Geographic Information Systems:an introduction

Week IIIData models cont’d

The Nature of Geographic Data

Last week

• Representing geographic data– E.g. continuous fields, discrete objects

• Data models– E.g. Raster, vector

Data Model Levels

IncreasingAbstraction

RealityReality

Conceptual ModelConceptual Model

Logical ModelLogical Model

Physical ModelPhysical Model

Human-oriented

Computer-oriented

Logical Data Models & Applications

• CAD• Graphical• Image• Raster• TIN• Geo-relational• Object

• Engineering design• Simple mapping• Image processing and analysis• Spatial analysis / modeling• Surface /terrain analysis / modeling• Geoprocessing geometric features• Features with behavior

Raster and Vector Models

• Raster – implementation of field conceptual model– Array of cells used to represent objects

– Useful as background maps and for spatial analysis

• Vector – implementation of discrete object conceptual model– Point, line and polygon representations

– Widely used in cartography, and network analysis

Rasters and Vectors

• How to represent phenomena conceived as fields or discrete objects?

• Raster– Divide the world into square cells– Register the corners to the Earth– Represent discrete objects as collections of one or more

cells– Represent fields by assigning attribute values to cells– More commonly used to represent fields than discrete

objects

Legend

Mixed conifer

Douglas fir

Oak savannah

Grassland

Raster representation. Each color represents a different value of a nominal-

scale field denoting land cover class.

Characteristics of Rasters

• Pixel size– The size of the cell or picture element, defining the

level of spatial detail

– All variation within pixels is lost

• Assignment scheme– The value of a cell may be an average over the cell, or a

total within the cell, or the commonest value in the cell

– It may also be the value found at the cell’s central point

                                                                                                                                                                                            

                                       

Vector Data

• Used to represent points, lines, and areas• All are represented using coordinates

– One per point– Areas as polygons

• Straight lines between points, connecting back to the start

• Point locations recorded as coordinates

– Lines as polylines• Straight lines between points

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ster

POLYGONPOLYGON

POINTSPOINTS

NODESNODESANNOTATIONANNOTATION

LINESLINES

Millbrook Lane

Vector Data

Vector - Land Records

GIS

Survey

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30.5’ 26.23’

20.37’ 26.23’

45.8

1’

45.8

1’

35.4

4’

R 10’

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Survey point

Computation

Link

Surveyed feature

Raster vs Vector

• Volume of data– Raster becomes more voluminous as cell size decreases– Raster is vaster

• Source of data– Remote sensing, elevation data come in raster form– Vector favored for administrative data– Vector is more appropriate when storing large amounts

of attribute information.• Software

– Some GIS better suited to raster, some to vector

Data Model Levels

IncreasingAbstraction

RealityReality

Conceptual ModelConceptual Model

Logical ModelLogical Model

Physical ModelPhysical Model

Human-oriented

Computer-oriented

Data Models and Spatial Analysis

• Raster v. vector?

• Picture of a large burn

• Picture of an urban landscape

• Picture of burning houses

• Map of burning houses

Geo-relational Model

Formats of feature data models in ArcGIS

• Georelational models– Coverages– Shapefiles

• Database Management System (DBMS)– Geodatabase

Coverages

Traditional model

for complex

(multiple feature types)

geoprocessing environments

Coverages

primary

composite

secondary

routes sections regions

annotation ticks links

.

Shapefiles

Simple, open,

“flatfile” based

format for single

data features

Geodatabase

Object-based

data model

where all features

are contained

in database;

features can

exhibit “behavior”

No compression Run length encoding (lossless)

Compression techniques

ADRG - National Imagery and Mapping Agency (NIMA)'s ARC Digitized Raster Graphics BIL - Band Interleaved by Line (image format linked with satellite derived imagery) CADRG - National Imagery and Mapping Agency (NIMA)'s Compressed ARC Digitised Raster Graphics (nominal compression of 55:1 over ADRG) CIB - National Imagery and Mapping Agency (NIMA)'s Controlled Image Base (type of Raster Product Format) Digital raster graphic (DRG) - digital scan of a paper USGS topographic map ECW - Enhanced Compressed Wavelet (from ERMapper). A compressed wavelet format, often lossy. ESRI grid - binary and ASCII raster formats used by ESRI GeoTIFF - TIFF variant enriched with GIS relevant metadata IMG - ERDAS IMAGINE image file format MrSID - Multi-Resolution Seamless Image Database (by Lizardtech). A

compressed wavelet format, often lossy.

