TOPOLOGY

Submitted by:-ROHIT KUMARCUJ/I/2013/IGIO/026

Submitted to:-Dr. SURAJ KUMAR SINGH

Definition- Topology basically refers the relationship between things, and in the realm of GIS, Topology refers to the relationship between spatial features or objects. Importance In GIS In terms of functionality, topology is important to GIS in (at least) three important way:

First, topology is necessary for certain spatial functions such as network routing through linear networks. Here the idea is that if line features do not share common nodes, that routes cannot be established through the network.

Second, topology can be used to create datasets with better quality control and greater data integrity. Topology rules can be created so that edits made to a dataset can be 'validated' and show errors in that dataset. An example would be the creation of a new manhole/sewer access feature outside a polygon dataset of road features.

Third, by creating topological relationships between feature classes, features can be shared across feature classes. In other words, if you open one dataset and edit/move a line feature that is shared between two feature classes, then both feature classes will be updated to reflect the edits. This is massively helpful for keeping datasets synchronized. An example would be a river feature that defines a administrative boundary (where the river moves over time), or the boundary of a municipal area and zoning polygons.

Topology rules There are many topology rules that you can use in creating spatial datasets. : Point in Feature Class X must lie within polygons in Feature Class Y, Polygons in Feature Class X must completely cover polygons in Feature Class Y, Lines in Feature Class X must intersect lines in Feature Class Y. For a more complete list of rules, see the ArcGIS Desktop Help (or Online Help), open the index and navigate to Topology Rules : Topology Rules (Editing in ArcMap).

Topology implementations With "coverages", a data format we haven't used very much, topology had to be "built" after each edit. But once topology was built, the dataset stored natively the node-id's shared by line features, and the line-ID's shared by polygons. Coverages are rarer and rarer now that most data providers use shapefiles and to a lesser extent geodatabases, but you still see them occasionally.

Shapefiles have a relatively unsophisticated implementation of topology. In fact, shapefiles do not store topological relationships. Rather, you can do topology-like operations (e.g. network routing) where the topological relationships are generated on the fly (e.g. connectivity between line features). Here is a nice document summarizing topology and shapefiles. Geodatabases provide the greatest topological functionality among the data formats supported by ArcMap.

Here is a summary of how topology is implemented: 1. Topology exists between Feature Classes in a Feature Dataset 2. A Feature Dataset can be considered an association of Feature Classes, but a Feature Dataset itself has spatial properties such as a spatial reference system and XY domain. Any Feature Class in a Feature Dataset must adhere to the same spatial properties as defined in the Feature Dataset. 3. Topology rules reside within a Feature Dataset. These rules have two characteristics: 1) the type of rule (within, intersecting, overlapping...) and 2) the feature classes that are members of that rule. 4. Most of the topological operations in ArcMap are available from two resources a. ArcToolbox : Data Management : Topology b. ArcMap : Editor toolbar : Advanced Editing : Topology

Applications Topology rules help create datasets with greater integrity - (i.e. no slivers between polygons, no unsnapped nodes between lines that should be connected, no twisted line features). 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).

Data Structure The most common topological data structure is the arc/node data model. 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.g. an arc that does not connect to another arc such as a dead end street. Isolated nodes, not connected to arcs represent point features. A polygon feature is comprised of a closed chain of arcs.

Data Organization And Storage In GIS software the topological definition is commonly stored in a proprietary format. 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.g. point, line, area. 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.g. no duplicate lines, no gaps in arcs that define polygon features, etc.

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.g. features that touch one another, and proximity, e.g. features that are near one another. 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, 2,000 forest stand polygons will require considerably longer to build the topology that 2,000 municipal lot boundaries. This is due to the inherent complexity of the features, e.g. lots tend to be rectangular while forest stands are often long and sinuous. 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. Without a topologic data structure in a vector based GIS, most data manipulation and analysis functions would not be practical or feasible. Topology facilitates the editing of shared features between different spatial layers and is a mechanism to ensure integrity with spatial data. However despite its importance in GIS, one major drawback of the topological data model is its static nature.

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