Address Reference Systems Introduction

An Address Reference System establishes the framework of rules, both spatial and non-spatial, adopted by an Address Authority for assigning addresses within the area it administers. The rules, in turn, provide the basis for address data quality tests that detect address anomalies and errors.

The Address Reference System includes, as needed, rules governing address numbering, street naming, block definition, subaddresses (suites, offices, apartments, etc.), and place names. The Address Reference System may also define address baselines, polylines, and breaklines to guide address numbering throughout the area. Finally, for identification and reference, an Address Reference System includes a name and identifier, the name of the Address Reference System Authority that administers it, the boundary of the area it administers, and reference to the official documents and maps where the rules are codified.

Address Reference Systems provide a framework for address assignment and for quality assurance of addresses. In order to use these within a Geographic Information System, the components of a system must be structured into a layer that includes the extent of the system (Address Reference System Extent), and the reference grids, lines or points that govern address numbering throughout the area. In many cases, such grids have been constructed as graphic features that are not structured in a way to make them useful for developing Address Reference System Axis lines, Address Reference System Axis Point Of Beginning locations, Address Reference System Reference Polylines, Address Reference System Range Breakpoints, Address Reference System Range Breaklines and for use in evaluating whether a specific address point falls in the correct place relative to the Address Reference System Rules. Thus it is important that the Address Reference System be created as intelligent geometry to provide the tools needed to evaluate any address point found within the Address Reference System. It should also, where appropriate, utilize existing centerlines or other existing features so that exact matching is possible.

Address Reference Systems differ in detail from locality to locality, but in the United States all Address Reference Systems fit into one of three broad categories: axial, linear non-axial, and area-based. The categories differ fundamentally in whether and how the street system governs address numbering, and secondarily in the elements needed to compose them. Figure 1 diagrams the types and elements. Table 1 lists for each Address Reference System type, the elements required and permitted to compose it.

Axial Type Address Reference Systems

In axial Address Reference Systems, address numbering is organized around axes. The axes may be thoroughfares, rail lines, rivers, or imaginary lines (such as section lines in PLSS areas, lines of latitude and/or longitude, or arbitrarily drawn lines). Address axes typically extend from a common point of origin (the local "zero" point for address numbers), and all numbers increase with distance from the point of origin.

The axes, in turn, define the zero point for numbering along streets that cross the axes. Most commonly, axial system organize the streets and address numbering into a grid. In a simple case, if Main Street ran north-south from the town square, and State Street ran east-west, then:

1. Address numbering for Main Street and State Street would increase as one proceeded away from the town square.
2. Address numbering for other north-south streets would begin where they cross State Street and increase in parallel with Main Street.
3. Address numbering for other east-west streets would begin where they cross Main Street and increase in parallel with State Street.

Often the geometric grid is interrupted or deformed by terrain, rivers, highways, rail lines, parks, or other major features. Occasionally there are more than four axes, or numbering does not begin at the same point for all axes.

Linear Non-Axial Address Reference Systems

In a linear non-axial Address Reference System, each thoroughfare is addressed independently of the other thoroughfares. There are no axes and there is no grid. Each thoroughfare has its own point of beginning for address numbering, and and numbers proceed according to an Address Reference System Numbering Rule from that point to the end of the thoroughfare or the boundary of the Address Reference System. Linear non-axial address reference systems are typically found in areas where the road network is sparse and intersections are few, or where topography dictates a street pattern that is aligned with contours of elevation.

Area-Based Systems

In area-based Address Reference Systems, Complete Address Numbers are not assigned along a thoroughfare, but within an area. Inside the area, Complete Address Numbers might be assigned according to a spatial pattern (around the block, for example), or by parcel or lot numbers, or chronologically as the buildings are built.

Area-based Address Reference Systems are rare in the United States, but they may be found in gated communities, housing projects, Puerto Rican urbanizations, trailer courts, small tribal settlements, military bases, small islands, campgrounds, and similar developments.

