Introduction GIS often represent spatial information with a two-dimensional x,y coordinate system. Some data linearly measured. In order to use the information in GIS, location of the linear data must be known. DS is method to use linearly referenced data with GIS layers measured in X-Y coordinates. Defined as the process of computing the location of events along a route. Route and Events are important elements to understand DS.

Route Definition: “A linear feature, such as a street, highway, or stream, used in a GIS, which also has a linear measurement system stored with its geometry” (Chang, 2014: 353)

3 Figure 17.8 An example of a geodatabase route feature class. For linearly referenced event data, route needs a measurement system. A geodatabase route feature class consists of a pair of x and y coordinates and an m value (as shown in Figure 17.8) x and y values locate linear feature in as a 2D coordinate system m is a linear measure which can depict the segmentation length Such route termed as “route dynamic location object” and ArcGIS called it “route feature class”.

4 Creating Routes Route links a series of segments together. Routes can be created interactively or through data conversion.

5 Figure 17.9 The interactive method requires the selection or digitizing of the line segments that make up a route (shown in a thicker line symbol). Interactive method

6 Data conversion method Routes can be created at once from all linear features or from features selected by data query. That is, one can go to the attribute table and select a route, or one can come up with a query that will do the selection

7 Types of Routes 1.Simple route (route follows one direction without loop or branch) 2.Combined route (a route joins with another route) 3.Split route (route subdivides into two routes) 4.Looping route (a route intersects itself)

Figure Interstate highway routes in Idaho. Combined Route For example, an interstate highway, which has different traffic conditions depending on traffic direction. Two routes can be built for the same interstate highway, one for each direction

9 Figure An example of a split route. Split route Example of a split route is the split of a business route from an interstate highway At the point of a split, two separate linear measures begin: one continues on the interstate highway, and the other stops at the business route

10 Figure A looping route divided into three parts for the purpose of route measuring. Looping route Each part is created and measured separately

Events Event are linearly referenced data that are usually stored in an event table. DS allows events to be plotted on a routes through a linear measurement system.

Types of Events Point events – Occur at a specific point location. e.g Accidents and stop signs. To relate line events to a route, you need route ID, the location measures of the events, and attributes describing the event. Line events – e.g Pavement conditions, and cover portions of a route. To relate line events to a line, you need route ID and the from-and to-measure.

Creating Event Tables There are two methods:  Creating an event table from an existing table that has data on route IDs and linear measures.  Creating an event table by locating point or polygon features along a route.

Figure 16.12: An example of converting point features to point event.

Figure 16.13: An example of creating a line event table by overlying a route layer and a polygon layer.

Application of Dynamic Segmentation Data Management Data Display Data Query

Data Management Useful in 2 ways: Different routes can be built on the same linear feature E.g. A transit network can be developed from the same street database for modelling transit demand Different events can reference the same route E.g. Can be used to store various environmental data with a stream for habitat studies

Data Display Once event table is linked to a route, it is georeferenced and used as a feature layer Similar to geocoding an address table Point symbols are used to display point events and line symbols to display line events

Data Query Both attribute data query and spatial data query can be performed on an event table and its associated event layer

Data Analysis Inputs: routes and events After conversion of events to an events layer it can be analysed as point/linear features E.g. Accidents concentration (analysed as point) analysis by dynamic segmentation

Data Analysis Analysis between two events layers e.g. accidents vs speed limit along a highway Produce an output: accidents locations and its associated speed limit along the highway Analysis gets complex with more layers