Parcel Mapping, Part I

This is the first part of a two-part article on parcel mapping. This two-part series is intended as an introductory primer on how to create, maintain and use parcel, or cadastral information in a geographic information system. The goal is to increase the surveyor's participation with GIS parcel mapping in order to improve the overall quality, accuracy, and long term viability of the cadastral layer.
In Part One, I will examine some of the ways that cadastral data are used in GIS, explain why the parcel layer is so helpful, and point out its capabilities. I will also discuss some observations about the surveyor's role in parcel mapping. In Part Two I will discuss the GIS principles that dictate what a good parcel layer consists of, illustrate a variety of technical approaches to creating the parcel, and suggest methods for improving and maintaining the parcels.

What Is the Parcel or Cadastral Layer?
The parcel layer is cadastral data that is the key to land ownership, parcel size, configuration, land use, improvement values, and other related information contained in federal, state, local government, or public and private agencies. For the purposes of this discussion, a parcel is defined as a contiguous single ownership interest (which may consist of more than one owner) in a legally described real property. A parcel can be comprised of one or more lots or portions of lots in a subdivision, an entire section (square mile of land) in the public lands survey systems, or a single closed figure with a metes and bounds description. Some examples are: a lot in a subdivision, or two adjacent lots owned by the same person, or an aliquot parts property such as, "all of section 23 in township…"
There are other definitions of a parcel but this is the most common usage in GIS. The reason for the "single and contiguous" requirement is a function of the GIS data structure, which will be explained in Part Two. The important thing to remember about the GIS parcel layer is that it is used in GIS in ways that are similar to how it has been (and still is) used in the hand-drafted and hand-drawn methods.
GIS differs from hand-drafted maps in various ways: GIS is easier and faster at performing searches and queries (for example, ownership, vacant lands, value, etc.), it is easier and faster to maintain and edit, and most importantly GIS facilitates the integration of the parcel data with other layers, such as soil type, slope, and infrastructure.

Who Uses Parcel Data and How Is It Used?
The list of users and uses of parcel data is limitless, but typical users are city and county governments, state agencies, departments of revenue, parks and natural resources, the Bureau of Land Management (BLM), the United States Forest Service (USFS), private companies, title companies, real estate brokers, land surveyors, sales and marketing, and utility and energy exploration companies.
Some examples of uses are: identifying who owns land along a highway corridor for a road widening project, property appraisals, or identifying all the conservation easements in the state.
Parcel data can give information on land uses such as agricultural uses, residential, park, industrial, or multi-family. Such information can help identify areas with a potential for development or protection.
The GIS parcel layer can identify landowners along a potential route, trail, or automatically generate a mailing list for notification or form letters. The parcel is used for rural addressing to help identify where improvements exist that need to be addressed, as well as who the owner is so that they can be notified of their new address. The ownership information can also be used to develop a list of residents to be evacuated in emergency situations, such as the wildfires in the American West this past summer.

Examples of Descriptions of Database Fields Typically Associated with a Parcel
• property type is a code that best describes the actual present day use
• improvement classification code identifies the classification of taxable property found within a parcel
• the number of living units present in the dwelling described on this card
• local zoning of the parcel
• the land value
• the total fallow acreage of the entire parcel
• the total grazing acreage of the entire parcel
• the total wild hay acreage of the entire parcel
• the total timber acreage of the entire parcel
• the total farmstead acreage of the entire parcel
• the total irrigated acreage of the entire parcel
• the code that describes the cost per acre to irrigation
• a code that describes the proximity of parking available
• the actual story height of the dwelling
• the code that best describes the type of roof