Table 1: Required, Optional, and Inapplicable Elements for Each Type of Address Reference System

Note: R - Required; O = Optional; NA = Not Applicable

Element name	Axial	Linear Non-axial	Area Non-axial
Address Reference System ID	R	R	R
Address Reference System Name	R	R	R
Address Reference System Authority	R	R	R
Address Reference System Extent	R	R	R
Address Reference System Type	R	R	R
Address Reference System Reference Document Citation	R	R	R
Address Reference System Rules	O	O	O
Address Reference System Numbering Rules	O	O	O
Address Reference System Block Rules	O	O	O
Address Reference System Street Naming Rules	O	O	O
Address Reference System Street Type Directional And Modifier Rules	O	O	O
Address Reference System Place Name State Country And Zip Code Rules	O	O	O
Address Reference System Subaddress Rules	O	O	O
Address Reference System Axis	R	NA	NA
Address Reference System Axis Point Of Beginning	R	NA	NA
Address Reference System Reference Polyline	O	NA	NA
Address Reference System Range Breakpoint	O	NA	NA
Address Reference System Range Breakline	O	NA	NA
Address Reference System Range Polygon	O	NA	NA

Address Reference System Identification, Extent, and Authority

The general elements identify an Address Reference System and establish the source and extent of its authority. These elements are required for every Address Reference System. The general elements are: Address Reference System ID, Address Reference System Name, Address Reference System Authority, and Address Reference System Extent.

• The Address Reference System ID provides a unique identifier (typically an integer) for each Address Reference System administered by an Address Reference System Authority. This, plus the Address Reference System Authority, should be unique throughout the United States. Any Address Reference System Authority may administer multiple Address Reference Systems. For example, a county may have more than one Address Reference System for unincorporated areas based on terrain changes, historical addressing patterns, or for other reasons. Cities may annex areas which have previously been addressed by a County or other Address Authority, and maintain the old Address Reference System. Other Address Reference Systems may be established in the future as an area develops.
• The Address Reference System Name identifies the Address Reference System in a way that is meaningful to users.
• The Address Reference System Authority element identifies the agency and/or jurisdiction with administrative responsibility for the Address Reference System.
• The Address Reference System Extent defines the geographic boundaries of the area within which addressing is governed by the Address Reference System. The Address Reference System Extent may or may not follow jurisdictional boundaries. There may also be areas within an Address Reference System that are excluded from that Address Reference System because they are addressed according to different rules.
• The Address Reference System Reference Document Citation states where to find the authoritative documents that officially establish the Address Reference System. The documents may include a map of the reference system showing the extent, address numbering system, axes, and other features; a statement of the addressing rules described below; an addressing procedures manual and forms; and an address ordinance.

Address Reference System Rules

The remaining elements describe the types of rules that might be adopted by an Address Reference System Authority to govern addressing processes. Due to the variety of local conditions and preferences, not all elements will be applicable to any given system, and all of these presented are optional elements. The rules are collected into the Address Reference System Rules, which incorporates the:

• Address Reference System Numbering Rules,
• Address Reference System Block Rules,
• Address Reference System Street Naming Rules,
• Address Reference System Street Type Directional And Modifier Rules,
• Address Reference System Place Name State Country And Zip Code Rules,
• Address Reference System Subaddress Rules.

Address Numbering Rules

Address numbering rules specify how numbers are assigned along thoroughfares, including what features are numbered. They govern when numbers increase, assign even and odd numbers to sides of streets, and specify the beginning points for numbering. They may also specify if and how address ranges relate to blocks.

• What Features are Given Address Numbers?

In addition to permanent primary structures, other features that can be numbered include vacant lots, secondary structures such as detached garages or farm outbuildings, temporary and seasonal structures, additional entrances of large buildings, non-structured uses such as open parking lots, and infrastructure features such as cell towers, pump and metering stations, substations and transformers.

• Increase and Interval Rules for Address Numbering

In the United States, address numbers increase according to one or more of three rules:

1. 1. Distance rule - numbers are assigned according to distance along the thoroughfare (e.g., 1000 numbers per mile, 500 on either side, or 2 per 10.56 feet).
   2. "Hundred block" Rule - where streets are laid out in a regular city grid, each block may be given a range of 100 numbers (50 per side), e.g. the 1400 block of Cherry Street. Within each block, numbers may be allocated by distance, or proportionally to the length of the block. If blocks have a fixed length (e.g ten per mile), then this rule can work just like a distance rule.
   3. Sequentially - properties or buildings are numbered sequentially, regardless of distance or blocks. The numbers may increase by twos, or they may increase by a larger interval (4, 6, 8, 14, etc.) to leave intermediate numbers for future divisions of land.

• Parity Rules

Parity rules assign even numbers to one side of the thoroughfare and odd numbers to the other side.

• Point(s) of Beginning for Numbering

In axial address reference systems, numbering begins where a thoroughfare intersects (or would intersect) its axis. In non-axial systems, the point of beginning is defined separately for each thoroughfare. Many non-axial systems follow the federal and state highway milepost practice of starting numbering at the southern or western end of the thoroughfare (or boundary of a jurisdiction), and increasing numbers to the north or east.

