Transportation planning agencies such as state departments of transportation (DOTs) and metropolitan planning organizations (MPOs) are increasingly relying on computer-based tools to help manage and analyze data, and to present study results to decision makers and stakeholders. Geographic Information Systems (GIS) represent a class of computer-based data management tools that have become increasingly useful to transportation agencies in a variety of applications. Among state DOTs, the most common applications of GIS include road inventory management, environmental studies, and dissemination of planning and operations information to the general public.

This overview section provides a general introduction to GIS, including a discussion of why GIS is important in transportation environmental applications, a brief historical review on the adoption of GIS in transportation, and pertinent federal laws and regulations. Additional information related to GIS in transportation can be found at FHWA’s GIS in Transportation website.

What is GIS?

In its simplest form, a Geographic Information System (GIS) is a computer-based data management system for storing, editing, manipulating, analyzing, and displaying geographically referenced information. However, effective use of GIS also requires good quality data, skilled personnel, and institutional arrangements to collect, share, and disseminate the data.

Geographically referenced, or geospatial, data describes anything that can be located in physical space, most typically with respect to earth’s surface, and therefore can be displayed on a map. There are a variety of methods used for specifying geographic location, including geographic coordinate systems such as latitude and longitude; linear referencing systems such as mileposts; street addresses; and locational proxies such as state, county, zip code or Census Tract.

GIS software allows a user to load and display different geospatial features (e.g., roads, bridges, county boundaries) together on the same map display. Each feature is typically displayed as a separate layer on the map. By viewing different features together on the same map, a user can answer questions about the spatial relationships between features, such as proximity (e.g. how close is a proposed road alignment to a wetland area); adjacency (e.g., what land parcels abut a road); containment (e.g., how many people live within a half-mile radius of a transit station); and connectivity (e.g., can I get to my destination using a specific route)?

GIS software also includes various navigation tools that enable a user to change the scale of the map display in order to view a larger geographic area (zoom out) or a smaller area in more detail (zoom in), or to move the viewing area to a different geographic location (pan).

Geospatial data typically includes other information in addition to its geographic location. A road feature database, for example, may contain its common road name (e.g., Main Street), a sign route number (e.g., U.S. Route 1), and various characteristics about the road itself (e.g., number of lanes, average daily traffic, pavement condition, etc). GIS software allows a user to point to a specific feature or group of features and display the attribute information associated with the feature. GIS software also allows a user to select subgroups of a feature based on attribute values (e.g., find all airports with more than 100,000 annual enplanements), or on locational relationships (find all airports within 100 miles of Atlanta, Georgia).

In addition to the basic capabilities of map display, navigation, query and selection, GIS software may include other functions such as:

    • Geospatial editing – move the location of a feature, change the shape or alignment of a linear or area feature; add or remove features.
    • Data capture and transformation – import geospatial data stored in different formats, map projections or datums; reproject data stored in different formats into a common map projection for display and analysis.
    • Cartographic (mapmaking) tools – change the color, style or symbology associated with a geospatial feature display (e.g., show all railroads as cross-hatched lines); add labels and annotation; add legends, North arrows and distance scales; print a copy of the map display.
    • Geospatial analysis – calculate distances and areas; create buffers around features (e.g., create a half-mile buffer around a specific roadway); calculate polygon overlays (i.e., the amount of area that is common to two geospatial area features).

Why is GIS Important in Transportation Environmental Applications?

Transportation is inherently a geospatial activity, involving the movement of people and/or things from one geographic location to another. Not surprisingly, therefore, much of the data needed to support transportation planning, operations, and policy decisions include location as a key attribute.

Transportation environmental applications are particularly concerned with the impacts of transportation infrastructure and operation on both the natural and human environment. Since many transportation-related impacts typically diminish with distance (e.g. road noise is most severe immediately adjacent to the road), relative location is a key factor in evaluating the presence and severity of the impact that a transportation facility has on its surrounding environment.

