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Spatial Reference Systems. UniPHORM - UNIGIS Josef STROBL Department of Geography - Salzburg University. Objectives. Appreciation of the importance of spatial referencing within OpenGIS context Orientation about mechanisms for unambiguous spatial referencing on the surface of the Earth

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Spatial reference systems l.jpg

Spatial Reference Systems

UniPHORM - UNIGISJosef STROBLDepartment of Geography - Salzburg University


Objectives l.jpg
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


Introduction l.jpg
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


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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!


  • Documentation of reference systems l.jpg
    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


    Coordinate systems overview l.jpg
    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)


    Coordinate system l.jpg
    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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    P(10,15)

    #17

    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)


    Cartesian coordinate systems l.jpg
    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


    Geographical coordinates l.jpg
    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


    Discrete georeferencing l.jpg
    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)


    Shape of the earth l.jpg
    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


    Utm universal transversal mercator system l.jpg
    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


    Metadata l.jpg
    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)


    Transformations l.jpg
    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“


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    Resources

    Additional information regarding spatial reference systems can be found in:

    • print publications

    • online references and tutorials

    • software

    • standards documents


    References l.jpg
    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.


    Online l.jpg
    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/


    Software l.jpg
    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!)


    Cen tc287 pr env 12762 l.jpg
    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


    Wrap up l.jpg
    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!


    Review questionnaire l.jpg
    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


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