• Block Rules and Address Range Rules

These rules derive from the increase and interval rules described above. The Address Reference System Block Rules define how the system is organized into blocks for addressing purposes, and whether blocks break at intersections and begin with a new series of numbers, or whether numbering is sequentially ordered along a street without regard to intersecting streets. Such rules also define what constitutes a block break, as many systems do not recognize alleys, or three-way (T) intersections as block breaks.

Address ranges are created using the low and high numbers for each block or other unit defined by the system. Rules pertaining to address ranges are contained with the Address Reference System Block Rules.

Street Naming Rules

Street naming rules define what Street Names may be allowed or prohibited, rules to prevent duplicate names, any language considerations, and whether Street Names must follow particular themes or orders (such as themes for names in subdivisions, or alphabetical or numerical orders).

Street Name Type, Directional, and Modifier Rules

The Address Reference System Street Type Directional And Modifier Rules govern the use of street types, directionals and quadrants, and modifiers in Complete Street Names. Street type rules might specify a limited list of approved types (such as the list in USPS Publication 28 Appendix C1), whether the type must precede or follow the street name, and whether specific types are reserved for thoroughfares with specific functional characteristics. Directional rules include whether a quadrant or cardinal direction (or rarely both) is required, optional or prohibited in an address, and, if so, whether it must precede or follow the street name and type. Modifier rules may allow or prohibit Street Name Pre Modifiers or Street Name Post Modifiers, or specify which modifiers are permitted.

Subaddress Rules

These rules, if included, cover the naming and recording of any subaddresses within structures, such as apartments, office suites, campuses, mobile home parks, industrial plants, malls and retail centers with multiple tenants, etc.

Place Name, State, Country and ZIP Code Rules

These rules define the specific allowable combinations of a Place Name, State, and ZIP code in the Address Reference System, and provide input to checking these elements for quality. Unlike other elements of the address, which must be defined locally, Zip Codes are defined by the USPS, and State Names and Country Names must conform to external standards (see State Name and Country Name for complete information).

Address Axis Rules

An Address Reference System Axis defines the points of beginning for address numbers for the streets that intersect it. The Address Reference System Axis pairs are often the "dividers" for quadrants, or directional designations. Finally, an Address Reference System Axis may also function as a "ruler" to define block breaks and address ranges for thoroughfares with similar directionality (e.g. north-south, or east-west streets) within the Address Reference System.

In theory, every street within an axial Address Reference System can be linked to an axis, either by intersection, or a virtual extension of the street centerline to the axis, or by interpolation (for streets that are set at an angle to the axes, and cannot be projected to intersect with only one of the axes). In practice, however, most jurisdictions with axial Address Reference System create a "grid" by using major through streets to create "blocks" of equal address ranges. For each Address Reference System Axis an Address Reference System Axis Point Of Beginning must be identified. These elements are used only within Axial systems.

Reference Polyline, Breakpoint, and Breakline and Polygon Elements

The Reference Polyline, Breakpoint, Breakline and Polygon elements are utilized primarily for quality assurance and address assignment purposes. These are optional elements used in Axial systems.

An address grid can be constructed by identifying the Address Reference System Range Breakpoints on a sufficient number of streets in the Address Reference System, and then joining equivalent breakpoints with an Address Reference System Range Breakline. By developing these breaklines, a set of areas are defined for each range of 100 (or some specified number of) numbers, and within them, shorter streets can be accurately addressed. If desired, the Address Reference System Range Breaklines can be used within a GIS environment to create polygons with equal address range values. These are then stored as Address Reference System Range Polygon. Streets used for the development of the breakpoints and breaklines (including the Address Reference System Axis elements) can be identified using the Address Reference System Reference Polyline element.

The Address Reference System Reference Polyline is illustrated below:

The Address Reference System Range Breakpoint is also illustrated below. The breakpoints are used in the construction of a grid by linking them into lines of the same value, and constructing range "contours".

The Address Reference System Range Breakline is illustrated below:

These breaklines can then be used as contours, creating grids in both directions, with cells that can display the appropriate address ranges in either or both grid direction. This is illustrated below:

Together, Address Reference System Axis, Address Reference System Reference Polyline, Address Reference System Range Breakpoint, Address Reference System Range Breakline, and Address Reference System Range Polygon form a geographic reference framework for the overall address numbering system within an axial Address Reference System. The framework guides assignment of new address numbers, and it provides the basis for important quality assurance tests.