COMPUTERS AND STATISTICS IN ARCHAEOLOGY Week 4. Geographic Information Systems (GIS) - 2 © Richard Haddlesey

Aims l WEEK 4 l Examine the differences between vector and raster graphics l Discuss which data types are best represented by each l Examine how ArcGIS treats vector graphics l Outline vector data sources l Create new vector themes by digitising in ArcGIS l WEEK 5 l Discuss the use of photography and satellite date in GIS l Outline raster data sources l Load and geo-reference aerial photographs in ArcGIS l Examine different display options when using raster images in ArcGIS

Key text Conolly J, Lake M Geographical Information Systems in Archaeology: Cambridge Manuals in Archaeology. Cambridge University Press. Cambridge Conolly J, Lake M Geographical Information Systems in Archaeology: Cambridge Manuals in Archaeology. Cambridge University Press. Cambridge Geographical Information Systems are a powerful technology that offer a host of analytical possibilities for investigating the spatial organisation of cultural and human- environment relationships (p31) Geographical Information Systems are a powerful technology that offer a host of analytical possibilities for investigating the spatial organisation of cultural and human- environment relationships (p31)

Data models Data models and data structures: [are] the digital representation of spatial phenomena Data models and data structures: [are] the digital representation of spatial phenomena A GIS works by manipulating the digital representations of real world entities…[this is known as a] data model A GIS works by manipulating the digital representations of real world entities…[this is known as a] data model (Conolly and Lake, 2006: 24)

Two main types of Data models in GIS l Vector –Objects represented as nodes (X,Y points) and connecting lines, attributes of objects attached as database tables l Raster –Geographical space represented as a grid of cells, numerical values represent attributes of each cell

Vector and Raster GIS Forest Lake Grassland Vector Raster Road Site

Strengths Vector l Spatial precision l Compact data storage l Scalable presentation l Object based l Database linkage Raster l Analytical capabilities l Surfaces l Continuous quantities l Pixellated data l Photographs

Vector applications Spatially referenced database applications (information about objects): Location maps Sites and Monuments Artefacts Mapping applications Triangulated Irregular Network (TIN) elevation models

Vector applications Downside: Very CPU intensive Boundedness Elevation is hard to express without a TIN

Raster applications Each cell can be given: Each cell can be given: A quantitative value that signals the mean elevationA quantitative value that signals the mean elevation a single attributea single attribute This is simple, in comparison to a vector, but that is its strength: This is simple, in comparison to a vector, but that is its strength: It can be mathematically manipulated and displayed much quicker than a vectorIt can be mathematically manipulated and displayed much quicker than a vector Fuzzy boundaries Fuzzy boundaries Map algebra Map algebra (Conolly and Lake, 2006: 28)

Raster applications Downside: Downside: Fixed resolution (not multiscalar)Fixed resolution (not multiscalar) Its difficulty in representing discrete entities (too blocky)Its difficulty in representing discrete entities (too blocky) Limited ability to handle multiple attribute data through a DBMSLimited ability to handle multiple attribute data through a DBMS (Conolly and Lake, 2006: 30-1)

The good news: Hybridisation Vector/raster systems l Most modern GIS software packages combine both vector and raster capabilities l Image processing l Aerial and satellite integration l ESRI desktop GIS suite l ArcView l ArcCatalog and ArcMap

The good news: Hybridisation Vector/raster systems Most modern GIS software packages combine both vector and raster capabilities Image processing Aerial and satellite integration DBMS integration ESRI desktop GIS suite ( Arc Info ArcEditor ArcView ArcCatalog and ArcMap ArcReader ArcGIS extensions

Hybridisation through layers in ArcView MSc lecture notes 2005/6

Software l CAD/CAM AutoCAD, Microstation l Image processing MicroBrain, ERDAS Imagine l Raster GIS Idrisi, Grass (Unix), Grassland, Spans, MapII (Mac) l Vector GIS ArcInfo, ArcGIS, MGE l Desktop mapping ArcView 3, MapInfo

Map objects l Point l Line/Polyline l Area/Region/Polygon l Text ABC

Database linkage Map objects = attributes

l Most mapping applications are intimately linked with a database of objects: Territories Historical records Sites Museum objects Features Excavated/collected artefacts Mapping to Database Attributes Graphical objects

Layers / themes

Why themes? l Logical breakdown of data - related objects in each theme l Assemble maps for different purposes by combining themes l Examine interaction between themes, create new themes Paper maps use symbolism to distinguish between layers, to compensate for the limitations of the technology

Data collection (1) l Conventional databases Point data Attributes for line & area objects l Digital vector data sources Geographically referenced –Worldwide, DCW –BGS –National (e.g. Ordnance Survey) –Other e.g. Developers plans etc Drafting / surveyors plans (DXF)

Data collection (2) l Map digitising l Considerations: Accuracy, costs, heads-up/down Projection, geodetic system, topology Topographic / projected Plans / unprojected Historical maps & plans

Data collection (3) l EDM, GPS & conventional survey Architectural survey l Field survey & recording Sites Survey units / material count Individual artefact locations l Surface collection l Excavation –Units & features as objects –3D / volumetric systems / voxels Vastly increased data requirements –Artefacts