Transcript Spatial Reference Systems

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)
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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)
P(10,15)
#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 ndimensional 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.
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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
© 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
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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
© 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 500000m
© 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)
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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/
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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!)
© 1998 UNIGIS
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
© 1998 UNIGIS