While the assessment and taxation of real property is as old as New York State itself, the tax maps as we know them today are a relatively recent invention.

In 1970 the Assessment Improvement Law was passed, which marked many of the changes to legislation that govern the modern standards of assessment and property taxation. A major part of the AIL was the mandate that each county was required to provide assessors and county tax agencies with accurate and current information concerning the status of real property within their jurisdictions. To accomplish this each county had to prepare and maintain approved tax maps, or update those that were already in use to a more modern standard. These tax maps were intended to provide a graphical representation of the boundary lines of the parcels found on the local assessment roll to assist the assessor in making more accurate appraisals.

Costs of preparing tax maps varied depending upon the size and complexity of the project. Such factors as the size of the county, total parcels contained within the assessing unit, density of “parcelization” and extent of urbanization had to be considered. It's estimated that the cost to prepare the initial tax maps was, on average, between $10 and $15 per parcel (Oswego County consists of nearly 60,000 parcels). In Oswego County this update to the tax maps was completed in 1975 and was based on accurate land base information derived from a 1974 aerial flyover and orthophoto mapping project.

In order to comply with the AIL the tax maps had to conform to a uniform measurement scale. The cities and villages were mapped at a scale of 1 inch = 100 feet or 1 inch = 50 feet while most of the towns were mapped at 1 inch = 400 feet with more densely populated township areas at 1 inch = 200 feet. In order to map the entire county the tax map was broken down into "sections" at the 1 inch = 400 feet scale. These sections were then further broken down into the smaller scale units as "subsections" where 4 subsections were needed for maps at the 200 foot scale, 16 subsections at the 100 foot scale, and 64 subsections at the 50 foot scale. This resulted in a total of 1,149 total section and subsection tax maps. In order to lay out all of the tax maps together to form the entire county you would need an area of approximately 10,053 square feet, or about 1/5 of an American football field.

After the creation of the modern tax maps in 1975, several mappers at the Real Property Tax Service Department (a new County office mandated by the 1970 Assessment Improvement Law) routinely updated the tax maps to reflect changes in ownership and property boundaries by hand. In 1996 Oswego County began the process of digitizing the tax maps into a computerized Geographic Information System (GIS). Today, the GIS tax maps are maintained by a single mapper who continues to keep the maps and the cadastral data in current condition.