Most information handled by government is land and parcel related, so the parcel data is important in an urban or county information system. "The parcel layer relies heavily on the base map and on the measurement data in legal descriptions. Once in operation, other transactions relate to the property layer itself, rather than its precursor layers. Building applications, land transfers, emergency response, as well as assessment functions, all rely on this layer." (Donahue 1994, 240)
There are several attributes that can make up the discrete parcel layer, that is, the basic layer that does not include the assessment information. The most important of these layers is the unique parcel identifier:
· Unique parcel identifier(UPI) (also, unique cadastral identifier, See Section 7.3.5) Every parcel needs to be assigned a unique identification number. The system for assigning this number should be legally defined and recognized by official entities as the primary reference for the individual parcel to all data and documents. Secondary parcel identifiers may be assigned for indexing per the needs of individual departments, but they all must be cross-indexed to the UPI. This connection between secondary and primary identifiers allows for multiple uses of the data.
Parcel identifiers should have the following characteristics:
Uniqueness
Permanence
Simplicity
Ease of maintenance
Flexibility
Reference to a geographic location
(IAAO 1988, 10)
It is ideal for the parcel numbering system to be economically implemented, structured for cost-effective maintenance and accessible for all users. With current GIS technology it is not necessarily crucial that the parcel identifier be tied to a specific geographic location (as stated above).
(Donahue 1994, 237)
Parcel identifiers fall under one of three categories: location, name-related, and alphanumeric. A location identifier denotes the location of the parcel such as is used in map-based, geographic coordinate, and rectangular survey identifier systems. In the map-based system, the map, block and parcel numbers of the assessment maps are incorporated into the parcel identifier. This system is not recommended for areas of rapid development. The geographic coordinate system locates a point on the surface of the earth based upon "its distance from each of two intersecting grid lines known as x and y axes." (IAAO 1988, 10) X and Y coordinates are used in the parcel number, usually reflecting the center of the parcel. This system, once understood, is simple, easy to maintain and upholds criteria of permanence and uniqueness.
· Location
-Map-based
-Geographic coordinate
-Rectangular survey· Name-related · Alphanumeric
Figure 4.2 Types of parcel identifiers
The third locational system is the rectangular survey system. It is more common to the Midwest and Western states. It bases the numbering schema on the US Public Land Survey System (PLSS). Township, range, section, quarter-section and quarter-quarter-section numbers, in addition to individual parcel identifiers are used to create the UPI in this system.
In the name-related identifier system of creating unique parcel identifiers, the names of individuals claiming ownership or interest in a parcel are used. The third type, alphanumeric identifiers, is composed of random numbers associated with each parcel. An index of tracts numbered sequentially is an example of a numbering system that uses this type of system. (IAAO 1988, 10)The city of Boston uses a ten-digit number as its unique identifier. Included are the Ward, Parcel and Subparcel identification numbers (See Figure 4.3). Amherst, another Massachusetts community, uses the Section, Block and Lot numbers in its UPI.
Name Description Type Parcel_id Each Parcel of Property in the City of Boston is assigned a unique 10 - character identifier consisting of a ward, parcel, and subparcel identification number. (0300275000) Example: Ward=(03), Parcel=(00275), Subparcel=(000) character Figure 4.3: Boston's parcel I.D. configuration (Boston 1996)
Additional core attributes include those that describe survey information:
This information is sometimes included within the unique parcel identifier. Despite incorporation into the UPI, it also usually exists as separate fields of the core parcel attribute database.
· Type (example: cadastral)
· Lot number
· Block number
· Township numberAnd others:
Supplemental information, such as those items listed below, are sometimes included in the core parcel database, although some of them may reside primarily in the tax assessor's database. Inclusion as a core parcel attribute depends on agency needs, in-house and GIS enterprise applications.· Plan (example: 71724 CLSR BC)
· Street address
· Status (example: Active)
· Legal description (example: Lot 122 Plan 71724 CLSR)
· Area
· Area unit (example: Hectares)
· Perimeter
· Ownership
· Deed reference #
· Plat reference #
Layer Name Type Color Description 10 Ease Line 17 Easement Boundaries 10 Road Line 7 Road Right of Way 17 Ease Text 17 Easement Text 17 Road Text 3 Road Name 20 Parcel Line 2 Parcel Boundaries 20 Plat Line 11 Previously Platted Boundaries 21 Dist Text 4 Parcel Boundary Distances 22 Bear Text 5 Parcel Boundary Bearings 24 Area
Text 5 Parcel Areas (in acres) 25 Temp
Text 252 Temporary Construction 25 Temp
Line 252 Temporary Construction 26 ID Text 6 Parcel Identification (key) Number 27 Name Text 3 Subdivision Name 28 Legal Text 1 Legal Description Text 29 Note Text 3 Miscellaneous Text 30 Forty Line 11 Quarter/Quarter Section Lines 30 Quarter Line 11 Quarter Section Lines 30 Section Line 11 Section Lines 31 Dist Text 7 Section Line Distances 32 Bear Text 7 Section Line Bearings 33 PLSS Shape 5 Section Corner Monuments 34 Area Text 7 Section Areas (in acres) 37 Name Text 7 Section Identification Text 40 Meander Line 4 Meander Lines
Figure 4.4: Sample parcel attribute table (Land 1999)
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