This year my columns have been about GIS data sources, including in-house information (BG, January 1995) digital files (BG, April 1995), and third-party tabular data (BG, August 1995). This column, the final in the series, focuses on images.

As used here, images are raster files that might or might not be registered to the earth's surface; more on this in a moment. Raster files essentially are computerized pictures made up of innumerable rows of tiny dots (pixels). Examples include information already in digital format (such as satellite imagery) and scanned images (building exteriors and interiors, floor plans, and even textual documents). Typical raster file formats (with the computer filename extension shown in parentheses) include Graphics Interchange Format (.gif), Tagged Image File Format (.tif), PC Paintbrush (.pcx), Windows bitmap (.bmp), SPOT satellite images (.bil) and Macintosh PICT images (.pct).

How are images useful? Images registered to the earth's surface, such as an aerial photograph, provide a GIS data layer that can then be viewed with other layers. For example, a brokerage company might have a points file of all the office buildings in a Metropolitan Statistical Area (MSA), together with a tabular file containing each building's name, size, age, number of tenants, and so on; the brokerage firm could create a GIS map in which the points are superimposed on the aerial photography image, with one or more attributes about each building written next to the appropriate point. This type of presentation might be preferable to, say, having the points plotted with street centerlines, census tracts, or other standard GIS layers.

That's the good news. The bad news is that accurately registering images to other GIS layers-i.e., raster files with vector files-can be complicated and time consuming. Sometimes the registration already has been completed by a vendor or other third party, such as registering a satellite image to the street centerlines of a given MSA. Absent the work being done by someone else, you will need a solid understanding of cartographic principles and appropriate data conversion routines in your GIS.

Images not registered to the earth's surface are far simpler to manage. The concept is to have one or more images linked to a point on a GIS map, so that you can retrieve the image(s) by clicking on that point. Using the previous example of a brokerage company's point file of buildings in an MSA, the company might store images of each building's exterior, lobby, and principal leasing agent; clicking on a building in a GIS display retrieves the images. Subject only to the limitations of the GIS software, anything that can be scanned can become an image retrievable through point-and-click.

Most-but not all-of the leading GIS software packages accommodate registered and unregistered raster files. Significant differences between GIS packages also arise in the routines available for registering raster files to vector files, and in the number of raster formats supported (.tif, .pcx, .bil, etc.). When purchasing GIS software, do not assume that all packages offer all the raster functions of interest to you.

Concerning data sources, some types of images can be ordered from vendors. The most frequently purchased raster files traditionally have been satellite imagery (usually from EOSAT or SPOT Image Corporation,) and digital orthophotoquads (from U.S.G.S). Today many types of raster maps are available directly from vendors or through the data catalogs of leading GIS software vendors; though too numerous to cover here, a salient example is scanned assessor parcel maps from, among others, TRW REDI Property Data in Riverside, CA and Metroscan in Sacramento, CA. A few firms also provide geocoded images of streets and building exteriors for selected cities across the nation, including Mandli Communications in Oregon, WI and GEOSPAN Corporation in Minneapolis, MN.

Despite the above availability of raster data, most real estate firms will create their own. The most likely methods will be: running a photograph or document through a high-resolution scanner; employing a black-and-white or color digital camera (currently about $800); or using a standard video camera in conjunction with computer software (currently about $300) that captures and stores individual frames from the video tape. Regardless of the method used, you will need to ensure that the scanner/camera/video output is in a format supported by your GIS software.

Homo sapiens, as a highly visual species, are drawn to maps and pictures. Not surprisingly, integrating vector and raster files in a GIS is a powerful combination indeed!