Beyond CAD GIS—The Tool for Cadastral Mapping

Abstract

The Rains County Appraisal District began its GIS program in the summer of 1999. New software, data, and ideals were needed to implement the program. Having never worked for an appraisal district before, I decided to visit the neighboring appraisal districts to help me understand my responsibilities. After having done so I had more ideas on what not to do than to do. I knew I wanted a GIS system instead of a CAD system. This paper will be helpful to any cadastral agency interested in starting a GIS program, either from scratch or by transition from a CAD system.

Introduction

I am writing from the perspective of a rural appraisal district in East Texas. The requirements and resources of your cadastral mapping system may be different. I am just beginning our GIS, and have a lot of work ahead of me before it is completed.

I have a degree in GIS/Cartography from Southwest Texas State University in San Marcos, TX. I had never worked for an appraisal district before, but I did have experience working with survey data, and property deed descriptions. I had a basic understanding of GIS concepts, but limited real world experience. This is my training ground.

This paper is not intended to be a one stop source for GIS implementation, but a guide to help appraisal districts and other cadastral agencies begin there own GIS program. It should serve as a non-technical introduction to GIS.

GIS Defined

GIS stands for Geographic Information Systems. It is much more than a graphics program. GIS maps can be called "smart maps" because they can be queried. It is a visual interpretation of a database (Eichelberg). This database contains two types of data, statistical and spatial. Data can be represented by three types: points, lines and polygons. It can be in one of two formats, raster or vector (What is a GIS, 2000).

Statistical data contains information about the feature being queried. Some examples of statistical attributes are square feet, class of building, value per square foot, ect. Location data gives real world coordinates of the feature, for example latitude and longitude, UTM, or state plane coordinates (What is a GIS, 2000).

The three ways data can be represented: points, lines, and polygons can vary depending on the scale of the map. A point places a feature at a given geographic location. It does not represent the area of the feature. An example of how I use point data is in locating the unincorporated communities, cemeteries, new constructions, etc. Polygons represent the total area of a feature. They are usually the best way to represent property lines, school districts, county boundaries, lakes, and any other data that requires a visual interpretation of area. Line data represent distance, and direction, but not area. I use lines to represent the roads of my county. A road could be a polygon if you have the time and resources to spare. Line data is quicker to generate, and in many cases just as effective as a polygon. Since most of my county is rural, and most property lines go to the middle of the road this is sufficient. However, when I get a subdivision plat I create the roads within it as polygons.

The two different data formats are raster and vector. A raster image is much like a photograph. The orthoimages, and USGS base maps that I have are all in raster format. Raster images are pixel based and lose resolution if the zoom level is too high. They are also larger than vector images. If you are familiar with graphics applications, images created with applications such as PhotoShop, or PaintShop Pro are all raster images. Vector data on the other hand can be zoomed in almost indefinitely with no resolution loss. Images created with applications like Freehand, and Adobe Illustrator are vector images. Raster images are often georeferenced with a world file. This file is a text file that assigns each pixel with a geographic location. If your image has a .tif extension, the world file would have the same name, but with a .tfw extension.

GIS is much more than software. It also includes the computer hardware, data, people, and methods. The hardware not only includes the computer itself, but the peripheral devices attached to it. These devices may include, but are not limited to, printers and plotters, digitizers, scanners, and GPS receivers. The data can be raster, vector, image, or tabular (database).

The human aspect includes administrators, managers, technicians, end users, and consumers. The methods are guidelines, specifications, standards, and procedures that dictate how GIS technology is applied (Esri, 1999).

If you have never been exposed to GIS theory before, I encourage you to look to other sources for more detailed information. If you have Internet access, a good place to start would be www.gislinx.com. Now back to cadastral mapping!

In The Beginning

Rains County is a small, rural county located about 70 miles east of Dallas. At 256 square miles it is one of Texas’ smallest counties, with a population of around 8,000 it has a very limited tax base. The previous cartographer was using Auto Cad to map the county. The quality of the work was such that I felt it better to start over, rather than spend years cleaning up garbage. If your county already has CAD drawings that you are happy with, ArcView, and many other GIS programs can easily import them and you will have a head start on your GIS implementation. For information on how to import CAD drawings please consult your software documentation.

Since I already had knowledge of ArcView, I was not interested in teaching myself AutoCad. The appraisal district had previously purchased ArcView 3.0. I was able to upgrade to the most current version with little cost. I am not going to tell you that you must use ArcView to have a cadastral GIS, but for Rains County it seemed to be the best software.

After deciding what software is best for your GIS, the next thing to do is to decide what your accuracy requirements will be. If your county already has digital survey data available you will be one step ahead. If you are from a county similar to Rains, you may be the first agency in the county generating such data.

Accuracy is relative to the task at hand. When considering your accuracy requirements it is important to remember the purpose and audience of your 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 by which pipelines and right of ways are determined. Accuracy costs both time and money. Two things that most appraisal districts are lacking. As long as I maintain a relative accurate parcel map, meaning the appraisers have no problem locating the properties in the field, I have done my job.

