A line is stored as a reference to two 'node' endpoints and a reference to 
a line type (e.g. straight line, circular arc, circle, smooth curve). 
A surface consists of one or more line primitives which form together a 
closed polygon. One exterior and several interior boundaries are possible 
for the same primitive. 
  
i | A TIN is a hybrid type of primitive. It contains a list of references to 
HU adjacent triangle primitives, to the three nodes belonging to the corners 
of the triangle and the surface normals at these points. 
b) Features 
A feature is a model of a real-world geographic element in the database. 
It is defined by a set of geometric primitives which in turn can be de- 
fined by other primitives. Features stored at the project level are 
consistent with the topologic type (e.g. all 'surfaces' are closed 
polygon) and topologically correct with respect to other features 
against which they are checked (e.g. no buildings on roads). 
c) Complex features 
  
Complex features are used to define an entity in terms of one or more 
other features. For example the complex feature 'parcel' could consist 
of the features 'parcel boundary', 'house' and 'garage'. The user can 
directly extract the entire content of a single or complex feature. 
d) Feature class 
The feature classification defines the logical organisation of the data. 
Topographic checks can be defined between elements of different feature 
classes. The topology of features is unique for a feature class and 
classes are also represented identically on a graphic display. A feature 
class can have subclasses, e.g. the feature class 'buildings' can have 
subclasses with 'single-storey buildings', 'two-storey buildings', etc. 
e) Partition 
A partition is a working subset of a project. Users interact with the 
system at this level to capture, edit, update and manipulate data. 
Partitions are extracted from a project by means of a partition defini- 
tion, which describes the selection of feature classes and how they 
will be presented, the security restrictions and the spatial extent. 
f) Project 
The definitive version of the database is held in the project and is 
updated when a partition is merged with a project. Each project can 
have several partitions. Feature classifications with the definition 
of attributes including range checking, topographic and topologic 
checks, partitions, graphic transforms and restrictions on operator- 
group access are created and maintained at this level. 
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