Creating A Network Dataset
To conduct a network analysis, you must first create a network dataset with roads (edges) and intersections (junctions). The ArcGIS software creates a network dataset from source features, such as lines and points. It then stores the connectivity between these source features. By themselves, features such as lines and points, are unaware of each other. For example, if two line features intersect, neither line is aware of the other. A network dataset keeps track of which sources features are coincident. It also has a connectivity policy, which can be modified to define further which coincident features are indeed connected. This connectivity makes it possible to model overpasses and underpasses without having the roads connect. While performing network analysis, the GIS will know which paths along the network are feasible. In the case of the overpass, the GIS will be aware that a turn is not allowed on the bridge, even though the two roads might intersect there.
Using Arc Catalog to Create a New Network Dataset
Open the ArcCatalog window in ArcMap and navigate to your Working folder. In the ArcCatalog Window, right click on your clipped roads shapefile and select New Network Dataset.
Note: If the Network Analyst extension is not currently activated, this option may appear grayed out.
The New Network Dataset window opens up and a default name is generated by the ArcGIS software ending in _ND. You may accept the default name if you wish and click next.
Click next to accept the default Yes for modeling turns in the network.
On the next page, you have the option to change the default connectivity settings. In most cases, you will not need to change these settings. By default, road segments connect to each other at endpoints. To check these settings, click on the Connectivity button. Make sure the source feature, the roads layer, is set to End Point. Then click Ok and Next.
When elevation fields are used, two edges that have the same endpoint locations, but different elevations remain disconnected. This is useful for modeling multilevel transportation infrastructure. This dataset does not contain a field with elevation information. Click Next to accept the default None for elevation settings.
The next window displays the attribute information for the network dataset. The ArcGIS software automatically tries to assign common cost attribute fields, such as Length and Minutes, to the network dataset. Check the fields the ArcGIS software will use by clicking on the Evaluators button.
This opens the Evaluators dialog window. From the drop-down menu next to Attributes, select Length. Under the Source Values tab, set the Value field to SHAPE_LENG. If you will recall, this is the field updated via Calculate Geometry earlier on. The Eureka_Roads source feature is listed twice, one for each direction. Change the Value field to SHAPE_LENG for both directions.
If you created a length field of your own, you may use that field for the length value instead.
From the drop-down menu next to Attributes, select Minutes. Again, you will see the Eureka_Roads source feature listed twice, one for each direction. Make sure the Value field for each direction is set to Minutes. If you will recall, this Minutes field was created earlier on and records the time it takes to travel. Then click Ok and Next.
On the next window, click Next to accept the Default Travel mode. On the next window, click Next to accept the default Yes for driving directions settings. The last window will display a summary of the network dataset settings and attributes. Click Finish to create the network dataset.
Click Yes to build the Network Dataset. Then click Yes to to add all feature classes that participate in the network dataset. Adding all the feature classes that participate in the network dataset is not necessary to our analysis, but it will allow us to take a closer look at the source elements that comprise the network dataset. You will remove these layers shortly.
Take a moment to inspect the network dataset. It uses the roads and junction layers as its source elements and manages the connectivity between them. This allows the network dataset to model direction and turns along the network. The network dataset links directly to the roads and junctions. Having the roads and junctions present in the table of contents unnecessary and clutters the map. Remove the road and junction layers from the table of contents, leaving only the network dataset (ND).