2.3. Non - Topological Vector Data

 Shapefile

A shapefile is a nontopological data structure that does not explicitly store topological relationships. However, unlike other simple graphic data structures, shapefile polygons are represented by one or more rings. A ring is a closed, non-self-intersecting loop. This structure can represent complex structures, such as polygons, that contain "islands." Shapefile stores nontopological geometry and attribute information for the spatial features in a data set. The geometry for a feature is stored as a shape comprising a set of vector coordinates. Because shapefiles do not have the processing overhead of a topological data structure, they have advantages over other data sources such as faster drawing speed and edit ability. This structure was very efficient from a data storage standpoint, but very inefficient for rendering polygons.  The shapefile format is a digital vector storage format for storing geometric location and associated attribute information. It is developed and regulated by ESRI as an open specification for data interoperability among GIS software products. The shapefile format was introduced with ArcView GIS version 2 in the early 1990s.  

The actual shapefile relates specifically to the .shp file, but alone is incomplete for distribution as the other supporting files are required. The three mandatory files have filename extensions .shp, .shx, and .dbf. Their description is as follows.

•.shp                Shape format; the feature geometry itself

•.shx                Shape index format; a positional index of the feature geometry to allow seeking                forwards and backwards quickly

•.dbf                The workspace containing shapefiles may also contain dBASE tables, which can store additional columnar attributes that can be joined to a shapefile's features              

Apart from the mandatory files there may be a number of other files that are stored in the same directory.

Spaghetti Model

It is the simplest vector data model in terms of implementation and non topological in nature. In spaghetti model, each point, line, and polygon feature is represented as a string of X, Y coordinate pairs with no inherent structure.  The entities are represented using a collection of points and lines with no real connections. It is a direct line-by-line translation of a vector map. The visual appearance of a map is captured by digitising lines and polygons without any additional information about which line connect to which or which polygon share common boundaries. In this model data input is very easy since it is similar to tracing the lines on the map using a digitiser until they have all been digitized. Since this data model stores graphical elements and not graphical entities, any polygons that lie adjacent to each other must be made up of their own lines, or stands of spaghetti. In other words, all polygons are stored as independent polygons and hence the intervening boundaries are digitized twice. This creates some redundancies within the data model and therefore reduces efficiency.

 



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