A geographic information system, or GIS, is a powerful data management system that provides users with a spatial understanding of locations and events by combining information as a series of data layers. GIS software provides the tools and functions that enable users to capture, store, retrieve, analyze, and display (map) spatial information. A GIS can integrate a variety of types of data, but most data layers in a GIS are considered “spatial data” because they are associated with specific locations on the earth’s surface (georeferenced) and linked to additional information (attributes) about that location. Spatial data can range from information that describes site-specific habitat conditions to information that depicts landscape-level events and conditions.
GIS is commonly used to locate or identify features of the earth's surface and to determine patterns of distribution. This includes examining the relationships that exist between features and phenomena over space and time if the appropriate data are available. The most common uses of GIS are for spatial data management, map production, site identification, spatial and temporal modeling, and statistical analyses.
Many different industries utilize GIS applications to display, examine, and model spatial information specific to their fields of study.
Examples of disciplines that make use of GIS applications include:
Many coastal resource managers use GIS to support natural resource management and habitat restoration, including recovery planning for endangered and threatened salmon populations.
The following data characteristics should be considered when working with a GIS:
Given the spherical nature of the earth, there are inherent difficulties in illustrating earth surface features on a flat, two-dimensional surface (computer screen or paper). To accommodate for this, mathematical formulas have been developed that convert the spherical coordinates of latitude and longitude to planar coordinates (like x,y values in feet or meters) on a flat surface.Each mathematical formula creates its own projection system, which distorts certain spatial properties in its own unique way. Different datums, or reference systems for measuring surface locations, are used to calculate planar coordinate values. Consequentially, users must be certain that geographic data are in the same projection system and datum so that the data layers are displayed appropriately in relation to one another.
Spatial data represent a portion of the earth’s surface such that each dataset has an inherent relationship between a certain distance on the map and the associated distance on the ground. Map scales may be written in a variety of formats including a ratio or fraction, and may indicate how many units on the earth’s surface are equal to one map unit (e.g., 1 map inch = 1 ground mile). The importance of considering scale when working with spatial data is that all datasets were created at a specific scale. When these scales differ, the information that a map provides may be misleading. A dataset created at a scale of 1 map inch equals 20 on-the-ground feet will provide more detail than a dataset created at 1 inch equals 2000 feet scale. When working with GIS datasets, it is important to know what scale will meet your mapping needs.
Spatial data come in two formats: vector and raster. The difference between the two formats lies in how they each store information describing the location of any feature type.
Vector format data stores information as one of the following:
Point features are single x,y coordinates. Examples of point feature data layers include water quality sampling locations, barriers to fish passage, and ecological restoration sites.
Line features are series of x.y coordinates. Examples of line feature data include roads, streams, and elevation contours.
Polygon features are series of x.y coordinates that start and end on the same coordinate. Examples of polygonal features include homogenous vegetation areas, administrative boundaries, and areas of landslide potential.
Raster format data is a cell-based representation of earth surface features. Each cell has a distinct value, and all cells with the same value represent a specific feature. Image and grid file types are stored in raster format.
Metadata is “data about data”. Specifically, metadata is additional textual information that makes GIS data more useful to those who work with it. Often there are certain parameters and technical considerations that are not easily conveyed with display of the geographic dataset, but are important to consider while using the data. This type of information is best provided in an associated text file that acts as a metadata record. Metadata files commonly come in .html, .xml, .txt, .doc and .pdf file formats. Examples of information provided in a metadata file includes a description of data development procedures, original creation date and updates, the data projection system and datum, attribute field definitions, explanation of attribute values, and data developer contact information. The Federal Geographic Data Committee (FGDC) has published guidelines for standardizing development of metadata.
GIS has many beneficial applications, however, like other technological systems, there are limitations to its use. For example, data for a specific area may lack spatial or temporal continuity. Data may be limited in distribution because of privacy issues. And GIS can be subject to misuse or misinterpretation. GIS professionals can avoid possible misinterpretation by clearly stating a data layer’s purpose and limitations in the metadata when developing GIS data, as well as using GIS data from other sources responsibly.