© 1998 UNIGIS Spatial Reference Systems UniPHORM - UNIGIS Josef 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

© 1998 UNIGIS 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)

© 1998 UNIGIS 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

© 1998 UNIGIS 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) P(10,15) #17

© 1998 UNIGIS 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

© 1998 UNIGIS 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

© 1998 UNIGIS 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)

© 1998 UNIGIS 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.

© 1998 UNIGIS 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!

© 1998 UNIGIS 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 m Thus the reference for elevation measures needs precise definition

© 1998 UNIGIS 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

© 1998 UNIGIS 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

© 1998 UNIGIS 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

© 1998 UNIGIS 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 m

© 1998 UNIGIS 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)

© 1998 UNIGIS 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, Defense Mapping Agency 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.

© 1998 UNIGIS Online Geographers‘s Craft (Peter Dana): /coordsys/coordsys.html /mapproj/mapproj.html /datum/datum.html The Map Projection Homepage:

© 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

© 1998 UNIGIS Standards International Standards Organisation –ISO TC211 European Standards Organisation –CEN TC287 The OpenGIS Consortium (OGC Inc.) –OpenGIS (see this chapter!)

© 1998 UNIGIS CEN TC287 pr ENV „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 :