Sept 95 Level of Govt: State, local Function: GIS Problem/situation: Agencies have a wide range of database applications, each requiring different levels of accuracy

Solution: Agencies need to decide their most critical needs and design accuracy accordingly

Jurisdiction: Texas; San Diego, Calif

Vendors: none Contact: Roddy Seekins, Texas Department of Information Resources

512/463-6581 or roddy.seekins@dir.state.tx.us Bill McGarigle Contributing Writer Several administrative, technical and political factors must be considered when deciding how much accuracy to have in a proposed GIS database. From an administrative point of view, the major considerations are the primary application and the price tag of the database - two closely related factors, given that cost rises almost exponentially as the accuracy of the database increases

Because electronic presentation accuracy definitions are different from paper products, it is important to clarify those distinctions. The accuracy of a hard copy map is defined by its scale. A Quarter Quad map on Mylar, for example, which is one-fourth the area of a standard USGS map (or quad, for quadrangle) has a 1:12,000 scale, which means one inch represents 12,000 inches, or 1,000 feet, on the ground. The electronic, or digital, version of that map retains its 1:12,000 scale, but since it is now a raster (video screen) product, its accuracy is defined in terms of resolution and horizontal distance (spatial data), rather than "scale." Even though the scale can be changed electronically with a flick of a mouse, the digital, ortho quarter-quad (DOQQ) has a resolution of one meter, with a spatial accuracy of plus or minus 10 meters - which means objects having a width of one meter or larger will be displayed with a positional accuracy of plus or minus 10 meters. Linear features smaller than one meter in width, but having one meter in length, may also be discernible. We will look at the importance of DOQQs for GIS database accuracy later in this piece

APPLICATIONS Agencies have a wide range of database applications, each requiring different levels of accuracy. Engineering needs the highest levels, perhaps quarter-meter resolution or even better. An environmental protection unit using GPS to locate rare and endangered species habitats require point accuracy to enable biologists to return to a specific breeding location. If regional planning groups are studying intelligent vehicle highway systems, they will need resolutions of three feet or better. Less accuracy may show the vehicle as being completely off the road. On the other hand, planning applications involving spatial relationships between features require much broader measurements. A comparison of different corridors for routing a new road or pipeline, for example, may call for a resolution of plus or minus 50 feet

Some applications require both fine and coarse resolution. As Roddy Seekins, statewide GIS Planning Analyst for the Texas Department of Information Resources, pointed out, "there is a trade-off for having too high a scale [resolution]. The larger your scale is, the more data you have to deal with, which is why in some cases an initial analysis is done using LANDSAT imagery with 30-meter resolution. Analysts will identify a problem area in vegetation, then step up to one-meter resolution." ACCURACY DICTATES BASEMAP The accuracy required for a particular GIS application usually dictates the original graphic, or basemap, from which the digitized version is made. A basemap can be simply a parcel outline or lot lines; it can be a Census TIGER [topologically-integrated, geographically-enhanced reference data] map, or a USGS 1:24,000 scale map. It can also be satellite imagery, black and white or false-color, infrared (CIR) aerial photography, from which digital, ortho quarter-quads (DOQQ) are made

A DOQQ is a highly-accurate digital version of the original imagery. It is produced from stereo pairs of aerial photos, either black-and-white or CIR

A high-resolution scanner translates the photographic image into electrical impulses, removes the distortions inherent in