Building and Analyzing Topology in Autodesk Map GI21-1

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1 December 2-5, 2003 MGM Grand Hotel Las Vegas Building and Analyzing Topology in Autodesk Map GI21-1 Alex Penney ISD Training Content Manager, Autodesk Professional Services, Autodesk Inc. Topology is one of the most intriguing features in Autodesk Map TM. It is relationships, driven by graphic properties, associated data, and spatial definitions that allow us to truly model the world around us. This course will cover when and how to use topology for a variety of problems and solutions. The Presentation The purpose of this presentation is to expose the student to the concepts, procedures and principles of topology in Autodesk Map. The 90 minutes are divided into 4 main sections: Concepts Preparing for Topology Creating Topology Analyzing Topology Concepts will cover all 3 Topology types in Autodesk Map; Node, Network and Polygon. Preparing for topology covers some of the principles surrounding object selection for topology and drawing clean up. Once prepared, the creation of topology will include discussion of how topology is stored in Autodesk Map and managing multiple topologies. Analyzing topologies is the final goal of the presentation. This handout represents many of the points covered in the presentation, but should not be considered a comprehensive article covering all aspects of Autodesk Map topology. Prerequisites For this presentation, the student should have a working knowledge of: Object Data Attaching and manipulating external databases Generating links to objects in the drawing file Autodesk Map Projects Object Queries Each of these features and concepts are used throughout the presentation. About the Speaker: Alex has worked in local government agencies and municipal districts providing GIS and data management solutions as an independent consultant. As the ISD training content manager for Autodesk, Alex managed the development of materials and trainers to Autodesk partners for more than 5 years. He has personally trained more than 1000 Autodesk Map users worldwide. Within Autodesk, Alex works directly with the Autodesk Map product development team to design and create training materials.

2 Autodesk Map Topology Concepts Topology in Autodesk Map can be thought of in terms of applying relationships between objects. These relationships include connectivity, direction and adjacency. Since we are working in a vector-based file, these relationships are applied to objects within our drawing file. The objects used by Autodesk Map topology can be all or some of the following; lines, polylines, arcs, points, text and blocks. Once a topology is established these objects are referred to as "links" for linear objects, and "nodes or centroids" for points, text and blocks. Node Topology Nodes can represent real world objects such as telephone poles, sewer manholes or wells. Depending on the scale and use of your map, nodes can even represent cities. Node topology can be applied to AutoCAD points and blocks. Node topology creates an association of these nodes through a named numbering system. Nodes in Autodesk Map do not actually understand relative distance or spatial relationships to one another, but they are associated by data. A spatial relationship of sorts can be determined through buffering within a Node topology. The figure below describes AutoCAD Points or Blocks with a topology applied. 2

3 Network Topology Network topologies can be used to represent real world objects and systems such as sewer collection systems, water distribution systems, street centerlines or electrical distribution systems. Any linear network can be represented using network topology. Network topologies make use of nodes and links. Nodes at each end define the links in a network topology. For example, in the figure below, Link A is defined as "from node 1 to node 2. This definition of a link provides an implied direction of the link. The definition of links through nodes also allows the definition of connectivity. For example, if node 2 is the endpoint for a connected link B, then link A and link B understand themselves to be connected through node 2. While in this figure there are objects representing nodes, objects are not required for a network topology to apply node definitions. Autodesk Map topology recognizes linear endpoints as nodes if no objects are specified. There is also the option of creating object nodes during topology creation. This will be covered in more detail in the Network Topology Creation section of this course. 3

4 Polygon Topology Polygon Topologies can be used to represent areas in the real world. Typical applications of polygon topology include parcels, counties, states, geologic areas, environmental habitats or voting districts. Polygon topology in Autodesk Map uses lines and points, text or blocks. The polygon topology uses the same concept of links and nodes as a network topology, with the addition of a centroid. The centroid is an AutoCAD point, text or block that resides anywhere within the definition of a polygon. The "polygon" is actually a connective system of individual links and nodes surrounding the centroid. There is a common misunderstanding that the polygon is actually a closed polygon as defined in AutoCAD. The centroid contains information about the links that surround it and define it as a polygon. The links contain information that defines polygons on either side. This left polygon and right polygon information defines adjacency. For example, in the figure below there are 3 "polygons" as defined within the topology. These 3 polygons are comprised of 3 centroids, 6 nodes and 8 links. The link defined as "from node 6 to node 5" separates polygon A and polygon C. The link definition of direction allows an understanding of left and right. In this case, polygon A is to the left, and polygon C is to the right, or more importantly, polygons A and C are adjacent. 4

