Topology is fundamentally used to ensure data quality of the spatial relationships and to aid in data compilation. Topology is also used for analyzing spatial relationships in many situations, such as dissolving the boundaries between adjacent polygons with the same attribute values or traversing a network of the elements in a topology graph.
Topology can also be used to model how the geometry from a number of feature classes can be integrated. Some refer to this as vertical integration of feature classes. Features can share geometry within a topology. Here are some examples among adjacent features:. In addition, shared geometry can be managed between feature classes using a geodatabase topology, for example:. A layer of polygons can be described and used in the following ways:.
This means that there are two alternatives for working with features—one in which features are defined by their coordinates and another in which features are represented as an ordered graph of their topological elements. Feedback on this topic? ArcToolbox : Data Management : Topology b. Applications Topology rules help create datasets with greater integrity - i. Topology facilitates the editing of shared features between different spatial layers.
Different data format have different implementations of topology with varying degrees of functionality. With data formats supported by ArcMap, Geodatabases have the greatest topological functionality. Not every GIS project really requires topology.
If you're just making a map of city locations and roads, then you don't really need topology. If you want to find the optimal path between five different cities, then topology is useful, but there are plenty of GIS projects where you don't really need topology or at least topology built into the datasets. This model contains two basic entities, the arc and the node. The arc is a series of points, joined by straight line segments that start and end at a node.
The node is an intersection point where two or more arcs meet. Nodes also occur at the end of a dangling arc, e. Isolated nodes, not connected to arcs represent point features. A polygon feature is comprised of a closed chain of arcs.
However, most software offerings record the topological definition in three tables. These tables are analogous to relational tables. The three tables represent the different types of features, e. A fourth table containing the coordinates is also utilized.
The node table stores information about the node and the arcs that are connected to it. The arc table contains topological information about the arcs. This includes the start and end node, and the polygon to the left and right that the arc is an element of.
The polygon table defines the arcs that make up each polygon. While arc, node, and polygon terminology is used by most GIS vendors, some also introduce terms such as edges and faces to define arcs and polygons.
This is merely the use of different words to define topological definitions. Do not be confused by this. Data Analysis Since most input data does not exist in a topological data structure, topology must be built with the GIS software.
Depending on the data set this can be a CPU intensive and time consuming procedure. This building process involves the creation of the topological tables and the definition of the arc, node, and polygon entities. To properly define the topology there are specific requirements with respect graphic elements, e. Advantages The topological model is utilized because it effectively models the relationship of spatial entities.
Accordingly, it is well suited for operations such as contiguity and connectivity analyses. Contiguity involves the evaluation of feature adjacency, e. The primary advantage of the topological model is that spatial analysis can be done without using the coordinate data.
Many operations can be done largely, if not entirely, by using the topological definition alone. This is a significant advantage over the CAD or spaghetti vector data structure that requires the derivation of spatial relationships from the coordinate data before analysis can be undertaken. Disadvantages The major disadvantage of the topological data model is its static nature. It can be a time consuming process to properly define the topology depending on the size and complexity of the data set.
For example, 2, forest stand polygons will require considerably longer to build the topology that 2, municipal lot boundaries. This is due to the inherent complexity of the features, e. This can be a consideration when evaluating the topological building capabilities of GIS software. Conclusion Topology is very important in GIS because it effectively models the relationship of spatial entities.
Moreover, it is well suited for operations such as contiguity and connectivity analyses. If you already have one polygon, it is possible with this option to digitise a second adjacent polygon so that both polygons overlap and QGIS then clips the second polygon to the common boundary.
When moving a vertex, all features that share that vertex are updated. A segment is a straight line formed between two vertices in a polygon or polyline geometry. The snapping distance black circle is defined in map units e. Search radius is the distance a GIS uses to search for the closest vertex you are trying to move when you click on the map.
In principle, it is quite similar to the snapping distance functionality. Snapping distance and search radius are both set in map units so you may need to experiment to get the distance value set right. If you specify a value that is too big, the GIS may snap to a wrong vertex, especially if you are dealing with a large number of vertices close together. Mainly designed for simplicity and for fast rendering but not for data analysis that require topology such as finding routes across a network.
Many GIS applications are able to show topological and simple feature data together and some can also create, edit and analyse both. In the section that follows we will take a closer look at Coordinate Reference Systems to understand how we relate data from our spherical earth onto flat maps! Outdated version of the documentation. Find the latest one here. The following list shows some examples of where topology rules can be defined for real world features in a vector map: Area edges of a municipality map must not overlap.
Area edges of a municipality map must not have gaps slivers. Polygons showing property boundaries must be closed. Undershoots or overshoots of the border lines are not allowed. Contour lines in a vector line layer must not intersect cross each other. Figure Topological Tools 1: 1 Topological editing to detect shared boundaries, when moving vertices.
Figure Snapping Distance 1: The snapping distance black circle is defined in map units e.
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