Different Uses of Parcels
Surveyors' uses of parcel data may differ from other users. A surveyor may be more interested in the history of a line and have more concern regarding the placement and geometry of a parcel than in its use or value. The surveyor's perspective is, in some ways, a more detail-oriented technical view of the construction and geometry of a parcel. The surveyor wants to know where a particular line originated, and its length and direction. Most other users may be less concerned about the specific geometry and more concerned about the attribute characteristics of the parcel. Most users are more interested in the database information associated with the parcel than they are in the parcel's geometry. The chart on page 49 lists some typical database fields associated with a parcel.
Figure 1 shows a sample of a parcel record. This example is a residential property with a three-bedroom house. A GIS query could be generated from any of the data fields listed, such as "show properties by condition" (good, fair, poor), or "show properties by heating type." More complex queries can be generated as well such as "show parcels that are zoned commercial, have no improvements and are on improved roads." Figure 2 shows an extract of rural land parcel information. Figure 3 shows an area of rural property classified by ownership code, with state lands in blue-green, federal lands in red, and private lands in brown. Figure 4 shows the same area classified by individual ownership, with each ownership in a separate color.
The charts and figures illustrate that the parcel layers contain far more than just location and geometry information, which is why the cadastral layer is so important to non-surveyors. The GIS can generate maps based on any of the database-coded information.
Depending on how the parcel layer was developed, it might not directly provide accurate location and geometry data such as: the exact location (coordinates) of the property corners, the bearing and direction of parcel lines, or acreages. That is, the parcel lines themselves may not be constructed using the historic and/or legal geometry. In these cases, the parcel configuration is a general representation of the legal geometry, in fact more of a cartoon. This does not diminish the parcel layer's usefulness for the previously described purposes, but it is a perspective that is at odds with the way surveyors regard parcels.

Different Uses
Not all parcels are created the same way. Another major difference between a surveyor's perspective of a parcel and GIS, is the use of projections and coordinate systems in GIS. Surveyors typically work in small areas when dealing with parcels, but those developing and using parcel data in a GIS cover more ground: a city, a county, a national forest, an ecosystem. When larger areas are the realm of interest, then a mapping projection must be adopted in order to bring continuity to the variety of data. Moving a parcel into a projection typically introduces issues of bearing rotation and scale, therefore changing the parcel geometry.
This does not mean, however, that the GIS cannot provide the legal and historic parcel geometry information. The legal bearing and distance, as described by the plat of survey or deed, can be an attribute of the line work or the parcel just as any other information may be. Alternately (or additionally) the GIS can have a hot-link from the parcel to the scanned image of the plat or survey or deed that created the parcel. Either of these methods allows the user to obtain the legally recorded information on parcel geometry or size by simply clicking on the GIS parcel map.

Issues and Concerns
Other concerns that surveyors have about parcels tend to revolve around regulatory applications. Zoning, floodways, wildlife protection, and other constraints may be placed on a parcel due to the parcel location. Usually these types of determinations are completed in a GIS by overlaying various layers upon the parcel layer. For example, the Federal Emergency Management Agency (FEMA) flood data which shows the 100-year and 500-year flood, may be overlaid to show whether or not a particular property is within the regulatory floodway, Figure 5. Surveyors are concerned that the GIS layers may not be spatially accurate enough to reasonably determine such locations, particularly when those determinations will have a financial impact on the property. Questions may arise: Are the parcel lines really in the right place and in correct configuration? Are the floodways correctly mapped? These are concerns for hand-drafted maps as well, but as stated before, the GIS facilitates these types of overlays, which highlights the differences between data sets. The good news is that GIS makes these spatial comparisons much easier than the compilation of hand-drafted maps. After all, these comparisons were being completed long before GIS came along, and will continue to be performed with or without the benefit of GIS.
One of the best remedies to address everyone's concerns about spatial accuracy (whether absolute or relative) is for more surveyors to be directly involved with building and maintaining parcels. Part Two will describe how that can be accomplished.

Editor's Note: For more information on flood hazard areas found on FEMA maps, see Wendy Lathrop's article The "Evil Zone A."

Rj Zimmer is the GIS Coordinator for Lewis & Clark County and and the City of Helena, Montana, and a Contributing Editor for the magazine.

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