lecture 2 _vector data model

7/25/2019 Lecture 2 _vector Data Model

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KAEA 4347 GIS FOR CIVIL ENGINEERS

L ECTURE 2


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Topic OutlineCoordinate systemsGeo-relational vector data modelObject-based vector data model

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Geographic coordinate system

GIS map layers must align spatially

GIS works with map represent features on the EarthssurfaceLocations of features on map are based on a planecoordinate system expressed in x- and y- coordinates

Locations of features on the Earth are based ongeographic coordinate system expressed in longitude &latitude

A map projection bridges these coordinates systems

The GCS: is the location reference system for spatialfeatures on the Earths surface. It is defined by longitudeand latitude. Both using angular measures.


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Datum: mathematical model of the Earth, which serves as

the reference for calculating geographic coordinates of alocations. The definition consists of an origin, theparameters of the spheroid and the separation of thespheroid and the Earth at the origin.

Map Projections: the transformation process fromspherical Earth surface to a plane.Map Projections

Transformation from the Earth surface to a flat surface always involvesdistortion, and no map projection is perfect. This is the reason of why

hundreds map projections have been developed for mapmaking.Commonly used map projections are: Transverse Mercator, LambertConformal Conic, Albers Equal-Area Conic

Projected Coordinate Systems: also called a planecoordinate system, is built on a map projection.


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Georelational VectorData Model


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Contents

Georelational Data ModelRepresentation of Simple FeaturesTopologyNon-topological Vector DataData Models for Composite Features


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Geo-relationalLooking at a paper map, wecan tell:

what map features are like.how they are spatially related toeach other.

Idaho borders Montana,Wyoming, Utah, Nevada,Oregon, Washington, andCanada, and contains

several Indian reservations.How can computer "see" thesame features and their 1relationships?


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Vector Data Preparation

The vector data model prepares data in twobasic steps:

First, It uses points and their x-, y-coordinates torepresent spatial features as points, lines, andareas.Second, it organizes geometry objects and theirspatial relationships into digital data files that thecomputer can access, interpret, and process.

So that the computer can process the data.


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Vector Data Models

The vector data model has undergone morechanges over the past two decades than any otheraspect of GIS.

ESRI, Inc. has introduced a new vector data modelwith each new software package:Coverage with Arc/Info,Shape-file with Arc View, andGeodatabase with ArcGIS.

The coverage and shape file are examples of thegeorelational data model, whereas the geodatabaseis an example of the object-based data model.


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GEORELATIONAL DATA MODEL

Geospatial data comprise the spatial andattribute components.

Spatial data describe the locations of spatialfeatures,whereas attribute data describe the characteristicsof spatial features.

The georelational data model stores spatialand attribute data separately in a split system:

Spatial data ("geo") in graphic files. Attribute data ("relational") in a relational database.


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REPRESENTATION OF SIMPLE FEATURES

The vector data model uses the geometricobjects of point, line, and area to representsimple spatial features.A point has 0 dimension and has only theproperty of location. It may also be called anode, vertex, or 0-cell.

A point feature is made of a point or a set ofseparate points.Examples: Wells, benchmarks, and gravel pits.


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An area is two-dimensional andhas the properties of area (size)

and perimeter. An area may contain holes,such as a national forest

containing private land parcels(holes).The existence of holes meansthat the area has both externaland internal boundaries.

An area is also called apolygon, face, zone, or 2-cell.


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Feature Representation andScale

The representation of simple features usingpoints, lines, and areas is not alwaysstraightforward.It can depend on map scale.For example, a city on a 1:1,000,000 scalemap may appear as a point, but the same citymay appear as an area on a 1:24,000 scalemap.


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TOPOLOGYTopology expresses explicitly the spatialrelationships between features.Topology is the study of those properties of

geometric objects that remain invariant undercertain transformations such as bending orstretching.

For example, a rubber band can be stretchedand bent without losing its intrinsic property ofbeing a closed circuit, as long as thetransformation is within its elastic limits.


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Adjacency and Incidence

If a line joins two points, the points are said tobe adjacent and incident with the line.

The adjacency and incidence relationshipscan be expressed explicitly in matrices.


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nodes 11 12 13 1411 0 1 0 112 0 0 1 013 1 0 0 014 0 1 1 0

1 2 3 4 5 611 -1 1 0 1 0 012 0 -1 1 0 -1 013 1 0 -1 0 0 -114 0 0 0 -1 1 1

Adjacency Matrix

Incidence Matrix

Node 11 joined node 12 by line 2.Line 5 is incident from node 14, andincident to node 12


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TIGERTIGER: Topologically Integrated GeographicEncoding and Referencing.In the TIGER database, points are called 0-cells, lines 1-cells, and areas 2-cells.The TIGER database includes the spatialrelationships between points, lines, andareas.


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Topology in the TIGER database involves O-cells orpoints, 1-cells or lines, and 2-cells or areas.


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Using the built-in spatial relationships, we canassociate a block group with the streets orroads that make up its boundary.

Likewise, we can identify an address on eitherthe right side or the left side of a street .


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ESRI's Coverage Model

Coverage is a topology-based vector dataformat.The coverage model supports three basic

topological relationships:Connectivity: Arcs connect to each other atnodes.Area definition: An area is defined by aseries of connected arcs.Contiguity: Arcs have directions and left andright polygons.


