Data Principles of GIS Recap This power point

Data Principles of GIS Recap This power point presentation and its contents are Mapping and Error copyright of the Archaeology Data Service and Law IPR WGKand (2002) Metadata Finding Data

Recap Attributes, footprints and topologies This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)

Understanding mapping precision and accuracy This implies power point its contents Precision thepresentation degree toandwhich a are copyright of the Archaeology Data Service and measurement is refined WGK (2002) Accuracy that the measurement taken is correct within the precision indicated

Understanding mapping precision and accuracy imprecise and inaccurate This power point presentation and its contents are copyright of the Archaeology Data Service Preciseand accurate WGK (2002) imprecise but accurate Precise but inaccurate

Understanding mapping scale Precision and accuracy are functions of how data are produced or gathered This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002) The perfect map is not 100% accurate or precise. It is FIT FOR PURPOSE

Understanding mapping precision and accuracy imprecise but accurate precise are and accurate This power point presentation and its contents copyright of the Archaeology Data Service and WGK (2002) precise but inaccurate imprecise and inaccurate

Understanding mapping representing the world This is power presentation and its are The map onlypoint a representation ofcontents the world. copyright of the Archaeology Data Service and There. WGK are 3(2002) main transformations that are required to transform the world into a map: • projection • generalization • referencing

Understanding mapping: projection There 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 10 km in range • Spheroid transformations are appropriate for navigation This power point presentation and its contents are across longer distances copyright of the Archaeology Data Service and • Ellipsoid are necessary for large scale WGKtransformations (2002) mapping (or mapping that uses GPS) BUT • The earth is irregular so there are different reference ellipsoids, spheroids and datums used for this transformation

Understanding mapping: projection Cylindrical projection Azimuthal projection This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002) Different projections have their own characteristics and uses. They all distort the properties of the Earth, but do so differently. Conic projection

Understanding mapping: error There are six main types of distortion in mapping: This power point presentation and • its contents are shape copyright of the Archaeology Data Service and • distance WGK (2002) • area • direction • scale • angle

Understanding mapping: error Different types of mapping handle map distortion differently This power point presentation and its contents are • conformal maps can used to Data Service and copyright of thebe. Archaeology describe. WGK shape(2002) well, but not area • equal distance maps compromise everything but distances • azimuthal maps protect angles

Understanding mapping: Error = 1: 1 error distortion is not constant Azimuthal projections distort from the centre This power point presentation. Error and its contents increases as a function ofare distance from the equator copyright of the Archaeology Data Service and WGK (2002) distort from the equator Cylindrical projections Error = 1: 1 180 KM W 180 Km E Error increases as a function of distance from the equator Scale is only exact on OS mapping at 2 central axes 180 km either side of the central meridian Error at central meridien = !: 0. 9996

Understanding mapping: error This power point presentation and its contents are copyright of the Archaeology Data Service and WGK The point is (2002) 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.

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 This power point presentation and its contents 1: 50, 000 OS are copyright of the Archaeology Data Service and Ellipsoid transformation WGK (2002) Casini Projection Aerial photo WGS datum

Principles of GIS: Intellectual Property Rights Fundamentals of copyright Mapping is very expensive to produce This power point presentation and its contents Copyright is protected very aggressively for GIare data copyright of the Archaeology Service and Aspects of transformation may Data also be copyright WGK Failure to(2002) comply may end in severe sanction Limited prescribed exemptions for education Raster is less restricted than vector Compiled images less restricted than uncompiled Desktop work is less restricted than networked Don’t just digitise your favourite map! Protect your own copyright!

Principles of GIS: Intellectual Property Rights Freedom of Information Environmentally sensitive places EIRThis power point presentation and its contents are Datacopyright Protection Laws of the Archaeology Data Service and Liability WGK (2002) Database Right

Principles of GIS: the importance of metadata Resource discovery metadata (DC / NGDF) Tracking changes Tracking IPR point presentation and its contents are This power Documentation copyright of the Archaeology Data Service and Guides to(2002) Good Practice WGK http: //ads. ahds. ac. uk/project/goodguides/g 2 gp. html

Finding Data: see your handouts This power point presentation and its contents are copyright of the Archaeology Data Service and WGK (2002)