Geographic Information Systems: an introduction Week III Data 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

Human oriented Computer oriented

Data Model Levels

Reality Conceptual Model Logical Model Physical Model Increasing Abstraction

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

Vector Data

Vector - Land Records

Surveyed feature GIS Survey Survey point Link / /

30.5’ 12 20.37’ 13 26.23’ 26.23’

/ / / 9 / Computation

Raster vs Vector

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

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.

– Some GIS better suited to raster, some to vector

Human oriented Computer oriented

Data Model Levels

Reality Conceptual Model Logical Model Physical Model Increasing Abstraction

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

Traditional model for complex (multiple feature types) geoprocessing environments

Coverages

Coverages

.

primary composite routes sections regions secondary annotation ticks links

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

Shapefiles

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

Geodatabase

Compression techniques No compression Run length encoding (lossless)

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 (type of Raster Product Format) (NIMA)'s Controlled Image Base

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

-

GeoTIFF

binary TIFF and ASCII raster formats used by ESRI 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 formats Geography 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 relational Microsoft Access based CAD solution attaching vector elements to a 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 SDTS

- Large complete Earth elevation model at 30 arc seconds - The USGS' successor to DEM

Other formats Binary Terrain

- The Virtual Terrain Project's Binary Terrain format

Well-known text

(WKT) – ASCII spatial projection description ( ESRI extension)

Well-known binary

(WKB) - Binary spatial projection description uses a *.prj

Geo-relational Model

Formats of feature data models in ArcGIS

• Georelational models – Coverages – Shapefiles • Object model – Geodatabase

Coverages

Traditional model for complex geoprocessing environments •Topologically correct •Data processing and spatial analysis •Complex data structure •Difficult interchange (E00) •Multiple feature types .

primary composite routes sections regions secondary annotation ticks links

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

Object-based data model where all features are contained in database; features can exhibit “behavior” •Multiple features stored as tables •Data processing topology •Object oriented instead of restricted to points, lines, polys •Resides in traditional database

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 sample

i,j

any two of the objects

z i

the value of the attribute of interest for object c the similarity of

i

’s and

j

’s attributes

i

w the similarity of

i

’s and

j

’s locations