Network dataset

A Network dataset is a GIS dataset that is designed to support network analysis. It typically consists of lines representing the routes of flow in the network, augmented with other features (such as junction points), topology, and attributes that model network-relevant properties such as impedance and capacity of flow. Network datasets are well suited to model transportation networks. With a network dataset, travel is typically allowed in two directions, whereas with a Geometric network, travel can only take place in one direction at a time.

Street Network Example
In the graphic to the right, a network dataset models the street network. The graphic highlights that one-way streets, turn restrictions, and overpasses/tunnels can be modeled. The analyses that are performed on the network, such as the route from stop 1 to stop 2, respect these and other network dataset properties.

Network datasets and Topology
For features in network datasets, connectivity rules (topology) are important. In a normal GIS layer, line features are generally unrelated to each other and have few, if any, rules regarding their connectivity. For example, if two line features intersect, neither line is aware of the other. Similarly, a point feature at the end of a line feature doesn't have any inherent information that lets it know about the line. Therefore, topology is a key element in a network dataset due to the need to understand relationships between features for network analysis. Consider underpass and overpass line features in a GIS. The GIS does not inherently know that one line goes under the other in the real world. Topology would therefore dictate this relationship and disallow a turn going from the underpass to the overpass and vice versa.

ArcGIS Information
When you perform an analysis using ArcGIS Network Analyst, the analysis always happens on a network dataset.

In ARC/INFO, coverages were used to create networks on the fly. In ArcView GIS, a persistent network was created when a network analysis function was run on a line shapefile for the first time. In ArcGIS, the network dataset stores this persistent network. You can save this network, modify its properties, and model a variety of networks using network datasets.

When creating a network dataset, the best option is to use a geodatabase. In a geodatabase, multiple features classes can be stored in a single feature dataset. As a result, the network dataset can support multiple sources and it is possible to create multimodal network datasets. A multimodal transportation network includes road, rail, and bus networks all together in one area for example the image of downtown Paris to the right. These datasets are used for more complex transportation scenarios. Often these multimodal network datasets have complex attribute information such as impedances, restrictions, and hierarchy. With the use of a geodatabase, this information can more easily be included in the model. .

Additional Links and References
Creating a network dataset [http://webhelp.esri.com/arcgisdesktop/9.3/index.cfm?topicname=what_is_a_network_dataset? What is a network dataset?] Understanding Connectivity