GIS can significantly enhance the analysis capabilities for transportation environmental applications in the following areas:

    • Visualization – Visualization of geospatial data on a map-like display is the most common and obvious application of GIS. By presenting large volumes of data together in the same map, a GIS makes it easier for someone viewing the map to recognize patterns, trends, and/or anomalies in the data (e.g., locating road sections with large clusters of fatal crashes or increasing density of the road network in urban areas). The human eye and mind can recognize these visual arrangements without having to understand the data that produced them, which also makes GIS an effective tool for presenting geospatial information to a non-technical audience.
    • Data Analysis – Spatial analysis tools provided in many GIS software packages allow users to measure distances between geospatial features; to select and summarize attribute data based on spatial relationships (e.g., what percentage of an urban area’s population is within one half mile of a transit stop); or to compute statistics such as spatial autocorrelation, which measure how similarities in attribute values can be explained by locational proximity.
    • Data Integration – GIS is a very powerful tool for merging data from different geospatial features (e.g., creating a database of “environmentally sensitive areas” from separate geospatial features of historic and cultural sites, endangered wildlife habitats, wetlands, and parklands). It can also be used to transfer attribute data from one geographic representation of a geospatial feature to a more accurate representation through a process known as conflation.
    • Platform for Data Collected using Remote Sensing Technologies – GIS provides a platform for viewing, analyzing, and integrating data collected using remote sensing technologies. Remote sensing technologies include airborne and satellite-based:
      • orthoimagery – for producing distortion-free images of visible features on the ground.
      • multi-spectral imagery – for detecting conditions outside the normal visible range, such as vegetation health, heat signatures, underwater features, or moisture content of soils.
      • LIDAR (Light Detection and Ranging) – for accurately measuring ground elevations and heights of features (structures and vegetation) on the ground.
      • GPS tracking – using the Global Positioning System (GPS) to automatically locate the position of a vehicle, person, or animal as it travels over an extended period of time. GPS has been used to determine the habitat extent of various types of wildlife such as wolves, caribou, and bears.

Remote imagery can be used to update and enhance other geospatial databases such as road layers, or can be used to create a new feature (e.g., potential wetlands or wildlife habitats). Moreover, remote sensing can be applied over an extensive geographic area at substantially less cost and time than ground-based field surveys. It therefore is a very efficient means of screening for potential environmental impacts in transportation corridor planning studies and early project alternative analyses.

A Brief History of GIS in Transportation

The development of GIS and its adoption for transportation applications closely mirrors the evolution of computer technology itself. The history of computer-based GIS dates back to the 1960s, with some of the earliest work conducted at the Harvard Graduate School of Design’s Laboratory for Computer Graphics and Spatial Analysis. These early GIS applications focused on area-based analyses, overlaying multiple layers of attributes associated with areas (e.g., population density, land use, etc.) in order to create composite thematic maps. Computer technology during this period was slow and expensive to run by today’s standards, and was limited in the amount of data that it could store and process. Consequently, early GIS data structures were designed to minimize unnecessary information in order to maximize processing efficiency; only those attributes essential to the specific application were included. Also, since each application was unique and the geospatial data were generally developed from scratch, data sharing was not an important consideration.

Throughout the 1970s, GIS technology was mostly limited to research institutions and large government agencies that had access to large mainframe computers. Beginning in the early 1980s, the next generation of computer technology – minicomputers and workstations – packaged much of the processing power of a mainframe computer into a smaller, more affordable hardware platform. At the same time, some of the early GIS researchers developed commercial versions of their GIS software and designed them to operate on specific minicomputers and workstations. This combination of minicomputer technology and commercialization of GIS software enabled many more organizations to begin experimenting with GIS, along with other computer-intensive applications such as computer-aided drafting and design (CADD), and database management systems (DBMS).

State DOTs began developing and maintaining computerized databases on highway and bridge characteristics, traffic volumes, etc. during the 1970s. Information contained in these databases was typically linked to a location on the physical facility using linear referencing methods. State DOTs also were responsible for preparing state and county highway functional classification maps, as required by FHWA regulations (23 CFR 470.105). Through the 1970s and 1980s, most highway maps were prepared using manual methods or computer-aided mapping programs that operated more like CADD systems than GIS. Access to commercial GIS software allowed some State DOTs (e.g., New York, Wisconsin) to begin applying GIS technology to help manage road inventory databases and create thematic maps for statewide planning. Information on some of the most common commercially available GIS software may be accessed on the GIS Software page on FHWA’s GIS in Transportation website.

