This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

RecapMapping and ErrorLaw and IPRMetadataFinding Data

DataPrinciples of GIS

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

RecapAttributes, footprints and topologies

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mappingprecision and accuracy

Precision implies the degree to which a measurement is refined

Accuracy that the measurement taken is correct within the precision indicated

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mappingprecision and accuracy

Precise and accurate

Precise but inaccurate

imprecise and inaccurate

imprecise but accurate

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mappingscale

Precision and accuracy are functions of how data are produced or gathered

The perfect map is not 100% accurate or precise. It is FIT FOR PURPOSE

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mappingprecision and accuracy

precise and accurate

precise but inaccurate

imprecise but accurate

imprecise and inaccurate

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mappingrepresenting the world

The map is only a representation of the world. There are 3 main transformations that are required to transform the world into a map:

•projection•generalization•referencing

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:projection

There are a variety of different projections used to turn the 3-dimensional world into 2 dimensions

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:projection

•Flat Earth models are appropriate for local survey mapping up to about 10km in range•Spheroid transformations are appropriate for navigation across longer distances•Ellipsoid transformations are necessary for large scale mapping (or mapping that uses GPS)

BUT

•The earth is irregular so there are different reference ellipsoids, spheroids and datums used for this transformation

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:projection

Azimuthal projectionCylindrical projection

Conic projectionDifferent projections have their own characteristics and uses. They all distort the properties of the Earth, but do so differently.

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:error

There are six main types of distortion in mapping:

•shape•distance•area•direction•scale•angle

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:error

Different types of mapping handle map distortion differently

•conformal maps can be used to describe shape well, but not area•equal distance maps compromise everything but distances•azimuthal maps protect angles

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:error

distortion is not constant

Azimuthal projections distort from the centre

Cylindrical projections distort from the equator

Scale is only exact on OS mapping at 2 central axes 180km either side of the central meridian

180 KM W 180 Km E

Error at central meridien = !:0.9996

Error = 1:1

Error = 1:1

Error increases as a function of distance from the equator

Error increases as a function of distance from the equator

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:error

The point is that the geo-spatial footprint necessarily contains error. It can’t be avoided, but can be managed.

This impacts on any questions we ask of our GIS and the nature of the answers that we get back.

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping:generalisation

For mapping to be useful certain known elements of the world are discarded. The greater the scale, the more is discarded. Shape, direction and connection may be lost.

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Error is inevitable!It can be managed but not eradicated:

it is easy to increase error, harder to reduce it

1:1250 “flat earth” survey

Casini Projection

1:50,000 OS

WGS datum

Ellipsoid transformation

Aerial photo

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Fundamentals of copyrightMapping is very expensive to produceCopyright is protected very aggressively for GI dataAspects of transformation may also be copyrightFailure to comply may end in severe sanctionLimited prescribed exemptions for educationRaster is less restricted than vectorCompiled images less restricted than uncompiledDesktop work is less restricted than networkedDon’t just digitise your favourite map!Protect your own copyright!

Principles of GIS:Intellectual Property Rights

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Freedom of InformationEnvironmentally sensitive placesEIRData Protection LawsLiabilityDatabase Right

Principles of GIS:Intellectual Property Rights

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Resource discovery metadata (DC / NGDF)Tracking changesTracking IPRDocumentationGuides to Good Practice

Principles of GIS:the importance of metadata

http://ads.ahds.ac.uk/project/goodguides/g2gp.html

This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Finding Data:see your handouts