5 Drawing Cleanup The idea of drawing cleanup goes well beyond the process of creating an ascetically pleasing map. While drawing cleanup features can certainly be useful for this, the primary importance of drawing cleanup is in the preparation of data for topology. For topology to function correctly, if at all, endpoints of lines must be coincident, crossing lines must be broken at the intersection (for polygon topology), overshoots and undershoots must be resolved and duplicate objects must be removed. AutoCAD commands are certainly available to Map users for drawing cleanup, but the size of most maps and complexity of the data generally prohibits this approach. Drawing cleanup features thus become an integral step in the creation of topology. This section is not intended to cover all the options and features of drawing cleanup, and in fact, is only presented because of the integral function drawing clean up has in the topology creation process. Typical Errors Depending on the source of your map and the methods used to produce it, you may find a myriad of errors present. Typically, a map that is produced using AutoCAD or Autodesk Map has been digitized into the system from an existing hardcopy. This is not as prevalent as it once was, but legacy data still surfaces. If a map has been produced with no fore-thought of topology, what we consider errors were not errors at all to the originator. These are typically crossing intersections, dangling links and pseudo nodes. Errors that are introduced due to a lack of attention or sloppy work are generally duplicate objects, clustered nodes, short objects and undershoots. If a 5 parcel polygon topology was built using the geometry in this diagram, the pseudo nodes, while not desirable, would not prevent the topology from completing. The crossing, dangling and undershoot links would cause a failure of topology build. There is an option during polygon topology creation that allows open polygons be ignored, but in this case, the crossing links would still prohibit successful topology creation. 5

6 Creating Topology Selecting Topology Objects Autodesk Map creates the topology model based on existing objects. Some objects can be automatically created to support topologies, but to take full advantage of topology, you will want to have better control of the objects used. Therefore, an important part of successful topology creation relies on the correct selection of objects to represent links, nodes and centroids. The methods used to create topology are in the form of a wizard. This section concentrates on object selection and some of the nuances surrounding topology creation. Naming and descriptions of topologies are intuitive and therefore not discussed. Note: It is strongly recommended that topology be created from objects that exist in the open drawing, and not from objects that have been queried into a project. When selecting objects included in a topology, the selection tools provide a manual selection option, or filters based on layers and/or blocks. In nearly every case, selecting the objects based on these filters is the most effective method. Keep this in mind when creating your geometry and layers. Be sure to organize your geometry by layers or block definitions that will accommodate the creation of topology. Manual selection methods are most often used for temporary topologies that consist of a subset of geometry. Node Topology Node topology is by far the easiest of topologies to build. There are no drawing cleanup requirements, and objects can be Points, Blocks or Text. Network Topology Network topology, similar to node topology has little in the way of drawing cleanup requirements. However, the network model that results from unwanted crossing links, undershoots and clustered nodes will not likely be the network representation you are looking for. Object selection for network topology includes; Links, Nodes and if nodes are to be created, or not. Actual node objects (points, blocks) are not required. In the absence of these objects, the topology model recognizes the link endpoints as nodes. If the model you are creating requires data associated with nodes, these objects will be required in the drawing, and selected as part of the topology creation process. An example of this could be a water distribution system with nodes representing valves having specific resistance values (on, off, CFS etc.). 6

7 Polygon Topology Polygon topology, as discussed in the Drawing Cleanup section, is very sensitive to clean geometry. While node topologies have no cleanup requirements and network topologies will build your topology regardless of errors, polygon topology will fail with the presence of errors. If you re unsure about the geometry in the drawing, this can be used to your advantage. If you want to verify the cleanliness of the geometry, attempt to build a polygon topology. If failures occur, the wizard provides an option to mark where the problems are. Selecting objects for polygon topologies include; Links, Nodes, Create New Nodes, Centroids, Create New Centroids and finally Error Markers. Links, nodes and creating new nodes is exactly the same as creating a network topology. In some cases, polygon topologies are created using links and nodes that are part of network, or other polygon topologies. This does not cause a problem. Centroids, as the name implies, are one of the most important components of the polygon topology. You will want to pay special attention when selecting, or creating centroids. If the polygon topology represents areas that have specific values, such as parcel numbers, voting districts, zoning, etcetera, then the presence of these objects within the boundaries of the links, and selecting them as the centroids will be required. For example, an AutoCAD block object with Object Data or SQL links can be used as the centroid. During topology analysis, this centroid serves as the bridge between the external data, and the definition of the polygon. In other words, the centroid has the link to the external data, and information about the polygon. Analysis tools provide access to both, and can combine the results. If a centroid is created during topology creation in addition to the intelligent point, there will be no bridge between the polygon and the external data. 7

