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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

Spatial Reference Systems

UniPHORM - UNIGISJosef STROBLDepartment of Geography - Salzburg University

objectives
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
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
need for spatial reference systems
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
local vs global referencing
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
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
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
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
direct vs indirect positioning

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#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
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
2d vs 3d systems
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
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
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
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.
geodetic datum
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!
elevation measurements
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
specific earth ellipsoids
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
map projections
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
types of projections
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
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
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
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“
resources
Resources

Additional information regarding spatial reference systems can be found in:

  • print publications
  • online references and tutorials
  • software
  • standards documents
references
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
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
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
standards
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
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
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
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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