Linear referencing datasets

Implementing linear referencing requires the use of two primary data types:

  • Route feature classes
  • Event tables

Dynamic segmentation uses events from event tables to locate and visually display these events along line features in a route feature class.

Route feature classes

A route feature class is a line feature class with a defined measurement system (such as distance or time). These measurement values can be used to locate events, assets, and conditions along linear features.

Route refers to any linear feature, such as a city street, highway, river, or pipe, with a unique identifier and a common measurement system along each linear feature.

Coordinate systems for linear referencing include m-values——(x,y,m) or (x,y,z,m).

In simple terms, feature vertices in route feature classes include m-values (x,y,m or x,y,z,m). These measured coordinates form the building blocks for route features. In route feature classes, line features have their x,y (or x,y,z) coordinates that describe location as well as a measurement (m) value along the line.

A collection of routes with a common measurement system is a route feature class. Each route in the feature class will also have a unique identifier. Line features with the same unique identifier are considered to be part of the same route.

Route feature classes are created and managed as line feature classes in the geodatabase. You can also use route feature classes from polyline shapefiles that include route identifiers and measured features.

Route feature geometry

Route features have a measurement system stored with their geometry. Each measured line's segments have x-, y-, and m- (measure) or x-, y-, z-, and m-values. When a measure value is unknown for a particular vertex, its m-value is recorded as NaN (not a number), as illustrated in this example:

Illustration of measures on line features

Simple linear features are represented by lines with one path. Complex linear features are represented by lines with many paths.

Simple and complex route systems with measures

It is important to note that although most applications use measures to represent increasing distances along a linear feature, measure values can arbitrarily increase, remain constant, or decrease along line features.

Measure values are independent of a feature class's horizontal coordinate system (and the vertical coordinate system as well if one is defined). This means that measure values are not required to be in the same units as the feature class's x,y,z coordinates. For example, features stored in a feature class whose coordinate system is Universal Transverse Mercator (UTM) where the unit is meters might store measure values in feet, miles, or time.

Event tables

Event tables contain information about assets, conditions, and events that can be located along route features. Each row in the event table references an event, and its location is expressed as measurements along named (identifiable) linear features.

There are two types of route events:

  • Point events—A point event, stored in a point event table, describes a discrete location along a route (a point) and uses only a single measure value to describe this discrete location—such as Mile 3.2 on I-91.
  • Line events—A line event, stored in a line event table, describes a portion of a route (a line) and uses both from- and to- measure values to describe this portion of a route—for example, Mile 2 to Mile 4 on I-91.

All event tables must contain a route identifier and measure location fields containing measure information. A point event table uses a single measure field to describe their discrete location. A line event table requires two measure fields (a from- and to- measure) to describe their location.

Route locations and their associated attributes are typically stored in an event table based on a common theme. For example, four event tables containing information on speed limits, year of resurfacing, present condition, and accidents could be included and used to dynamically locate events on one route feature class.

An event table can be any tabular data source that ArcGIS Pro supports. This includes geodatabase tables, delimited text files, dBASE, and database management system (DBMS) tables accessed via an Object Linking and Embedding database (OLE DB) connection.

Event table example

Hydrologists and ecologists use linear referencing on stream networks to locate various types of events, as illustrated in the example below. The route feature class for streams provides measures along the streams using river reach mile. Point and line event tables record the route ID and location along each river reach. These event tables can be used to locate point and line events.

Line and point event table example

Storing features using relative locations

With linear referencing, locations along linear features are referred to in terms of their route measure, or distance from a known point. For example, it often makes sense to describe the location of an accident as occurring at 12 miles from the beginning of the interstate rather than at a GPS coordinate such as 1,659,060.25, 1,525,238.97.

To determine a location along a linear feature, a system of measurement is required. When a measurement system is stored along with a linear feature, any location along that linear feature can be expressed in terms of the measure values.

Illustration of accident location along a linear feature

In addition to making data more intuitive, storing data as a relative location along a linear feature has the added benefit of ensuring that spatial phenomena you know to fall on a linear feature is located on the feature. For example, in the absence of a very accurate basemap, locating accidents using x,y coordinates may end up displaying accidents that do not fall on the road network as they should. This will not happen if the accidents are located using linear referencing.