8 Topology Object Data All topology models in Autodesk Map are defined by Object Data. Building a topology using the selected objects, Autodesk Map automatically generates 4 Object Data Tables specific to the topology and attaches data to the objects. The following describes some of, but not all Object Data tables associated with a topology. Refer to the Autodesk Map Topology Concepts section and compare the conceptual topologies to these Object Data tables. You will see a fairly straight-forward definition emerge. 349 Link Object Data Node Object Data Polygon Centroid Data Polygon Link Object Data While the Object Data tables seem like an obvious and convenient place to store object specific data, it is not recommended that this be done. An example of object specific data may be to use the resistance field of the link data to enter a speed value (for a street). Topologies are often recreated or edited. When this is done, all specific data you entered will be replaced with default values used by topology. A much better solution is to create a separate table, or external database link to store and link data to the object. When analyzing the topology, these alternate tables can be used in place of the default values. When recreating, or editing the topology, the alternate tables, and their links will be unaffected (assuming the geometry is not deleted or broken). 8

9 Managing Topologies Autodesk Map allows an unlimited number of topologies to be written into a file. It even allows multiple topologies to be written to a single object or sets of objects within a drawing file. When considering the size of some maps and that topology adds to the memory overhead of any Autodesk Map project, it becomes obvious that there is a need to manage topologies. Autodesk Map makes this easy by allowing the user to select topology models to load and unload from memory at any point in a drawing session. Topology Analysis The obvious point of creating and managing topologies is to analyze them. Methods of analysis include: Path Trace, Flood Trace, Buffer, Dissolve and Overlay. Path Trace Path trace is the process of allowing Autodesk Map to find the most efficient method to traverse our network from one location to another. The simplest path trace is a "shortest distance" path trace that identifies the shortest path from one point to another using only the distance values found on the links. More sophisticated path trace analysis may utilize data representing resistance values from external databases or Object Data. This data may include cfm ratings in a water distribution network, coefficients in a collection system, length values on the objects, or a combination of all of the above. Using the default values for resistance (length), this figure shows the shortest distance between each point. Flood Trace Flood trace behaves in much the same manner as a path trace and has the same criteria available for use. The difference between Path and Flood is that Flood allows the entire network to be considered from a single location, with parameters defined for limits of travel. Using the same start point as the above path trace, this flood trace identifies all links that can be traversed within 3000 feet. 9

10 Buffers Buffers can be created using node, network and polygon topologies. The buffer values can be static, or can be derived from data associated with the objects being buffered. The result of a buffer is a polygon topology that represents the distance from the object(s) being buffered. Using the path trace from above, the resulting topology has been buffered with a distance of 100 feet. The result of the buffer analysis is a single polygon with a centroid. Overlays Overlays involve analyzing how multiple topologies affect each other. There are several options for overlay, including; intersection, clip, identify, erase, paste and union. When defining an overlay, the source and overlay topologies are selected, along with the operation. The following shows a simple intersection overlay with the parcels as source, and the circle (which is a topology) as the overlay. The results are polygons within a new topology that have adopted geometry from both according to the intersect. 10

11 Using Multiple Analysis While many analysis functions require only a single procedure, such as flood trace, or a single overlay, using some creativity along with a solid understanding of the tools, you can generate some very sophisticated analysis from a series of operations. This may include using path trace, buffer, overlay, topology query and report, all within a cohesive series of procedures. Summary 90 minutes is not a long time to describe the features and principles behind Autodesk Map topology. The presentation is meant to introduce you to the basic concepts of topology along with some solid foundation knowledge of procedures in the preparation, creation and analysis of topology. The presentation also exposes you to some interesting contortions to answer a few challenges. Topics worth continued study that are not covered in any detail, either in the presentation, or handouts are: Topology queries Loading topology Creating objects during topology load Overlay options Editing Topology Using custom expressions for topology analysis 11