To began your mapping. The most important data will be your base map. The basemap defines the boundaries of your project, and provides control points to help keep your maps accurate. Two good sources of basemaps are the USGS 7.5’ digital quad maps, and digital orthophotos.

Orthophotos for my county where not yet available, I started my GIS project with the USGS quad maps. I purchased the Sure!Maps Raster from www.suremaps.com/. They are based upon the 7.5’ USGS quad maps. They provided seamless coverage of my entire county, and came with a utility that allowed me to export them in a variety of projections and datums. They can be exported in TIF or GeoTIFF format, and are readable by most software including AutoCAD, ArcView, ArcInfo, and MapInfo. The USGS quad maps contain such features as political boundaries and cemeteries that the orthophotos alone do not delineate.

The USGS quad maps where a great start to begin our GIS, but many of the USGS 7.5’ quads for my county had not been updated since the middle of the 1950s. There are newer roads, and the incorporated areas had grown since that time. We needed a more up to date basemap to do our parcel maps on. Orthophotos seemed like the ideal choice, but the cost to have new ones flown was too cost prohibitive to even consider.

In November of 1994, The Texas GIS planning formed a partnership with the USGS, state and local agencies, and private industry to produce low cost color Infra-red (CIR) digital orthophotos statewide. The project became known as the Texas Orthoimagery Program (TOP). The USGS and the Texas Department of Information resources (DIR) covered 75% of the cost. Private industry and local governments covered the remaining 25%. 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 additional quads could be purchased for the cost of $50 (Steede-Terry, 1997).

Each 7.5’ quad was covered by 4 orthophotos, thus each orthophoto can became a 3.75’ Digital Ortho Quad (DOQ). Uncompressed the 1 meter resolution DOQs use 158 megabytes of disc space, a CD can hold up to 4 images. They are also available in 2.5-meter resolutions. The 2.5-meter resolution DOQs requires about 8 megabytes of disk space. A CD can hold 64 2.5 meter DOQs. The 1 meter CD also contains a DEM file for each 7.5’ quad (Steede-Terry, 1997).

Product Specifications

Most Pentium class computers should have no problem handling the 3.75’ DOQ’s, but if you plan to view more than one on screen at a time it would be wise to look at image compression. Two options for image compression are Mr. Sid, and ErMapper. Information on Mr. Sid can be found at www.lizzardtech.com. I have used ErMapper's free image compression utility that is available at www.ermapper.com. You will need the extensions for your software. These are available free to from the ErMapper web site. ArcView already supports Mr. Sid images. To use them you will need to go to the extensions dialog box and make sure the option is turned on. The ErMapper compression utility has worked great for me. I have been able to compress the DOQs to about 8 megabytes per 3.75’ quad with very little resolution loss. I can easily fit my entire county on one CD.

Adding More Data

Now that you have your base map, it is time to start adding more data. I was able to locate most of the data that I needed on the Esri web site for free at www.Esri.com/data/online/tiger/index.html. The data is the 1995 US TIGER data. The accuracy requirements and cost were acceptable for our GIS, but if you need more accurate data it may be necessary to consult with a commercial vendor.

The freely available TIGER data was easily rectified over the DOQs. The roads were all line data, and it took about a week to rectify them. The roads in Rains county are classified into five classes: US Highways, State Highways, Farm to Market Highways (FM), county roads, and private roads. Each class is archived as its own layer, but I merged them all into one layer to keep my view from being cluttered.

The TIGER data included many other layers in addition to the roads. I was also able to use the county boundary, streams, water boundaries, incorporated outlines, school districts, and water bodies. Data availability will vary by county, but here is a complete listing for my county.

Now that I had all my base data assembled I was almost ready to begin my parcel mapping. Depending on the number of plats available for your county you may be able to begin mapping. Rains County is a rural East Texas county. There are few subdivisions, and many large tracts of land that are described only by meets and bounds. Many of these parcels have been passed down from generation to generation and have not been surveyed since the 19^th century, or early parts of the 20^th century. These survey descriptions 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. Through diligent research, and educated guessing I try to trace these back to the original survey lines. For those of you unfamiliar with varas, it is old Spanish measurement equal to about 2.77 feet.

Unlike most states, Texas is not broken up into section, township and range. Spain, Mexico, and France surveyed most of Texas before the section, township and range system was created. When Texas became and independent nation, then a state, all the previous land grants were honored. The original land grants are called abstracts, and they have unique sizes and shapes. This can make it difficult to map parcels in rural Texas counties that have not been platted. It was a necessity to locate a digital copy of my county abstract map. When I was trying to locate a digital copy of my county abstract map the only source I could find compatible with ArcView was from Tobin International of San Antonio, Texas. They have data for other states and countries as well. Their web site is www.tobin.com. The Texas State Rail Road Commission now has their data available in ArcView format. Their web site is www.rrc.state.tx.us.

Now that all of my base data is assembled, I am finally ready to begin my parcel mapping.

