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2 Welcome. I m Colin Childs from Educational Services at ESRI in Redlands, California, and I teach a host of ArcGIS classes ranging from geodatabase design to working with Spatial Analyst. Today I will be exploring network analysis and the different types of networks available with the ArcGIS software. This discussion is tailored to users of the ArcGIS software who may deploy geometric network functionality or the Network Analyst extension functionality to model directed or undirected network flow. You ll briefly hear what the differences are and how to decide which is the appropriate one to use for your specific application. Networks pervade our daily life. We drive from home to work on street networks. We cook our dinners with natural gas delivered through gas utility lines. We catch up with news and send s through the Internet. We even visit our relatives by flying on airline route systems. Networks channel flow. Certain phenomena flow in a continuous field across a region, such as rainfall or temperature. But rainfall is collected into streams, and nearly all resources that we process, all goods that we manufacture, flow in a constrained way, carried along the networks of streets, pipes, cables, and channels. A network is a one-dimensional system of edges that collect junctions to which resources, communication, and people are transported. Three common applications of networks are in transportation, for the use of rivers, and in utility networks. Networks model the transportation of people; the resources such as water, electricity, gas; and communication. Networks constrain flow to edges, such as streets and rivers, and they join at junctions, such as intersections and confluences. The geodatabase has two core network models. The network dataset, optimized for undirected flow (especially written for transportation), and the geometric network model, which implements directed flow systems for activities (such as river networks and utility lines). Geometric networks: A geometric network uses custom network features simple edges, complex edges, and junctions to model the parts of the network. Complex edges are used to model a compound set of edges and junctions, such as the electrical distribution line with connected services. With geometric networks, connectivity is updated whenever any network feature is added or removed. When you edit features in a geometric network, the logical network is continuously updated for you. Copyright 2007 ESRI. All rights reserved. 1
3 Geometric networks have connectivity rules that control which network objects can be connected properly to each other. For example, a 2-inch pipe may connect to a 3-inch pipe via a reducer. Analytical solvers use weights in a geometric network to perform trace functions, useful for utility and hierographic applications. Solvers in the geometric network include downstream tracing, upstream tracing, isolation tracing, and path tracing. Network Datasets: Network datasets are more analogous to a geodatabase topology. A network dataset has network sources that are defined on simple features. These features undergo no modification. They may also participate within its topology, if you so choose. With network datasets, connectivity is updated on demand when you rebuild the entire network. Network datasets, beta model, and directed flow networks: They allow flow in any direction, and in fact, the resource makes the decision. They employ terms as well to model restrictions, and may also allow you to incorporate barriers through which the network will not pass. Network datasets have a rich attribute environment that uses costs, descriptors, descriptions, and hierarchies. Network datasets were optimized for large transportation networks. One of the key differences between a geometric network and a network dataset is how network connectivity is maintained. In a geometric network, it is initially made up of simple line and point features, which become simple junctions and simple edges, that may be aggregated to create complex edge features within the geometric network. Once created, network connectivity in a geometric network is continuously updated to doing any edits that you make. In the network dataset, the network sources have their connectivity discovered on demand. Both geometric networks and network datasets have logical networks. Logical networks are a series of tables that represent the network elements and how they connect to each other. For geometric networks, the logical network is incrementally updated through any edit. For the network dataset, the logical network is regenerated with every build of the network that you may execute. Network datasets represent the core geodatabase network model for representing undirected flow networks, and as I mentioned, particularly for transportation networks. Copyright 2007 ESRI. All rights reserved. 2
4 Network datasets also encapsulate decades of GIS modeling experience that allow us to optimize street datasets for network analysis. Later you ll see that the network dataset has a rich attribute model that can be used to represent cost impedances, that allow you to incorporate connectivity rules, to group features, to allow hierarchies to be set, to incorporate elevation fields, one-way restrictions, and turns. The network dataset model is a sophisticated model that allows analysis such as optimal routing and allocation, and even functions on multimodal systems, such as combined street, subway, and pedestrian traffic. Let s take a brief look at some of the differences between network datasets and geometric networks. As mentioned, network datasets are used largely for transportation modeling, whereas geometric networks are better suited for utility and natural resource modeling. Analysis on a network dataset will return pathfinding or allocation, whereas analysis done on a geometric network generally returns tracing. So the geometric network analysis is largely a trace-type function. The sources of data in a network dataset make up junctions and edges and turns. These are essentially feature classes in feature datasets, shapefiles, or feature classes. The sources of data for a geometric network are initially the same (except the turns), and they make up simple edges, simple junctions, and complex edges within the geometric network. Connectivity in the network dataset is reactive and refreshed with a rebuild of the network. Connectivity in the geometric network is dynamic and updated each time you execute or perform an edit. Network datasets allow us to incorporate turns. In other words, the ability to follow two or more connected streetlines, and model more natural traffic scenarios, such as restricted left or u-turns. Geometric networks do not support turns at all. In a network dataset, topology is an option to be used on the sources of data for the network dataset. In the geometric network, the data sources may not be part of any topology. Network datasets have rich attribute models that support costs, descriptors, restrictions, and hierarchies. Geometric networks have a simple attribute model that really only allows you to specify weights. The workflow for a network dataset is that network productivity is built on demand, similar to that of database topology. Once a source feature class has been edited, your network becomes invalid and needs to be rebuilt. Copyright 2007 ESRI. All rights reserved. 3
5 For geometric networks, the workflow is in the form that the network is continually maintaining the connectivity, so there s no need to execute any rebuild. Network datasets allow us to incorporate multimodal systems through connectivity groups. Geometric networks do not support multimodal systems at all. Well, as you notice then, the network dataset is the new core geodatabase data model, implemented by a network analyst to represent undirected flow networks, largely used for transportation. The network dataset encapsulates a lot of modeling experience and is optimized for street datasets. You can use this data model to model cost impedances, connectivity rules, groups, hierarchies, elevation fields, one-way restrictions, and even turns. And also, the modeling angles have to do with sophisticated analysis, such as optimal routing and allocation, and even incorporate a multimodal system. For further resources, please check our instructor-led training courses at This discussion touches on topics that are covered in our two-day instructor-led class titled, Working with ArcGIS Network Analyst. Thank you for tuning in to this session of our ESRI Instructional Podcast Series. Stay tuned for future broadcasts. Copyright 2007 ESRI. All rights reserved. 4