© 1998 UNIGIS

Spatial Reference Systems

UniPHORM - UNIGISJosef STROBL

Department of Geography - Salzburg University

© 1998 UNIGIS

Objectives

• Appreciation of the importance of spatial referencing within OpenGIS context

• Orientation about mechanisms for unambiguous spatial referencing on the surface of the Earth

• Overview of specific spatial reference systems employed in central Europe

© 1998 UNIGIS

INTRODUCTION

• Every spatial feature needs to be referenced to a location for GIS use

• Spatial reference systems provide a framework to define positions on the Earth‘s surface

• We are used to working with coordinate systems, but due to the Earth‘s irregular, spherical shape this can become intricate

© 1998 UNIGIS

Need for Spatial Reference Systems

• Clear definition scheme required for geodata exchange and interoperability

• This description needs to be coupled to geodata by sets of metadata– to permit flexible georeferenced visualization– to permit correct measurements– to permit operations between datasets

based on different reference systems

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Local vs global referencing

• Local coordinate systems used to be sufficient for some maps and plans:

• local origin with no given global reference• mostly cartesian systems, no projection info

• Universal interoperability is only feasible within globally unequivocal reference systems

• DO NOT USE LOCAL SYSTEMS!

© 1998 UNIGIS

Documentation of reference systems

• All paper maps are supposed to contain complete documentation (projection, location, scale, orientation etc.)

• This often gets lost in the digitizing process!• All geospatial data sets to be accompanied by full

documentation:– complete georeferencing information– source, temporal and scale information– validity and quality information

© 1998 UNIGIS

Coordinate systems overview

• Rules for identifying the position of each point in space by an ordered set of numbers:

• Systems:– Cartesian: coordinate values locate a point in

relation to mutually perpendicular axes– Polar: coordinates locate a point by angular

direction(s) and distance from center. – Spherical: point on surface located by angular

measurements from center (latitude, longitude)

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Coordinate system

• Coordinate systems are defined by– number of dimensions (1, 2 or 3)– sequence/name of coordinate values (x, y, z)– unit scaling factor and system (meters)– origin of axes– direction of axes

• Coordinate systems can be based on a geodetic reference (datum) and a map projection

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Direct vs. Indirect Positioning

• Two methods to position points relative to the surface of the Earth:

– direct position: position based on coordinates

– indirect position: position not using coordinates (e.g. street address)

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#17

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Cartesian coordinate systems

• Named after mathematician René Descartes• Mutually orthogonal system of straight axes as

a complete reference framework for n-dimensional spaces

• Axes intersect at system‘s origin• Metric, continuous measurement along axes• Projections of spherical surfaces result in

2-d cartesian systems

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2D vs. 3D systems

• Most GIS are 2D or 2.5D

• Many GIS operations are not defined in 3d space

• Increasingly, we need to handle 3D data, even if we don‘t fully use them

• Visualisation of 3D data sets is currently more important than analysis

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Geographical coordinates

• Specify position on a spherical surface relative to rotational (polar) axis and center

• Angular (polar) measurements– Latitude: angle from equatorial plane ±90°– Logitude: angle from Greenwich meridian ±180°

• For planar display on a map a „projection transformation“ is needed

© 1998 UNIGIS

Discrete georeferencing

• Coordinate systems represent spatial extent in a continuous measurement system.

• Most everyday spatial references use „names“ for places and locations, thus referring to „discrete entities“:– placenames, administrative units– natural features with determined, bounded extent– (actually, the location of a raster cell is based on a

discrete reference, too)

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Shape of the earth

• Sphere – simple, for small scale work

• Ellipsoid– improved adjustment to ‚real‘

shape

• Geoid– not a geometrically, but

physically (gravity) defined body.

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Geodetic Datum

• Origin relative to Earth mass centre• x-axis relative to Greenwich• z-axis relative to Earth rotation axis• y-axis (to complete right-handed system)• based on specific ellipsoid (e.g. Clarke),

this may be scaled• = 7 parameters!

