The Brick Township parcel map.
The county tax administrator stated in the mandate cover letter ". . . we strongly recommend that you invest in the computerization of the appraisal data that is collected during this revaluation and a system be implemented to maintain this data on an ongoing basis." Brick Township applied this recommendation to the mapping process as well as the assessment data.
Automation of The Tax Maps
In April 1987 a complete set of municipal tax maps was submitted to the division of taxation. The revaluation mandate required the municipality to "take all necessary steps to update the tax maps in accordance with the instructions issued by the director" (of the division of taxation) to meet the regulations in effect at the time.
The tax maps were originally prepared in the mid-1950s and approved by New Jersey at that time. No map revisions had been submitted for review/approval since. The division of taxation's initial review of the "current" maps issued a response stating that they had "discovered numerous deviations from the tax map regulations" and required "that all necessary revisions be made to the entire set before resubmitting for final inspection." The "set" consisted of 220 sheets.
At this point GIS became a real consideration for the township. Officials authorized a middle-of-the-road approach from the options presented for consideration. These ranged from manually redrafting the entire set of tax maps to meet regulation standards, to full GIS-capable base/parcel mapping. In December 1987 a small portion of the township was automated as a pilot study to finalize system configurations and staff needs.
The advantages of computerized mapping were obvious. Consistent standards and easy continual updating, which had been dreams in the past, were now available. With the ability to "tag" assessors' records to computerized map parcels, the GIS goal was solidified. The pilot study feasibility report and final findings recommended that the municipality join with other municipalities and private enterprise to establish an ongoing program to develop a multipurpose GIS.
In June 1988 the township received delivery, installation, and training of the initial computerized mapping system. This consisted of a 386/25 PC with 120-MB hard drive, high resolution graphics, a 36-by 48-in. digitizer, and an 8-pen plotter. Digitizing the entire set of municipal tax maps then began. A continuous dialogue with division of taxation officials was set in motion to ensure compatibility with regulation standards. To accelerate the program, and meet the revaluation mandate deadline, a second similar PC was authorized for acquisition and installed in December 1988.
A systematic development plan was prepared for the automated mapping program to move toward GIS. In September 1989, authorization was given for acquiring three additional PCS to utilize mapping data with existing record attributes in the planning board, land use, and engineering offices.
In July 1991 the division of taxation issued approval of the computerized tax maps for revaluation purposes. The tax mapping revision/maintenance process has operated continually since that time; cost to develop - $200,000.
GIS Base Mapping
The limitations of using the township-wide composite of tax map sheets as a GIS base map were quickly made apparent. While attempting to create both an index and a street map from the composite, it was determined that a quality base map was crucial to the further development of the GIS program. Various funding mechanisms, cooperative efforts, and other agency participation options were evaluated to assist with the continuation/development of the mapping program - out of the tax map environment into the GIS world.
Realizing both the tremendous need and potential for township-wide GIS development, the utility authority assumed the leading role from this point forward. Agreements were prepared between both township and authority for GIS/mapping development. Multi-agency cooperative efforts were established from the local to the federal level. Accurate base mapping and utility infrastructure management were the priorities. Parcel mapping became an easily obtainable "by-product" of the already existing digital tax maps.
The 26 sq mi of Brick Township were prepared for digital photogrammetric base mapping. Global positioning system technology was utilized to establish precise geodetic mapping control monumentation throughout the township. A natural color aerial overflight was completed in April 1993. The utility authority partnered with four other participating agencies (both public and private) to share the base mapping development costs. They were: The Township of Brick, The County of Ocean, New Jersey Bell Telephone Company, and New Jersey Natural Gas Company; cost to develop - $300,000.
Digital topographic mapping was prepared to meet national map accuracy standards at a hard copy output scale of 1 in. = 100 ft (1:1,200) with 2-ft contour intervals. All mapping was developed within the latest horizontal and vertical datum standards (NAD 83 and NAVD 88). The New Jersey State Plane Coordinate System was utilized for geographic orientation. The digital data were delivered on a single CD-ROM in AutoCAD (AutoDesk, Inc., Sausalito, California) drawing format, comprising 195 tiles covering the entire township.
Parcel Mapping Development
The importance of a township-wide parcel map coverage was recognized by the Brick Township Municipal Utility Authority (BTMUA) and the township. The parcel map "layer" was to hold parcel map information in a contiguous township-wide coverage in NAD 83. The geographic data were to be held in topological data structures, thus allowing for spatial analyses. The parcels' attribute data were to come from the tax assessor's MODIV database.
After researching and evaluating several different methods for digital parcel map creation, BTMUA selected and approved the following procedure:
* ArcCAD software (Environmental Systems Research Institute [ESRI], Redlands, California) was selected for the project. ArcCAD was chosen because of its capability to work directly with AutoCAD, and add GIS functionality to it.
* The digital tax maps (in CAD format) were to be utilized as the graphics data source.
* The tax maps were to be geo-referenced to match the digital base map.
