| GIS Goes Public
An innovative approach to data conversion saves Summit County, Ohio more than $5 million.
By Robert N. McLaughlin An innovative, unconventional approach to data conversion saved Summit County, Ohio, more than $5 million - and more than 60 percent - of projected costs to implement GIS. The county not only saved money, it saved time and brought the benefits of GIS to the public sooner.
Integrating raster and vector technologies allowed the county to have a publicly accessible GIS in just six months.
The Summit County GIS is a cooperative effort involving the county auditor, the county engineer, and the county executive. Auditor James B. McCarthy stressed throughout system development that "the public access system must deliver information directly to the public in a user-friendly manner."
Summit County has founded its GIS project with an eye towards better public service and an emphasis on direct access by the taxpaying public. The GIS was developed to deliver information to the public as a priority, not as an after thought, and is providing available information on residential and commercial properties within Summit County.
Unlike other projects around the country, the GIS was designed for immediate use and access by the public. A user at one of the system's terminals can call up an index map of the entire county. Selecting a township or city calls up a raster image of the old map book index sheet, with cross-reference symbols and vector parcel centroids superimposed on it. Pointing to a cross-reference symbol brings up a current map, while the parcel centroid brings up parcel data. Users can access the appraisal database, zoom, pan, and print out maps and tabular data.
In April 1994, Woolpert of Dayton, Ohio was selected to create the base map which supports this project. The base mapping to support this project was developed by photogrammetric mapping techniques. First, a digital elevation model was compiled to generate 2-foot ground contours countywide. Vector planimetric information was compiled, consisting of road pavement edges, hydrography (rivers, creeks, streams, and so on), buildings and railroads. The primary base map developed was county-wide, color digital orthophotography - ortho-rectified to the DEM. Orthophotography for Akron and the surrounding urban areas was developed at 1"=100' scale with a 0.5-foot pixel resolution. For the remaining, less urbanized areas of Summit County, the orthophotography was developed at 1"=200' scale with a 1.0-foot pixel resolution.
Rapid-static Global Positioning System (GPS) surveying techniques, established by McCoy and Associates of Akron, Ohio, were used to determine first-order positions for 272 concrete monuments throughout the county. These monuments and additional photo ID points were targeted to serve as ground control for the photogrammetric mapping. Rapid-static GPS surveying uses multiple-channel L1, L2 and P-code capable receivers that allow short satellite observation times. The observation time depends on many factors, including the number of satellites, multi-path errors, available observables on L1 and L2, atmospheric conditions and so on. Static GPS surveying generally requires more than an hour of data collection, but rapid-static surveys have been conducted with just five to 20 minutes of data collection. The average observation time for this first-order survey was 20 minutes per ground control point. The final accuracy for the survey was 1:100,000. Positions were also determined for 78 additional monuments through a cooperative effort with the Summit County Engineer's Office. All vertical accuracies met the GPS equivalent of third-order requirements or better.
After the final coordinates were determined for the ground control points, analytical aerial triangulation was used to extend and density ground control for 1"=800' scale color aerial photography. All photogrammetric compilation to develop the DEM and the vector planimetric mapping was performed using 1"=800' scale color aerial photography - approximately 950 stereo models in all.
During the DEM development, the edges of road pavement and hydrographic features were compiled as elevation breaklines. Compiling these features as breaklines allows the DEM to interpolate elevations between measured points on the line. The elevations interpolated along a breakline will not be influenced by any other measured elevation points outside the breakline. This allows an observable change in the terrain to be accurately modeled by the DEM and assures that the generated contours will accurately represent those observable changes. Breaklines are also placed at all ridges, tops and bottoms of embankments and valleys, bases and tops of hills, retaining walls, and any other terrain feature denoting an observable change in the terrain. After breaklines are compiled, additional elevation points are placed at a density sufficient to accurately represent the shape of the ground.
After photogrammetric compilation of the DEM and planimetric features, 2-foot contours were generated from the DEM. Addition of road and water body names and development of a street centerline network completed the vector mapping, which was delivered as ARC/INFO coverages.
Imagery for the color digital orthophotography was scanned from film diapositive prints of the aerial photography. For the 1"=100' scale mapping areas, diapositives of the 1'=1,600' scale photography were scanned. All image scanning was performed at a 15 micron (1,693 dpi) resolution. For the 1'=800' and 1'=1,600' scales of aerial photography, this yielded ground resolutions of 0.47 feet and 0.94 feet, which were subsequently resampled to 0.5-foot and 1.0-foot resolutions, respectively.
Each scanned image was then ortho-rectified to the DEM, and a process of interactive mosaicking was performed to create a set of continuous images throughout the county. In this process, natural ground features such as road and fence boundaries are selected as the border between adjacent digital images. The mosaicking process was done interactively, enabling the workstation operator to select the optimal points for joining images, rather than automatically, which always connects images at their midpoints.
