3161 Gis Data Models

GIS Data Model

GIS Data Model

GIS data model refers to data storage concept in GIS (nonreferenced). How it is stored?

URP 3161: GIS

Ahsanul Kabir

GIS Data Model

Vector data model

The vector data model represents geographic features similar to the way maps do. Points represent geographic features too small to be depicted as lines or areas; lines represent geographic features too narrow to depict as areas; and areas represent homogeneous geographic features.

URP 3161: GIS

Ahsanul Kabir

GIS Data Model

Vector data model

In a vector data model, each location is recorded as a single x,y coordinate. Points are recorded as a single coordinate. Lines are recorded as a series of ordered x,y coordinates. Areas are recorded as a series of x,y coordinates defining line segments that enclose an area, hence the term polygon, meaning ‘many-sided figure’.

URP 3161: GIS

Ahsanul Kabir

GIS Data Model

Vector data model With x,y coordinates, you can represent points, lines and polygons as a list of coordinates instead of as a picture or graph. In the figure below, for example, the coordinate pair 3,2 represents a point location (building); the coordinate pairs 1,5 3,5 5,7 8,8 and 11,7 represent a line (road); and the coordinate pairs 6,5 7,4 9,5 11,3 8,2 5,3 and 6,5 represent a polygon (lake). The first and last coordinates of the polygon are the same; a polygon always closes. These coordinate lists represent how geographic features are stored in a computer as sets of x,y coordinates.

URP 3161: GIS

Ahsanul Kabir

Vector data model

GIS Data Model

To keep track of many features, each is assigned a unique identification number or tag. Then, the list of coordinates for each feature is associated with the feature’s tag

URP 3161: GIS

Ahsanul Kabir

Vector data model

GIS Data Model

Arc-node Data structure The arc-node data structure stores and references data so that nodes construct arcs and arcs construct polygons. Nodes define the two endpoints of an arc; they may or may not connect two or more arcs. An arc is the line segment between two nodes. An arc is composed of its two nodes and an ordered series of points which define its shape, called vertices. Nodes and vertices are represented as x,y coordinates.

URP 3161: GIS

Ahsanul Kabir

Vector data model – arc-node data structure

GIS Data Model

For example, polygons A and B in the left diagram are represented by a series of connected coordinates. In the diagram on the right, nodes are created where the lines intersect, arcs are created between the nodes, with vertices providing shape, and polygons A and B are constructed from the arcs.

URP 3161: GIS

Ahsanul Kabir

Vector data model

Topology

GIS Data Model

Topology explicitly defines spatial relationships. Creating and storing topological relationships has a number of advantages. Data is stored efficiently, so large data sets can be processed quickly. Topology facilitates analytical functions, such as modeling flow through the connecting lines in a network, combining adjacent polygons with similar characteristics, identifying adjacent features and overlaying geographic features. The arc-node data structure 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

URP 3161: GIS

Ahsanul Kabir

Vector data model - Topology

GIS Data Model

Connectivity Arc-node topology is supported through an arc-node list. The list identifies the from and to nodes for each arc. Connected arcs are determined by searching through the list for common node numbers. In the following example, it is possible to determine that arcs 1, 2, and 3 all intersect because they share node 11. The computer can determine that it is possible to travel along arc 1 and turn onto arc 3 because they share a common node (11), but it’s not possible to turn directly from arc 1 onto arc 5 because they don’t.

URP 3161: GIS

Ahsanul Kabir

Vector data model - Topology

Area definition

GIS Data Model

Recall that 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. In the illustration below, polygon F is made up of arcs 8, 9, 10 and 7 (the 0 before the 7 indicates that this arc creates an island in the polygon). Each arc appears in two polygons (in the illustration below, arc 6 appears in the list for polygons B and C). Since the polygon is simply the list of arcs defining its boundary, arc coordinates are stored only once, thereby reducing the amount of data and ensuring that the boundaries of adjacent polygons don’t overlap.

URP 3161: GIS

Ahsanul Kabir

Vector data model - Topology

Contiguity

GIS Data Model

Two geographic features which share a boundary are called adjacent. Contiguity is the topological concept which allows the vector data model to determine adjacency. Recall that the from-node and to-node define an arc. This indicates an arc’s direction, so that the polygons on its left and right sides can be determined. Leftright topology refers to the polygons on the left and right sides of an arc. In the illustration below, polygon B is on the left of arc 6, and polygon C is on the right. Thus, we know that polygons B and C are adjacent. Notice that the label for polygon A is outside the boundary of the area. This polygon is called the external or universe polygon, and represents the world outside the study area. The universe polygon ensures that each arc always has a left and right side defined.

URP 3161: GIS

Ahsanul Kabir

GIS Data Model

Vector data model - Topology

URP 3161: GIS

Ahsanul Kabir

TIN Data Model

GIS Data Model

A TIN dataset contains points with x, y, and z values and a series of edges joining these points to form triangles. The triangular mosaic forms a continuous faceted surface, which can be used to analyze and display terrain and other types of surfaces. TINs offer an alternative to the raster data model for representing surfaces.

URP 3161: GIS

Ahsanul Kabir