File based feature model

File based feature model is divided to

1) coverages

2) shapefiles

Both coverages and shapefiles employ to georelational data model. They store the vector data for the features in

a file and use unique identifiers to link feature to attributes

.

Database management system (DBMS) feature

models

It is the geodatabase model . In this model features are stored as rows in a relational database table. The rows in

the table contain both the coordinates and the attribute

information for the features.

coverage

A coverage is a georelational data model that stores vector datait contains both the spatial (location) and attribute (descriptive)

data for geographic features. Coverages use a set of feature

classes to represent geographic features. Each feature class

stores a set of points, lines (arcs), polygons, or annotation (text).

Coverages can have topology, which determines the relationships

between features.

coverage

Coverage features

Coverage contain primary, composite and secondary feature types.

Primary features in coverage are:

1) Label points

2) Arcs

3) Polygons

Label points

Label points represent individual point features.

For example wells.

Label points also link attributes to polygons, each polygon in a coverage has a single label pointwith its feature ID

number, located in the center of the polygon.

Arcs

Arcs are connected sets of line segments, with nodes at the end points.

A single arc can stand alone (road), or line networks such as streams.

Arcs can also be organised into polygons that represent areas, such as soil type.

Nodes

Nodes : are the endpoints of and connections between arcs.

Nodes have attributes, such as valve in a network of water mains.

They are important for tracking how features in coverages are connected to each other (topology).

Composite features

Composite features

1) Routes

2) sections

3) regions

They are built from the primary features

Routs and sections

Routs and sections are linear features that are composed of arcs and parts of arcs.

Routs define paths along an existing linear network such as the route from a house to an airport along a street

network.

Sections

Because points of interest on a network are not always at nodes, sections identify partial arcs. They record how far

along a given arc a route begines or ends.

Regions

Regions are area features that are composed of polygons, regions can be discontinuous.

For example: the mainland and an island can be mapped as two polygons, but they can belong to the same region.

Secondary Features

Secondary features include :

1) Tics

2) links

3) Annotation

Tics and links dont represent geographic objects but they are used to manage coverages

Tics

Tics are geographic control points. Every coverage has a feature class containing tic points, which represent known

real-world coordinates.These tic points help define the

extent of a coverage; they do not represent any actual

data points within the coverage .

Links

Links are displacement vectors that arte used to adjust the shape of coverages . For example: to match the

edges of adjacent coverages. Links consist of a from point and to a point.

Tics and Links

Tics and links dont represent geographic objects but they are used to manage coverages

Annotation

Annotation is used to provide text about geographic features on map.

Annotation can be positioned at a point, between points , or along a series of points.

Annotation is used to make maps easier to read and understand.

Coverage topology

When you stand on a hill and look down on a landscape, you can easily identify intersecting streets and adjacent

properties. The mathematical logic a computer uses to

identify these relationships is topology.

Coverage topology

Topology defines spatial relationships between connecting or adjacent features in geographic data. The

principle in practice is quite simple: spatial relationships

are expressed as lists (for example, a polygon is defined

by the list of arcs comprising its border).

Coverage topology

Creating and storing topological relationships have a number of advantages:

1) Data is stored efficiently, so large datasets can be processed quickly.

2) Topology facilitates analytical functions, such as modeling flow through the connecting lines in a network, combining adjacent

polygons with similar characteristics,

3) Identifying adjacent features, and overlaying geographic features.

The topological structure of a coverage supports three major topological concepts:

Connectivity: Arcs connect to each other at nodes.

Area definition: Arcs that connect to surround an area define a polygon.

Contiguity: Arcs have direction and left and right sides.

Connectivity

Connectivity is defined through arc-node topology. This is the basis for many network tracing and path finding

operations. Connectivity allows you to identify a route to

the airport, connect streams to rivers, or follow a path

from the water treatment plant to a house.

Connectivity

In the arc-node data structure, an arc is defined by two endpoints: the from-node indicating where the arc begins

and a to-node indicating where it ends. This is called arc-

node topology.

arc-node topology

Area definition

An area is represented in the vector model by one or more boundaries defining a polygon. Although this sounds

counterintuitive, consider a lake with an island in the middle. The lake actually has two boundaries: one that

defines its outer edge and the island that defines its inner edge. In the terminology of the vector model, an island

defines an inner boundary (or hole) of a polygon

polygon-arc topology

The arc-node structure represents polygons as an ordered list of arcs rather than a closed loop of x,y

coordinates. This is called polygon-arc topology.