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Application Entities 
Within each application that uses GIS, 
there are application entities that require 
modelling. Most applications model the 
elevation of the earth’s surface. Foresters 
model stands and openings; municipalities 
model parcels and road networks; fisheries 
experts model watersheds and stream 
networks. All of these are called 
application entities. 
In addition to extracting the application 
entities, other design decisions are made at 
the application level. These include 
accuracy requirements, units of resolution 
of both graphic and non-graphic data, 
coordinate definitions in terms of 
projection, spheroid and datum. 
e.g., between rivers, highways, lakes. 
Features are often classified according to 
the complexity of information required to 
represent their positions as: 
• Point features such as manholes or 
well sites. 
• Linear features such as a roads or 
streams. 
• Polygonal features such as lakes. 
GIS Constructs 
Application entities require a GIS construct 
to represent them. Most such entities can 
be represented by either surfaces, features, 
or a combination of both. 
A surface defines a single value for every 
position within an area of interest. 
Common GIS surfaces include elevation, 
slope, aspect. These varieties of surfaces 
have been computer-modelled for many 
years. More recently, with the advent of 
imagery, other surfaces are becoming 
prevalent - including not only raw or 
geocoded image data, but also resultant 
surfaces such as vegetation indices. 
A second type of GIS construct is a 
feature, representing a distinct geographic 
object. Most applications require 
differentiation between various features;