Interactive mosaicking eliminates significant tonal differences between adjacent images. For example, there won't be a discernable light-dark boundary across a parking lot when the parking lot is taken from the left side of one image and the right side of another. In addition, interactive mosaicking prevents the loss of buildings that fall on a border created by automatic mosaicking which is caused by the relief displacement inherent in any aerial photograph. For example, displacement sometimes makes tall buildings appear to fall away from the center of an aerial photograph. By assuring that image borders do no pass through any buildings, all the buildings can be shown.
Another special step taken for the orthophotography was to modify the DEM data around bridges to prevent them from appearing warped on the orthophotography. The DEM data serves two purposes for this project: to ortho-rectify the digital images to produce the digital orthophotography, and to generate 2-foot contours that meet National Map Accuracy Standards. But developing a DEM to produce contours causes bridges that are above the actual ground surface to appear warped. Developing a temporary copy of the DEM for contours and placing new elevation points around the bridges prevented the warping. The new elevation points averaged the height between the bridge and the ground below.
Orthophotography was delivered in .TIF (Tag Image File) format, both as 24-bit and 8-bit images. Resampled data sets were also delivered at 4-foot, 8-foot, 16-foot, 32-foot pixel sizes to decrease display time for larger areas.
The base map is the foundation of the Summit County GIS project. This data provides the other GIS project participants; the County Engineer and Executive's office, an immediate landbase from which they can draw information from and can construct information. The photos also provide a foundation on which the tax maps can be rectified as well as a visual reference that the public can easily understand. The decision to go with color orthophotos versus black and white is driven by the desire to make the data as clear and useful to the public as possible.
The next step in the implementation process was to address the data conversion and maintenance processes. The existing maps needed to be scanned in and label points needed to be created for the map cross-references and parcel labels. Because Summit County needed to continue with its day-to-day work, one of our goals was to retrain existing staff rather than hire additional people for the GIS. We choose to use a local scanning company, AAScan, for the conversion. They developed routines and procedures to rasterize and cleanup the images and to digitize in points for all of the labels using AutoCAD. The county developed its own routines and procedures to import and validate the data converted by AAScan. One of the windfall benefits of the conversion process was that the tax maps were compared to the real estate maps and to the appraisal database. All of the discrepancies identified by this process were then researched and resolved thereby improving the overall quality of both the county's GIS and appraisal databases.
The image data is comprised of simple black and white images of the old tax maps stored in a TIF format using group IV compression. During maintenance these images are converted into the ESRI grid format. The map cross reference symbols are ESRI point coverages. The parcel coverage contained both the centroid label points and any parcel boundaries that have been updated since the conversion. The raster data beneath the map updates is, of course, erased. The Tax Map department has also started to capture additional information that used to be left off of the maps due to clutter. They now capture and retain street centerline, allotment boundaries, and easement boundaries for new allotments. They also place combine lines in separate coverage in anticipation of the day that the parcel data is turned into polygon data.
The Public Access System was developed in ArcView with ESRI's Avenue scripting language. The majority of the development work was done by ESRI of Redlands, Calif., with minor enhancements being made by the Summit County GIS staff. The map maintenance is done with ESRI's ARC/INFO, the Image editor and the COGO toolkits.
The Public Access System starts out with an index map of the entire county. From there, the user selects one of the townships or cities causing the application to display the raster image of one map book index sheet. Superimposed on the image are map cross-reference symbols when selected, the map represented by that symbol replaces the current map. If the user points to one of the parcel centroid symbols, a small subset of the parcel data is displayed. The user may then elect to access the appraisal database directly from the 3270 screen.
At this point the ArcView portion of the application transfers control to the HLLAPI portion of the application via its Inter-Application Communication (IAC) calls in Avenue. The HLLAPI programs raise the 3270 window to the foreground and initiate a lookup using the parcel number identified in ArcView. Conversely, if a user looks up a parcel on the 3270 screen and would like to see the map associated with that parcel all they need to do is press the PF4 key. The HLLAPI program captures this keystroke and then collects key pieces of information off of the 3270 screen. The ArcView IAC calls are used once again. This time, however, the data is passed back to the ArcView application which then proceeds to do a lookup and displays the map and parcel identified on the 3270 screen.
The user has full control over the map display, they can zoom in, zoom out, pan or revert to the previous map. They also have the ability to print out the portion of the map they are viewing along with selected values from the database. Should the user want a printout of the full appraisal database for the selected parcel, they can use the same print key they had used in the past. This means the public will no longer need to be bounced back and forth between the Tax Map department and the Real Estate department to collect their information.
First and foremost, the goal of this project is to provide better access to the public. We want to put in place a mechanism to get the data into the hands of the public. As the project progresses, we continuously re-examine everything, we ask ourselves why, and our vendors why not. With the help of PlanGraphics of Frankfort, Ky., who delivered the most cost-effective solutions, we believe the results speak for themselves. We have improved the quality and availability of data to the public, simplified the property transfer process for the public, and streamlined the county workflow. This was all accomplished in about one third of the time estimated, and for about one third the estimated cost. About the Author:
Robert N. McLaughlin is the GIS director for the Summit County Auditor's Office. He may be reached at 216-643-2732. Back |
