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GIS--The Tool for Cadastral MappingBy Brian McLaughlin, Chief Cartographer, Rains County Appraisal DistrictEditor's Note: The Rains County Appraisal District began its GIS program in the summer of 1999. The author used new software, data, and ideals to implement the program. In the BeginningLocated about 70 miles east of Dallas, Rains County is a rural county of just 256 square miles. This makes it one of the smallest counties in Texas. With a population of around 8,000, it also has a very limited tax base. The County's previous cartographer used AutoCAD to map the County. The quality of the work was such that I felt it was better to start over rather than spend years cleaning up garbage. For counties that already have good CAD drawings, ArcView GIS and many other GIS programs can easily import these CAD drawings, providing a head start on GIS implementation. Since I already had knowledge of ArcView GIS, I was not interested in teaching myself AutoCAD. The appraisal district had previously purchased ArcView GIS 3, so upgrading to the most current version cost little. For Rains County, ArcView GIS seemed to be the best software for a cadastral GIS. Choosing GISAfter choosing the best GIS software for the job, the next thing was to decide what the accuracy requirements would be. Having digital survey data available for a county will put you one step ahead. However, if you are from a county similar to Rains County, you may be the first agency generating such data. Accuracy is relative to the task at hand. When considering accuracy requirements, remember the purpose and audience for these maps. As a cartographer for an appraisal district, my job is to help the appraisers locate properties in the field and locate which parcels are being divided into smaller parcels. I am not making maps that will be used to lay pipelines or determine rights-of-way. Accuracy costs both time and money-two things that most appraisal districts are lacking. As long as I maintain a relatively accurate parcel map, meaning the appraisers have no problem locating the properties in the field, I have done my job. Basemap DevelopmentThe most important data to begin the GIS was the basemap. The basemap defines the boundaries of the project and provides control points to help keep maps accurate. USGS 7.5' digital quad maps and digital orthophotos are two good sources of basemap data. Orthophotos for Rains County were not yet available, so the GIS project was started with the USGS quad maps. Sure!MAPS RASTER maps based on the 7.5' USGS quad maps provided seamless coverage of the entire County and came with a utility that exported the maps in a variety of projections and datums. Quads can be exported in TIF or GeoTIFF format and are readable by most GIS software. The USGS quad maps contain features such as political boundaries and cemeteries that the orthophotos alone do not delineate. The USGS quad maps were a great start to begin our GIS, but many of the USGS 7.5' quads for Rains County had not been updated since the middle of the 1950s. Since then new roads and growth in incorporated areas meant more up-to-date data was needed. Orthophotos seemed like the ideal choice, but the cost to have new ones flown was too prohibitive. Using DOQQ DataIn November 1994, the Texas Geographic Information Council passed a resolution to support the development of low-cost color infrared (CIR) digital orthophoto quarter quadrangles (DOQQs) State-wide and formed a partnership with the USGS, State and local agencies, and private industry to produce them. The project, known as the Texas Orthoimagery Program (TOP), was structured so that the USGS and the Texas Department of Information Resources (DIR) covered 75 percent of the cost and private industry and local governments covered the remaining 25 percent. That partnership brought the cost of each 7.5' orthoquad to about $900, easily within the reach of most budgets. After the initial cost was covered, the orthoimages became part of the public domain and the price for standard deliverable products was reduced to $50. Four 3.75' DOQQs were required for each 7.5' quad. Uncompressed at one-meter resolution, a DOQQ uses 158 megabytes of disc space, and a CD-ROM can hold up to four images. At 2.5-meter resolution, a DOQQ requires only about eight megabytes of disk space, and each CD-ROM can hold 64 DOQQs. Though most Pentium class computers should have no problem handling DOQQs, image compression may be a good idea when viewing more than one DOQQ at a time. Two image compression options are available. An extension that comes with ArcView GIS supports the MrSID image compression format developed by Esri business partner LizardTech. ERMapper also offers a free image compression utility that I used to compress the DOQQs to about eight megabytes per 3.75' quad with very little resolution loss, allowing the entire County to fit on one CD–ROM. Adding More DataWith the basemap established, it was time to start adding more data. I obtained most of the data I needed from the Esri Web site. Census 2000 TIGER/Line Data is available to download free of charge. The accuracy requirements were acceptable for our GIS, though more accurate data can be obtained from a commercial vendor. The TIGER data was easily rectified over the DOQQs. The roads were all line data that took about a week to rectify. The Rains County roads are classified as U.S. highways, State highways, farm to market (FM) highways, County roads, and private roads. Though each class is archived in its own layer, all layers were merged into one layer to simplify the View. The TIGER data included many other layers that were used: county boundary, streams, water boundaries, incorporated outlines, school districts, and water bodies. Though data availability varies by county, there was a complete listing for Rains County. Four Smokes NorthWith all the base data assembled, it was almost time to begin parcel mapping. Rains County is a rural east Texas county with few subdivisions and many large tracts of land that are described only by metes and bounds. Many of these parcels have been passed down from generation to generation and have not been