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Elevation measurements

• Elevation ‚above sea level‘ is based on the physical (gravity) surface of the Earth

• Differences between this ‚normal‘ and the geometrically defined ellipsoid height based on a specific geodetic datum can reach 50-100m

• Thus the reference for elevation measures needs precise definition

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Specific earth ellipsoids

• Over time, dimensions of ellipsoids have been refined and adjusted for best fit in different regions on Earth

• Usually specific ellipsoids are given the name of the mathematician / surveyor in charge and are specified as– semi-major and semi-minor axes a,b – or a and 1/f, where f=a/b

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Map projections

• A map projection is defined by– name of projection– type of projection (e.g. cylindrical - using

different reference bodies)– description (applicable parameters

depend on type of projection)– ellipsoid / datum parameters

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Types of projections

• Important types of projections are:– planisphere: whole earth is „unwrapped“ onto

a plane one way or another– azimutal: part of earth‘s surface is projected

onto a plane– conical: part of earth‘s surface is projected

onto a conical shape and then flattened– cylindrical: same thing with a cylindrical

shape

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UTM: Universal Transversal Mercator System

• Worldwide the most important projection system for large scale mapping

• Transversal („horizontal“) cylindrical proj.• Cylinder is repositioned for better fit at every

6° longitude, starting from the international dateline going east:

• Zones 1-60, each 6° wide around central meridian• central meridian is scaled to <1 to disperse error• central meridian set to constant value of 500000m

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Metadata

• Describing all spatial reference details for a geospatial data set in a structured and standardized way.

• Indispensable for– all kinds of data transfers– interoperability

• Part of ISO / CEN / OGC work (see below)

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Transformations

• Changing towards a target projection is either done on-the-fly or by generating a new, projected geospatial dataset.

• Several different situations:– from geographical coordinates to projection– from a source projection, via geographical

coordinates, towards target projection– vector data projection: „forward“– raster data projection: „backward“

© 1998 UNIGIS

Resources

Additional information regarding spatial reference systems can be found in:

• print publications

• online references and tutorials

• software

• standards documents

© 1998 UNIGIS

References

• Maling, D.H. ... chapter in ‚Big Book‘• Maling, D.H. Coordinate Systems and Map Projections-2nd

edition. Oxford: Pergamon Press, 1992

• Bugayevskiy, Lev M. and John P. Snyder. Map Projections: A Reference Manual Taylor & Francis, 1995.

• Defense Mapping Agency. 1991. World Geodetic System 1984 (WGS 84) - Its Definition and Relationships with Local Geodetic Systems, 2nd Edition. Washington, DC: Defense Mapping Agency (DoD).

• Snyder, John P. Flattening the Earth-Two Thousand Years of Map Projections. Chicago: University of Chicago Press, 1993.

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Online

• Geographers‘s Craft (Peter Dana):http://www.utexas.edu/depts/grg/gcraft/notes/coordsys/coordsys.html

http://www.utexas.edu/depts/grg/gcraft/notes/mapproj/mapproj.html

http://www.utexas.edu/depts/grg/gcraft/notes/datum/datum.html

• The Map Projection Homepage: http://everest.hunter.cuny.edu/mp/

© 1998 UNIGIS

Software

• Blue Marble Geographics– Calculator, Transformer

• ArcView GIS– Use View/Properties for on-the-fly projection

from LatLong, or Projector! extension

• GeoMedia– Projections flexibly defined in MS Access

(.mdb) tables

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Standards

• International Standards Organisation– ISO TC211

• European Standards Organisation– CEN TC287

• The OpenGIS Consortium (OGC Inc.)– OpenGIS (see this chapter!)

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CEN TC287 pr ENV 12762

• „Geographic information - Referencing - Direct position“

• Document CEN/TC 287 N 585• Defines basic concepts related to

coordinate position information• Gives necessary guidance to use

reference systems for geographic information

© 1998 UNIGIS

Wrap-up

• With OpenGIS, spatial reference systems are a VERY important topic once again

• GIS specialists need detailed knowledge of projections and coordinate systems

• For larger scales and greater accuracy, we need more in-depth treatment of spatial reference systems!

© 1998 UNIGIS

Review questionnaire

To start the review questionnaire please click to the following address:

http://www.geo.sbg.ac.at/projects/UniPhorm/quiz/quiz_spatref.htm