* The tax maps were then to be converted from CAD drawing files to PC ARC/INFO coverages with topology.
At this point the need for hardware arose. The BTMUA acquired a 486DX66 PC with 32 MB of RAM and 3 GB storage space, and a HP 650C inkjet plotter. The township and the consultants acquired similar hardware.
The process of converting the CAD tax maps into a geo-referenced GIS coverage was neither quick, nor easy. Here are several steps along the way:
* In an ArcCAD session, a tax map file and a base map file were brought together simultaneously. The tax map file appeared in the origin of the map, whereas the base map file was in its proper coordinate position.
* All elements of the tax map (entities) were moved to their proper location. Generally, about five to ten adjustments were needed to ensure a proper fit. We will deal later with the issue of what was considered a proper fit.
* Not all layers in the tax map were considered important for the parcel map. Thus, the title block, secondary lot lines, noah arrows, street names, etc., were not displayed (frozen.)
* A deliberate ArcCAD procedure was developed to convert the block and lot lines into parcel polygons, and to carry over the block and lot numbers from the tax map as attribute data associated with the polygons. Because of the introductory character of this article, we will not explain the procedure in full detail. The procedure includes data format conversion, correcting topological errors (normally and naturally abundant in CAD files), and numerous topological overlays. As a result, every parcel/polygon in the cove rage was identified with its block and lot number. We will see later why that was important, and what exceptions we had to make.
* When two or more tax maps/coverages were brought together, a series of procedures known as "edge-matching" had to be performed.
Following the same script, three different GIS teams started to build the parcel map coverage, working on specifically assigned areas. Those teams were the BTMUA GIS team, the township's GIS, and their GIS consultants - Owen, Little & Associates, Inc. The project is still under way, and we look forward to reporting its successful completion in a future issue of PUBLIC WORKS.
What Remains To Be Done?
Once all tax map files are properly converted, i.e., when the parcel map coverage is built, for it to be useful it has to carry the MODIV database. We have designed and tested the following procedure for the process of associating the two databases.
Every polygon in the parcel map coverage is uniquely identified by its block and lot number. So is every record in the MODIV database. (There are some exceptions here, for example, multiple condominiums on one parcel. We will deal with those later.) Using some basic relational database management techniques, we will generate a field in each database storing the value of each record's/polygon's "unique identifier." Then, we will merge the two databases based on the value in that specific field. Ideally, every parcel/polygon will receive its MODIV data.
Ideally. In the real world we live in, there are errors - computer errors and human errors. We are prepared to deal with those errors, we know they are imminent. We expect 10 to 15 percent discrepancies in the databases, which will result in mismatches. Luckily, because of GIS's power, those mismatches are easy to identify. Still, we expect that at least 3,000 records (from an entire database of over 30,000) will have to be revisited one by one, and the source of discrepancies identified and corrected. While tedious, this process will provide very important data check and feedback to the agency where the error occurred.
There is a special isolated case of multiple mismatches that we know will occur, with no errors involved. This is the case of multiple property ownership, condominiums being the classic example. Normally, a GIS expects a one-to-one relationship between spatial and non-spatial data. That means that one parcel has one owner, which is not always the case.
Several approaches can be taken to circumvent the issue of multiple property records tied to one parcel. One is to store the condominium footprints in a different coverage and relate the condominium property records to that coverage. Both coverages can be displayed together, and this data management trick will be transparent to the end user. We took that approach for the Brick GIS. Another approach is to establish a one-to-many relationship between two tabular databases, and display both at all times.
Spatial Accuracy And Precision
We used the digital base map to georeference the digital tax map files in a process we call "visually best-fitting." After testing several flavors of this process, the BTMUA decided to do best-fitting on a block by block basis, if necessary. Naturally, that kind of fit is not perfect (yields about 5 ft precision at best.) We determined, though, that this process will give the best return for cost, time, and effort invested. Higher precision could be attained by best-fitting on a lot by lot basis, but that approach was considered too time-consuming and costly. Estimated cost based on the shared method (already having the data in digital format) - $2.00 per parcel.
Now is a good time to mention that the parcel map coverage is a "live" database. It is dynamic, as many of its components change over time - ownership records change, subdivisions occur, etc. The spatial component of the coverage is also live and open to fine-tuning and adjustment. A survey of a property can, and should, be used to update and improve the spatial accuracy of the parcel map layer. The same applies for property corners' coordinates collected by BTMUA's GPS unit.
All this is only the beginning. A complex set of dynamic databases such as the township's parcel map coverage has to be maintained and updated regularly. Procedures have to be developed for all updates, and their relationship to the tax map updates. That is, until GIS coverages are recognized and accepted as new and better tax maps, which they already are.
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|Title Annotation:||geographic information systems used in digital mapping|
|Author:||Garnett, Richard E.; Entchev, Atanas E.|
|Article Type:||Case Note|
|Date:||Aug 1, 1996|
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