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Coverage Data Structure

The coverage modelincorporates the topologicalrelationships into the

structure of feature data.The data structure of apoint coverage containsfeature identificationnumbers (IDs) and pairs ofx- and y-coordinates.


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The data structure of a linecoverage


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The data structure of polygoncoverage


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Topological Polygon Data Layer


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Contiguity of Topological Polygons


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Geo-relational Polygon Dataset


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Importance of Topology

Topology-based data sets require additionaldata files to store the spatial relationshipsbetween features.Topology has at least two main advantages.

The first is the assurance of data quality.It enable us to detect lines that do not meet correctly

polygons that are not closed properly.avoid incomplete features and ensure data integrity.

Second, topology can enhance GIS analysis.


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NONTOPOLOGICAL VECTORDATA

Shapefile is a standard nontopological dataformat.

Geometry of a shapefile is stored in two basicfiles:

The .shp file stores the feature geometry.

The .shx file maintains the spatial index of thefeature geometry.


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NONTOPOLOGICAL DATAs

Advantages

They can display more rapidly on thecomputer monitor than topology-based data.

This advantage is particularly important for peoplewho use, rather than produce, GIS data.

They are nonproprietary and interoperable,meaning that they can be used acrossdifferent software packages.


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DATA MODELS FOR COMPOSITE FEATURES

Composite features refer to those spatialfeatures that are better represented ascomposites of points, lines, and polygons.ESRIs coverage model, for example,includes such composite features as TINs(triangulated irregular networks), regions, and

routes.The inputs to a TIN include point, line, andarea features.


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DataStructureof A TIN


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TIN Surface of Death Valley, Califo


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TIN Surface of Death Valley, Califo


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TOPOLOGY RULES


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Object Based Vector Data Model

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4.1 Object-based Data Model4.2 The Geodatabase Data Model

4.4 Topology Rules refer to previous slide ppt

4.5 Advantages of the GeodatabaseData Model

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Georelational data model is a split systemObject-based model stores spatial andattribute data together rather than in a splitsystem

Geometry (spatial data) stored as an attributealong with other attributesEliminates use of split system and need for

data synchronization

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4.1 Object-Based Data ModelWindows environment

Menus, icons, etc. instead of command lineModel treats spatial data as objects

Object can represent a spatial feature (road or lake)Object can also represent a layer or the coordinatesystem on which the layer is based

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Two Differences between Georelationaland Object-Based Models

1. Stored in single system rather thansplit

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Figure 4.1The object-based data model stores each land use polygon ina record. The Shape field stores the spatial data of land use

polygons. Other fields store attribute data such asLanduse_ID and Category.

A Land Use Data Set


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A Major BreakthroughUsing a single system is a majorbreakthrough because software

developers must regularly deal withissues of data storage and data filestructure.

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Second difference between georelational andobject-based data models

2. Allows spatial feature (object) to beassociated with properties and methods

Property - an attribute or characteristic of anobjectMethod - a specific action that can be

performed on an object

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4.1.1 ClassesSet of objects with similar characteristicsHierarchical structureFeature class - data set that storesfeatures of the same geometry type inthe data base.

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Figure 4.2The Geometry property of the Feature classcan differentiate the object types of point , line ,and polygon .


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4.2 The Geodatabase Data ModelThird major ESRI data model followingcoverage model of 1980s and shapefile

model of 1990s ArcObjects - collection of thousands ofobjects, properties, and methods

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4.2.1 Geometric Representation ofSpatial Feature

Uses geometries of point, polyline, andpolygon to represent vector-based spatialfeatures

Point - simple feature with a point or multipointfeature with a set of pointsPolyline - set of line segments which may ormay not be connectedPolygon - Made of one or many rings

Ring - set of connected, closed, nonintersecting linesegmentsSee Box 4.1 , page 65 of text 12


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4.2.2 Data Structure

Geodatabase data model distinguishesbetween feature classes and feature datasetsFeature class

Stores spatial data of the same geometrytype

Feature datasetStores feature classes that share the samecoordinate system and area extent

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Feature Classes and Feature

DatasetsFeature class is like a shapefile in havingsimple features

Feature dataset is similar to a coverage inhaving multiple datasets based on the samecoordinate system and area extent

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Figure 4.8In a geodatabase, feature classes can be standalone featureclasses or members of a feature dataset.


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4.5 Advantages of the Geodatabase

Data ModelTake advantage of functionalities from object-oriented technology

Convenient framework for storing andmanaging GIS dataEliminates complexity of coordinating between

spatial and attribute components of databaseCustom objects may be develope d

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TUTORIAL QUESTIONS


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TUTORIAL QUESTIONS

1) Name the three data formats that ESRI has developed forvector data over the past 20 years.

2) The geo-relational data model uses a split system to storevector data. What does a split system means?

3) Name the three types of simple features used in GIS and theirgeometric properties.

4) Explain the importance of topology in GIS5) What are the main advantages of using shapefiles?6) Explain the difference between the georelational data model

and the object-based data model

7) Describe the difference between the geodatabase data modeland the coverage model in terms of the geometricrepresentation of spatial features

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lecture 2 _vector data model

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