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National Land Parcel Data: A Vision for the Future
support a new real estate tax cycle. In other cases, inconsistent maintenance cycles are a result of local tax assessment rules. For example, in many local governments the tax bill reflects a property’s status in the previous year. Transactions that occur during the year are by law not included in the tax rolls until the following year even though ownership or geometry may have changed.
The rate of increase of new parcel creation is about the same as the rate of population increase. Even though the general trend of parcel population density is about 2 people per parcel, in fast-growing areas, highly urbanized areas, or in rural areas, these numbers vary (Stage and von Meyer, 2006b). New parcels are reformed from existing parcels through splits and combinations as well as created in new plats and subdivisions. This often requires sorting through many documents to identify and interpret the information. The motivation for a local government to maintain these transactions is to support the local business processes such as real estate taxation, permitting, and law enforcement and public safety programs that require current and accurate information.
5.1.2 External Distribution of Parcel Data
Most local parcel data programs maintain a production and a publication version of their parcel data. The publication version is one that is made publicly available. The production version may contain much more detailed data, such as detailed survey work. In the most sophisticated programs the internal production system is maintained in real time as property transactions are recorded. Therefore, it is possible that applications within a county may be based on current data, but the community may update its external offerings only on a monthly, quarterly, or even annual basis. Even if the data in the local digital parcel layer are as accurate and current as possible, any data stored separately at a state or national level can easily get out of sync. It is technically possible to create a system where the data at a national level exactly match the data available locally, but the cost and administrative burden must be balanced against the need for real-time currency.
5.1.3 Quality of Data in Existing Digital Parcel Maps
Incomplete, out-of-date, and inaccurate data often exist in digital parcel maps. At the local level, parcel maps primarily support local property taxation and are usually adequate for that purpose. However, their underlying base maps can be many years old and they are often georeferenced incorrectly, do not align properly with high-resolution orthophotography, and may be internally inconsistent due to the original sources and methods used to create the data. Poor quality control, especially in terms of geographic accuracy, is understandable because of the cost of producing highly accurate data. Figure 5.2 shows how parcel data from King County, Washington, does not line up well with aerial imagery because of issues encountered during the conversion of hard-copy maps to digital format. Other errors can occur because the existing hard-copy maps may have been developed from “assumed” reference systems, such as presuming that Public Land Survey System sections are square or that individual parcel boundaries follow visual evidence such as fence lines or water edges.
There are many different approaches to the creation of parcel polygons that vary substantially in cost. Very few communities can afford the cost of compiling parcel data directly from the legal description on a deed because of the labor-intensive process of interpreting and resolving discrepancies in legal descriptions found in many deeds. Often the more exacting method using coordinate geometry and topology rules is employed for a subset of parcels such as those that have been recently surveyed and compiled on a plat of survey where the distances and bearings can be relied upon as being consistent and accurately presented. In most other cases the parcel fabric is created by digitizing existing hard-copy sources. In most jurisdictions, it is not legally required that the parcel legal descriptions in deeds be compiled and verified by a land surveyor. This often leads to