views of the underlying geographic information can be constructed and used as "windows into the geographic database" to support query, analysis, and editing of geographic information. Maps can also be used to access geographic modeling tools that are used to derive new information.

GIS maps are interactive and help to communicate vast amounts of information. You can reach "through" an interactive map to present any set of information that helps your end users meet their missions and do important work.

See How maps convey geographic information for more information about mapping and visualization.

A GIS has a comprehensive set of analytic and data transformation tools to perform spatial analysis and data processing.

GIS includes a large set of geoprocessing functions to take information from existing datasets, apply analytic functions, and write results into new result datasets. There are numerous spatial operators, such as the Buffer and Intersect tools shown here, that can be applied to GIS data.

Each geoprocessing tool takes existing information as input and derives a new result, which can be used in subsequent operations. This ability to string together a logical sequence of operations so that you can perform spatial analysis and automate data processing—all by assembling a model—is one of the key elements of GIS.

What is GIS?

A geographic information system (GIS) is a system used to describe and characterize the earth and other geographies for the purpose of visualizing and analyzing geographically referenced information.

Many have characterized GIS as one of the most powerful of all information technologies because it focuses on integrating knowledge from multiple sources (for example, as layers within a map) and creates a crosscutting environment for collaboration. In addition, GIS is attractive to most people who encounter it because it is both intuitive and cognitive. It combines a powerful visualization environment—using maps to communicate and visualize—with a strong analytic and modeling framework that is rooted in the science of geography.

This combination has resulted in a technology that is science based, trusted, and easily communicated using maps and other geographic views.