Topology - Its all about relationship...

Based on some research, any two people in the world are connected by at most seventh level of connection. It can be acquaintance, friend, relative, or other. Topology is more or less similar. ????♂?

If we say that a school is two kilometers from a my home, then two features are related by a distance. If we say that a school is located near the garden, then two are related by an adjacency relationship. There are many ways features can be related to each other.

In a GIS, there are two ways of handling relationships. One way is to derive them on-the-fly when requested by the user. This is the approach used when we invoke the distance calculation command to compute the distance between two arbitrary points.

Another way to handle a relationship is to derive the relationships ahead of time, store them in the database, and retrieve them when required. This approach is suitable for relationships that take considerable time to derive and are relatively static. Topological relationships are ideal candidates because their derivations require fairly intensive calculations, and once derived would remain valid until the next update to the geometric data. It is common practice, therefore, to pre-derive the basic topological relationships and store them in the database. In essence, this approach trades storage for response time.

Non-topological file format - At the time of data creation, particular file format is storing only coordinates and attributes but relationships are not stored. Since this file format is not storing relationship, it saves in terms of diskspace. Editing and display functions are relatively faster.

Topological file format - At the time of data creation, particular format is storing object relationship along with their coordinates and attributes. Now when file is storing relationships, it normally occupies relatively larger diskspace. Since file is big, the editing and display functions are relatively slower.

Then what is the fun of having relationships... ??

When relationships are stored natively in database, spatial analysis becomes faster and easier to develop. Imagine you want to find out shortest distance between two point of interest. In this case, if relationships are already stored, then development will become easy and analysis will perform faster. Developing the same function in non-topological file formats will be very complex as they don't have relationships in database and they have to rely on coordinates of the features (point, line and polygons). Topological database allows rule based editing and helps in generating more cleaner data. You can define rules like, State Highway must be connected to National Highway. Another rule can be two pipelines in Gas Network must be connected through Valve.

Three basic topological relationships are : connectivity, adjacency, and enclosure.?Connectivity?describes how lines are connected to each other to form a network.?Adjacency?describes whether two areas are next to each other, and?enclosure?describes whether two areas are nested. There are many other relationships as shown below which are derived from these three basic relationships.

Now lets come back to our question: Is shapefile a topological data format?

Answer - No. Shapefile does not store relationship in database so it is non-topological data format.

Hope you are finding this useful. Happy learning. ??