In 1987, AASHTO held the first GIS for Transportation (GIS-T) Symposium. Its purpose was to bring together transportation professionals to identify and discuss potential applications and unresolved issues in using GIS technology in transportation. A second symposium was convened in 1989, and the event has been held annually since then. Since its inception, this conference has helped measure the rate of adoption of GIS in State DOTs, and has provided a showcase for innovative applications, technologies, and research.

Perhaps the most significant catalyst for adopting GIS technology by the transportation community was the creation and widespread dissemination of the Census Topologically Integrated Geographic Encoding and Referencing (TIGER) database in conjunction with the 1990 decennial Census. TIGER was the first nationwide database of combined geospatial features including roads, railroads, rivers and other water bodies, and political boundaries. Furthermore, the Census Bureau took a proactive role in marketing TIGER to potential user groups, including planners, demographers, and business marketing groups. TIGER/Line files were released using standardized, well documented formats, on CD-ROM media, and at very low cost. The Census Bureau also worked with GIS software vendors to develop programs for translating TIGER/Line files into the vendors’ proprietary formats.

By providing a nationwide database of key geospatial features, TIGER significantly reduced the labor intensive and costly task of building geospatial databases from scratch. This made it much easier for smaller transportation agencies like MPOs to begin using GIS to display and analyze recently collected Census demographic data for input into transportation planning models, and for many more State DOTs to investigate use of GIS for planning applications.

Another key factor was the “coming of age” of microcomputers, with increased processing power and a de facto standard operating system (i.e., Windows), which enabled software developers to produce application programs that would run on different hardware platforms. Although most major GIS software developers were initially wary about migrating their commercial GIS software from workstations to microcomputers, by the early 2000s nearly all commercial GIS developers had produced a fully functional microcomputer version of their GIS software.

Access to GIS functionality using a microcomputer significantly increased the adoption of GIS technology both within and across transportation agencies. Smaller State DOTs and MPOs no longer had to purchase dedicated GIS workstations, and could therefore make GIS available to a larger number of agency staff working in different application areas, rather than limiting access to a few GIS specialists. According to information collected through the AASHTO GIS Symposium, the number of State DOTs that had established an officially recognized GIS unit within their organizational structure increased from less than 20 percent in 1990 to 100 percent by 2007. Significant increases in GIS use also occurred at MPOs, and to a lesser extent in the planning divisions of transit agencies, although these increases are less well documented.

The U.S. Department of Transportation (USDOT) also played a significant role in promoting the use of GIS for transportation applications. In the early 1990s, FHWA used GIS to develop the National Highway Planning Network (NHPN), a network database of the nation’s major highway systems, which is used to display and maintain information about the National Highway System (NHS) and the Strategic Highway Network (STRAHNET). Around the same time, legacy FHWA databases, such as the Highway Performance Monitoring System (HPMS) and the National Bridge Inventory (NBI), were modified so that they could be linked to the NHPN. These enhancements enabled what were previously just tabular summaries to be mapped, providing a more informative picture of the geographic distribution of national transportation facilities, and improving the quality of the data.

In 1995, an Office of Geographic Information Services was established within the USDOT’s Bureau of Transportation Statistics (BTS). Major goals of this office were to promote the use of GIS technology within the transportation community, encourage and facilitate data sharing partnerships for transportation geospatial data, and serve as a focal point for dissemination of national transportation geospatial information. This office created and continues to disseminate the National Transportation Atlas Database (NTAD), a compilation of national level geospatial databases of transportation networks, facilities and related areas, which is updated annually. It also functions as the federal lead agency for the coordination of transportation data standards within the Federal Geographic Data Committee (FGDC). FGDC is an interagency committee that promotes the coordinated development, use, sharing, and dissemination of geospatial data through the National Spatial Data Infrastructure (NSDI).

As adoption of GIS technology has grown among state DOTs, particularly over the past decade, GIS activities are typically divided between basic core functions and applications. Two essential components of a state DOT’s core GIS program are (1) a geospatial database of the state’s road network, and (2) one or more linear referencing systems to link the agency’s legacy databases to the road network.

Most state DOTs have devoted considerable effort to developing, maintaining, and updating these two components. Approximately one third of the state DOTs also have begun developing an enterprise data warehouse to consolidate all of the agency’s geospatial data using GIS as the integrating platform. Additionally, about two thirds of the state DOTs are actively engaged in developing web-based GIS applications to facilitate access to agency data and plans, both internally among agency staff and externally to the general public.