Types of Parcel identifiers
There are several classes or parcel identification systems. They fall into four basic types; (1) Geographic, (2) Map-Block Based, (3) Intelligent Key, and (4) Non-Intelligent Key. These approaches have all been implemented across the country and have distinct advantages and disadvantages.
A geographic key is a special category of location-based intelligent key. Typically, it is the coordinate value of a point within the parcel (e.g. the centroid). The value is typically based on the projection used for parcel mapping or is converted to a non-projected coordinate system such as latitude/longitude. Automated routines can be developed to generate the parcel ID point, ensure that the point is located within the parcel polygon, and harvest the coordinate values for the PIN.
Another version of the geographic coordinate is the US National Grid (USNG) number. The USNG Standard has been adopted by the Federal Geographic Data Committee (FGDC) and provides a system to label points uniquely to one-meter accuracy. There may be some parcels that are smaller than one meter by one meter, but these would be a limited set and could be handled by an exception rule. The parcel identifier based on the USNG would provide a nationally unique index value and an approximate location.
Another variation of geographic based parcel identifier is to interleaved values (e.g. NNNEEENNEENE). These values cannot be used directly for a location but may be useful to create a unique identifier.
Advantages: The geographic PIN is located inside the parcel polygon and can be used as point representation of the parcel data. The coordinate value can be generated from the parcel geometry. General location and adjacency can then be derived from the coordinate values.
Disadvantages: Generation of a geographic key requires that all parcels be mapped. Certain parcel configurations or generation scenarios (e.g. parcel combines, parcel splits) can create situations where the parcel identification uniqueness is difficult to guarantee if strict adherence to the generation scheme is observed. The length of the parcel identification will also vary in situations where additional identifying information needs to be incorporated (e.g. elevation, floor). In these situations, involving vertical parcels, the coordinate system-based identifiers often begin to break down. This is caused by the stacking of points or the necessity to incorporate an elevation value, resulting in a three-dimensional key. This approach is also sensitive to the map projection and the datum used and requires a GIS-based foundation to drive the assignment of the key. Non-convex parcel geometries can result in centroids that lie outside of the boundaries of the parcel. These situations must be resolved using alternate methods of centroid calculation.
Existing Systems: Statewide geographic PINs have been implemented in North Carolina and Massachusetts.
The block-based parcel identification is sometimes referred to as the Sidwell or tax lot approach. In a block-based system, the jurisdiction is divided into blocks or map sheets and each block is assigned a unique identifier. Lots or tax parcels within each block are then assigned a unique number. In some systems, subsequent splits and combinations are identified using suffixes. These systems are easily recognized as they often have hyphens separating the components and are most commonly used in urban settings or in jurisdictions that have historically used a map/sheet-based system.
Advantages: The general location of the parcel is easily discerned from the block number as users become familiar with the numbering system and the hyphenation makes the parcel identification values more human-readable. The number can be used without hyphenation, no hyphens making it more “machine ready”. Value-based sorting arranges parcels into blocks groups.
Disadvantages: Block reconfiguration requires the creation and assignment of new block codes. If there are parcels between blocks, the system requires standardized rules for assigning a single block code to those parcels. This approach is not ideal for large land areas without clear urban block divisions and as implementation would require an indexing or tiling scheme. It is also possible to reach the limitation on unit numbering within a block, though this is rare in properly designed systems.
Existing Systems: New York and Chicago use a modified block-based system.
An intelligent key is a synthetic value generated from selected information. In the Public Land Survey System (PLSS) states, the location within the PLSS Section or quarter section is often used as a base for the intelligent key. Other commonly used values are the parcel location within a block, the tax use classification, exempt status, and unit number. Intelligent keys are common and attempt to convey human-readable information about the property through the key itself.
Advantages: This is the most user-friendly option for parcel identification. An intelligent key can be derived from any number of existing geographic characteristics (e.g. municipality, council district, land use, tax category, location, address) and assignment can be based on these characteristics, eliminating the requirement of pre-assignment parcel mapping.
Disadvantages: This approach can result in transient keys; keys that must be re-derived or re-assigned if any of the constituent values change. Improper maintenance of the keys across formerly interoperable systems can lead to issues with referential integrity; if a key change is not propagated to the systems and services relying on it, it can cause workflow disruption. The nature of the intelligent key approach means that any changes other than the creation of a new parcel could cause an update trigger. Standardized rules and workflows must be developed alongside the intelligent key system to address these changes.
Existing Systems: Many PLSS states use an intelligent key. Many jurisdictions that had intelligent keys are migrating away from these systems.
The non-intelligent sequential number is generated algorithmically and assigned to a record by a database management system (DBMS). It conveys no information about the parcel and contains no intelligence. Numbers can be assigned to any unit of land identified as a parcel. Because the value is assigned by the DBMS, no user intervention or input is required. This can facilitate synchronous electronic workflows supporting large numbers of users and large flows of data such as eRecording or electronic document management solutions.
Advantages: This approach offers the greatest automation potential as there is no need to map the parcel prior to assigning the pin. This approach provides machine-readable values that are independent of the DBMS or mapping system used. It can be adapted to changing generation criteria, such as the addition of non-taxable or unmapped parcels. This approach is easy to implement and maintain.
Disadvantages: This is the least human-readable/user-friendly approach. The generated values contain no derivable information; the adjacency or similarity of values does not convey information about the geographic location of the parcels, parcel history, or other characteristics.
Existing Systems: Two of the largest jurisdictions using this system are Atlanta and Houston. Most jurisdictions that drive the parcel number from the CAMA system or from the document reference use a sequentially assigned non-intelligent number and carry a non-intelligent number even if the jurisdiction assigns an alternate identifier.
There are parcel identification creation and data maintenance issues with each approach. The following sections describe some of the underlying technical and use considerations for any parcel identification system.