surveyed since the 19th or early 20th century. The survey descriptions for these parcels do not contain precise points of beginning and have such vague descriptions as "four smokes north" or "300 varas to a rock in the road" (a vara is an old Spanish measurement equal to about 2.77 feet). Diligent research and educated guessing were required to trace these parcels back to the original surveys. Unlike most states, Texas is not organized by section, township, and range. Spain, Mexico, and France surveyed most of Texas before the section, township, and range system was created. When Texas became an independent nation and then a state of the United States, all the previous land grants were honored. The original land grants, called abstracts, have unique sizes and shapes, making it difficult to map parcels in rural Texas counties that have not been platted. Tobin International of San Antonio, Texas, was the source for an ArcView GIS-compatible digital map of the abstracts for Rains County. With all of the base data assembled, parcel mapping could begin. Adding Parcel DataSubdivision plats can be digitized using digitizing tablet or can be scanned and digitized on-screen. Plats should be georeferenced in the same projection and datum as the basemap. The ArcScripts section of the Esri Web site contains free extensions that allow you georeferencing scanned images in ArcView GIS. Once the scanned image is georeferenced, the lot boundaries can be digitized on-screen. Properties that only have metes and bounds descriptions require a different technique. I used PMCgis Toolbox, available from PMC Promap Corporation, an Esri business partner. Two other commercial extensions are available from CEDRA. All these extensions add metes and bounds descriptions directly into ArcView GIS using coordinate geometry (COGO). Parcel data creation for Rains County has begun. Each parcel is a unique polygon named after the parcel identification number so that it can be queried and data can be attached to it. Using the existing CAD maps, the parcel must be manually located and the attribute data obtained from a separate database. Rains County uses a program called Automated Central Appraisal District, supported by The Software Group of Plano, Texas, to maintain all the parcel attribute data. After migrating to ArcInfo, parcel maps will be linked directly to this database. This database exports files in .dbf format that can easily be joined to an ArcView GIS table or imported into other GIS software. My goal is to complete 25 percent of the County's parcels before linking the database to the parcel map. The FutureGIS technology enables appraisal districts to manage parcel data in a manner not possible with hand-drawn or CAD-generated maps. The ability to query alone is enough reason to convert to a GIS system. The amount of time saved by being able to instantly locate a particular parcel is enormous. An appraiser can easily determine if a parcel is located in a floodplain, if it has water frontage, or if it is located along a highway. All these conditions affect the value of a property. Square footage and acreage can be calculated on the fly. Database connectivity saves map space and reduces clutter because every bit of text-owner's name, book and page number, or size-does not need to be squeezed onto the map of the parcel. All this information can be stored in the database and displayed by clicking on the parcel. A cadastral GIS is more than a parcel map. New maps can be created quickly and selectively. Maps can be created of just commercial properties, just homesteads, or just residential properties. Imagination and database design are the only limitations. The Bexar County Appraisal District, located in San Antonio, Texas, has one of the best cadastral GIS systems. It is featured in the book Zeroing In: Geographic Information Systems at Work in the Community. The Customer Query System created by the GIS staff allows home owners to compare their property's value to the assessed value of similar and/or surrounding properties and view recent sales figures. Before the Rains County Appraisal District can take advantage of advanced GIS techniques, more parcels must be mapped. The hardest part of this time-consuming process is deed research. More hours are spent studying the deeds than entering the parcels. Fortunately, Rains County has a progressive board of directors that has provided the best equipment the County can afford. The process will be speeded up with the use of a GPS receiver. ConclusionThe first step to GIS implementation is proper planning. Careful planning will save time, money, and disappointment down the road. It is better to proceed slowly than do too much too quickly and make mistakes that are irreversible. I am constantly evaluating my methods to ensure that our GIS will be the best that it can possibly be. I evaluate my long-term plan every year and my short-term plan every four months. I attend professional seminars and visit with other appraisal districts and GIS users. The County has a very limited budget, so my philosophy is to make the best use of limited resources and find creative ways to obtain needed items. Before beginning to assemble the basemap and database, list the system requirements, paying close attention to the accuracy requirements. Fixing mistakes that should have never happened is doubly hard. Consider launching a pilot project to test the methods and procedures before committing to a project area. In recent years computer hardware and GIS software have gotten more powerful and more affordable. GIS software has become more user friendly. While specialized degrees are helpful, they are not required to take advantage of this technology. If you can play computer solitaire, you can probably become a GIS user with practice and a little education. If you work with maps in any format, GIS can make you use them more effectively-and in ways you never dreamed of before. The question is no longer can we afford GIS, but can we afford not to implement this technology? About the AuthorBrian McLaughlin has a degree in GIS/cartography from Southwest Texas State University in San Marcos. ResourcesFor more information, please contact |
