2.3.2.3 Method
Tax Assessors, working in a Tax Assessor's Office, are responsible for accurately, uniformly, and fairly
judging the value of all taxable properties in their jurisdiction. Details about properties are maintained on
a tax assessment roll that includes information such as ownership, address, land and building value, and tax exemptions. The Assessor's Office is also responsible for processing applications for tax abatement, in cases of overvaluation, and exemptions for surviving spouses, veterans, and the elderly. Figure 2.8 shows some aspects of a tax assessment GIS in Ohio, USA.
A GIS is used to collect and manage the geographic boundaries and associated information about properties. Typically, data associated with properties is held in a Computer Assisted Mass Appraisal (CAMA) system that is responsible for sale analysis, evaluation, data management, and administration, and for generating notices to owners. CAMA systems are usually implemented on top of a database management system (DBMS) and can be linked to the parcel database using a common key (see Section 10.2 for further discussion of how this works).
The basic tax assessment task involves a geographic database query to locate all sales of similar properties within a predetermined distance of a given property. The property to be valued is first identified in the property database. Next, a geographic query is used to ascertain the values of all comparable properties within a predetermined search radius (typically one mile) of the property. These properties are then displayed on the assessor's screen. The assessor can then compare the characteristics of these properties (lot size, sales price and date of sale, neighborhood status, property improvements, etc.) and value the property.
2.3.2.4 Scientific foundations: principles, techniques, and analysis
Scientific foundations
Critical to the success of the tax assessment process is a high-quality, up-to-date geographic database that
can be linked to a CAMA system. Considerable effort must expended to design, implement, and maintain the geographic database. Even for a small community of 50000 properties it can take several months to assemble the geographic descriptions of property parcels with their associated attributes. Chapters 9 and 10 explain the processes involved in managing geographic databases such as this. Linking GIS and CAMA systems can be quite straightforward providing that both systems are based on DBMS technology and use a common identifier to effect linkage between a map feature and a property record. Typically, a unique parcel number (in the US) or unique property reference number (in the UK) is used.