About GIS

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 Applications

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 various 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: natural resource mapping and management; emergency response; urban and regional planning; and education, health, and human services. Many coastal resource managers are now using GIS to support management and habitat restoration activities related to fisheries, including recovery planning for endangered and threatened Pacific salmon populations (see the GIS for Planning section for more information).

GIS Limitations

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. Additionally, privacy issues can sometimes limit distribution of data. GIS data may also be subject to misuse or misinterpretation. For example, the historic and current salmonid distribution data layers included in the SRP Interactive Mapping Application on this Web site maps entire streams if salmonids are or were recorded as present in any reach, even though very few creeks have passage all the way to the headwaters. GIS professionals can avoid possible misinterpretation of data by clearly stating the purpose and limitations of the data set in the metadata file when developing GIS data, as well as by using GIS data from other sources responsibly. For information on other factors that should be considered when working with GIS data, see Other GIS Considerations.

The Salmonid Habitat Restoration Planning Resource (SRP) GIS

As part of this Web site, a customized GIS (the SRP GIS) has been created with tailored spatial data for use with Environmental Systems Research Institute's (ESRI) desktop ArcGIS® software and Internet mapping server software, ArcIMS®. (To learn more about these software packages, visit ESRI's Web site).

The SRP GIS includes over 200 data layers covering the San Mateo and Santa Cruz areas of Central California. These data layers have been specifically selected for their physical, biological, ecological, management, or socioeconomic relevance to planning for salmonid recovery in this region. Most have been acquired from public agencies and non-profit or watershed groups, while some were created specifically for this project to fill critical data gaps. The data layers have been organized into 13 thematic categories based on the nature of the data:

• Base Layers (those generally used for creating base maps, including some with state-wide coverage)
• Climate and Weather
• Ecosystem Management
• Environmental Quality
• Hydrology
• Infrastructure
• Jurisdictional Areas
• Land Use, Land Cover and Vegetation
• Physical Geography
• Salmonid Distribution and Habitat
• Socioeconomics
• Watershed Assessments (data from local watershed assessments)
• Wildlife

The SRP GIS was developed and managed using ESRI's ArcGIS (ArcView 8) software. The data layers compiled into this customized GIS are available for download and use in either ArcGIS or ESRI's ArcView 3.x desktop GIS software. See the SRP GIS Data Inventory for more information about the data layers and to begin downloading them. To learn more about how these spatial data can be used in planning for the recovery and management of threatened salmonid populations in Central California, go to the GIS for Planning section.

For those users who are new to GIS mapping, or want a quick look at some of the spatial data, an ArcIMS application for the SRP GIS has been developed. This on-line mapping application provides users with the tools to examine specific locations and events of interest as they relate to salmonid resources in the San Mateo and Santa Cruz watersheds, directly through this Web site (go to Interactive Mapping).

Other GIS Considerations

When working with GIS data, in general, all users should take into account the projection, scale, data format, feature type, and metadata for a data layer. The following definitions and specifications will help GIS users implement their specific mapping and analysis efforts when using the SRP GIS.

Projection . 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. All spatial data in the SRP GIS are projected to the Universal Transverse Mercator (UTM) Zone 10 North coordinate system, using the North American Datum of 1983 (NAD83).

Scale . 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 (i.e. 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. Datasets in the SRP GIS vary in scale, and their use depends on the intended purpose of each analysis. The scale at which the spatial data were created can be found in the metadata file (link to below) accompanying each dataset. When working with these datasets, it is important to know the scale that will meet your mapping needs.

Data format and feature type . 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 (point, line, polygon). Vector format data stores information about points as a single x,y coordinate, lines as a series of x,y coordinates, and polygons as a series of x,y coordinates that start and end on the same coordinate (see example). 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 (see example).

Geographic data representing each of the three possible vector types (points, lines, and polygons) of the earth's surface can be found in the SRP GIS. Examples of point feature data layers include water quality sampling locations, barriers to fish passage, and ecological restoration sites. Examples of line features include roads, streams, and elevation contours. Examples of polygonal features include homogenous vegetation areas, administrative boundaries, and areas of landslide potential. Multiple file formats are also included in the SRP GIS, since ArcGIS software is able to integrate a variety of data types. The SRP GIS includes shapefiles, grid files, image files, coverages, and additional tabular data (in dBase file format) that are joined or related to certain spatial data layers.

Metadata. Metadata is essentially "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 include 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 (see FGDC's Web site). All spatial data layers in the SRP GIS are accompanied with metadata files and were developed (or edited) according to FGDC standards.

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