Adding Parcel Data

Subdivision plats can be digitized either with a digitizing tablet, or scanned and digitized on screen. You will need to be sure to georeference the plats so that they will appear where they are suppose to be on your basemap. If you are using a digitizing board please refer to your software documentation to learn how to do this. If you are using ArcView there are free extensions available at the Esri web site that allow you to georeference scanned images. Please go to www.Esri.com, then click on user scripts on the menu to the left. After you have a georeferenced image, you can then digitize the lot boundaries on screen.

Properties that only have meets and bounds descriptions will have to use a different technique. There are two commercial extensions that will allow you to add meets and bounds descriptions directly into ArcView. The process is known as coordinate geometry, or COGO. One is called Cedra AvCad, they also have another extension called AVParcel. AvParcel has many additional features that allow you to manage your parcel data. For more information please visit www.cedra.com. There is a new tool on the market called PMCgis Toolbox. It is available from Promap; their website is www.promap.com. PMCgis contains more parcel editing features than AvCad such as donut polygons, and polygon splitting. It is the extension I currently use.

I am just beginning to create my parcel data. Each parcel is a unique polygon, and is named after the parcel identification number. This can allow it to be queried so I do not have to waste time looking for my parcel like I do in a CAD drawing. It will also allow me to attach data to each parcel. Currently we have to look for the parcel on our old CAD maps, then look up the attribute data in a separate database. We use a program called Automated Central Appraisal District to maintain all of our parcel attribute data. It is supported by The Software Group of Plano, Texas. Their web site is www.tsgweb.com. When we migrate to ArcInfo we will be able to link that database with our parcels maps. The database is also exportable in .dbf format, which can easily be joined to an ArcView table, or imported into other GIS software. My goal is to complete 25% of our parcels before I link the database with the parcel map.

The Future

GIS technology enables appraisal districts to manage their parcel data in a manner not possible with hand drawn or CAD 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 flood plain, if it has water frontage, or is located along a highway. These are all conditions that will effect the value of a property. Square feet and acreage can be calculated on the fly. The database connectivity saves a lot of map space, thus reducing clutter. There is no need to try to squeeze every bit of text into a parcel area such as owner's name, book and page number, or size. All of these features and more can be displayed simply by clicking on the parcel.

A cadastral GIS is more than a parcel map. New maps can be created quickly. You could create maps of just commercial properties, homesteads, residential properties, and more. The imagination and database designs are the only limitations.

The Bexar County Appraisal District, located in San Antonio, Texas has one of the best cadastral GIS. It is featured in Esri's book Zeroing In. The GIS staff created the Customer Query System that allows home owner's to compare their property values to surrounding ones and properties of similar values in the county. They can also view recent sales figures and compare that to their current value. This system allows the customers and the appraisers to determine if the values are out of line. Before the Customer Query System many staff hours where required to generate the same information (Mitchell, 1998).

Before the Rains County Appraisal District can take advantage of advanced GIS techniques, more parcels must be mapped. This is a time consuming process. The hardest part is doing the deed research to find a point of begging. More hours are spent studying the deeds than entering the parcels into the computer. Fortunately we have a progressive board of directors that has provided the best equipment they can afford. I hope to spread up the process by using a GPS receiver in the field. This will save hours of research, and speed up the process of creating our GIS.

Before you Begin

The first step to GIS implementation is proper planning. Careful planning will save time, money, and disappointment down the road. It is better first proceed slowly, than do too much —too quickly and make mistakes that are irreversible. I am constantly evaluating my methods to insure that our GIS will be the best that it can possibly be. I keep a long-term plan that I evaluate every year, and a short-term plan that I evaluate every four months. I attend professional seminars, and visit with other appraisal districts and GIS users to both gain and share ideals. Since we have a very limited budget, my philosophy is to make the best use out of the limited resources that we have, and find creative ways to obtain items that we need. Before you begin to assemble your basemap and database make a list of the requirements that you have, paying close attention to your accuracy requirements. It is hard to go back later and fix mistakes that should have never happened. You should consider starting with a pilot project to test your methods and procedures before committing them to the entire project area.

Conclusion

In recent years computer hardware and GIS software have gotten more powerful, and more affordable. At the same time software has gotten more user friendly. Specialized degrees are helpful, but no longer 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?

Acknowledgements

References

Eichelberger, Pierce, Maturing GIS: Anecdotes, Examples, and Lessons.*

Mitchell, Andy, Zeroing In: Geographic Information Systems at work in the Community. Pages 28-36. Redlands, CA: Esri, 1998.

Steede-Terry and Bury S, Andy, Texas Orthoimagery Program: DOQQ Processing Techniques and Applications for the State of Texas. EOM, 1997. Available from TOP.

GIS. GIS Day 1999 poster. Available from Esri: www.Esri.com, 1999.

What is a GIS. Available online: www.gislinx.com/whatisgis.shtml, 2000.

* Exact origin of article unknown. Copies available upon request.

Brian McLaughlin
Chief Cartographer,
Rains County Appraisal District
765 Hwy 69 N
Emory, TX 75440
(903) 473-2391
(903) 473-4040-Fax
Brianscott@argontech.com