Vector formatsGeography Markup Language (GML) - XML based open standard (by OpenGIS) for GIS data exchange DXF - Contour elevation plots in AutoCAD DXF format Shapefile - ESRI's open, hybrid vector data format using SHP, SHX and DBF files Simple Features - Open Geospatial Consortium specification for vector data MapInfo TAB format - MapInfo's vector data format using TAB, DAT, ID and MAP files National Transfer Format (NTF) - National Transfer Format (mostly used by the UK Ordnance Survey) TIGER - Topologically Integrated Geographic Encoding and Referencing Vector Product Format - National Imagery and Mapping Agency (NIMA)'s format of vectored data for large geographic databases. GeoMedia - Intergraph's Microsoft Access based format for spatial vector storage. ISFC - Intergraph's Microstation based CAD solution attaching vector elements to a relational Microsoft Access database Personal Geodatabase - ESRI's closed, integrated vector data storage strategy using Microsoft's Access MDB format Coverage - ESRI's closed, hybrid vector data storage strategy. Legacy ArcGIS Workstation / ArcInfo format with reduced support in ArcGIS Desktop lineup

Grid formats (for elevation)USGS DEM - The USGS' Digital Elevation Model DTED - National Imagery and Mapping Agency (NIMA)'s Digital Terrain Elevation Data GTOPO30 - Large complete Earth elevation model at 30 arc seconds SDTS - The USGS' successor to DEM

Other formatsBinary Terrain - The Virtual Terrain Project's Binary Terrain format Well-known text (WKT) – ASCII spatial projection description (ESRI uses a *.prj extension) Well-known binary (WKB) - Binary spatial projection description

Geo-relational Model

Formats of feature data models in ArcGIS

• Georelational models– Coverages– Shapefiles

• Object model– Geodatabase

Coverages

primary

composite

secondary

routes sections regions

annotation ticks links

.•Topologically correct

•Data processing and spatial analysis•Complex data structure

•Difficult interchange (E00)•Multiple feature types

Traditional model for complex geoprocessing environments

Coverages

Shapefiles

Simple, open, “flatfile” based format for single data features

•Single feature type within one file•No topology•Easy to transport•Open

Shapefiles

Geodatabase

•Multiple features stored as tables•Data processing topology•Object oriented instead of restricted to points, lines, polys•Resides in traditional database

Object-based data model where all features are contained in database; features can exhibit “behavior”

Geodatabase

Topology

• Field within mathematics.• The study of the general abstract nature of continuity or

"closeness" on spaces. • System administrators actually describe computer

networks as being topologic– Bus topology– Ring topology– Star topology

• Leonhard Euler– The solution of a problem relating to the geometry of position 1736. – Euler was aware that he was dealing with a different type of geometry where distance was not

relevant.

Consider the problem of building a fusion reactor which confines a plasma by a magnetic field.

The solution of a problem relating to the geometry of position

Topology

• Science and mathematics of geometric relationships– Simple features + topological rules– Connectivity– Adjacency– Shared nodes / edges

• Topology uses– Data validation– Spatial analysis (e.g. network tracing, polygon

adjacency)

Topology and GIS

• Topology in GIS is generally defined as the spatial relationships between adjacent or neighboring features.

• Two types of topology:– Polygon topology– Line topology

• three advantages of incorporating topology in GIS databases:data management, data correction and spatial analysis

Data correction and storage

Topology and spatial analysis

• Adjacency • Connectivity (network models) • Containment

Polygon Topology Model

Polygon Topology Contiguity

Definitions

• A method of analysis is spatial if the results depend on the locations of the objects being analyzed– move the objects and the results change– results are not invariant under relocation

• Spatial analysis requires both attributes and locations of objects– a GIS has been designed to store both

Spatial Autocorrelation

• Spatial autocorrelation is determined both by similarities in position, and by similarities in attributes– Sampling interval– Self-similarity

Tobler

Spatial autocorrelation measures

n number of objects in the samplei,j any two of the objectsz the value of the attribute of interest for object ic the similarity of i’s and j’s attributesw the similarity of i’s and j’s locationsi,j

i,j

i