Digitally, the basic elements of a geologic map in the vector model are carried as:
     Lines-  polylines,
     Areas-   polygons,
     Symbols-   points.
A polyline is a series of nodes or points with x and y coordinates, with instructions to the software to draw segments between them. A polyline has a beginning and ending node. A polygon is defined to the software as a closed polyline where the coordinates of the first and last node are the same, and carries instructions to the software that the area thus defined is to be displayed a in specified way. A point is simply a point defined by an x, y and possibly z in the coordinate system of choice. 
 

PUTTING IT TOGETHER--BUILDING A GEOLOGIC DATASET AND OBJECT TOPOLOGY
In order to extract maximum value from the geologic map as a dataset, as well as to display the map correctly it is necessary to know certain specific information about the geometric and spatial relationship of the map objects.   For example, if a contact or fault is connected to another contact or fault, or to a specific polygon (or area of a specific rock type,) or where lithologies border one-another. This type of information is often referred to as the topology, which can be defined as the relationship in space between the objects in a spatial database. Traditionally there are two ways of carrying this information in the database; both methods require the digital geologic map (geologic object dataset) to be constructed in the same manner. The two traditional topology models are:

   Explicit Topology- This is the original Arc Info model (not Arc View) in which each object carries as an explicitly defined attribute the unique ID of the polygons and polylines which border it. 

   Implicit Topology- This is the method used by MapInfo and other software including Arc View, in which the bounding and adjacent features to an object are calculated on the fly based on the sharing of nodes with the same coordinates. 

While there are advantages and disadvantages to both methods, the fundamental implication for creating the digital geologic dataset (map) for both is that the linework (collection of contacts, faults or area-defining or area-bounding linear features) is captured one time only.   That specific linework is then used to construct the areas or polygons (lithologic areas) that comprise the map. The result is that all sets of adjacent features are properly node sharing, necessary for satisfactory execution of the Implicit topology model, and additionally, if done from within Arc Info, each object will be explicitly attributed with its adjacent objects' identifier. Secondarily, any dataset thus consists of both the polygons and the linework (polylines) used to build those polygons. Both will be attributed parts of the dataset. Constructing the polygons from constituent polylines is known as "building" the polygon data.

In order to produce the desired outcome most efficiently, data capture and building of the objects of a digital geologic map should proceed with this working model from the very beginning.  Cleaning or transforming an existing dataset that has not been properly constructed can be a labor intensive but necessary precursor to effective use. 

The properly built and constructed dataset will allow powerful investigation of the data with a minimum of technical expertise, thus placing a fast and enlightening tool in the hand of the geologist who is ultimately most familiar with the data and in the best position to interrogate it to productive ends.