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Spatial Reference Systems

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### Spatial Reference Systems

UniPHORM - UNIGISJosef STROBLDepartment 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
- Overview of specific spatial reference systems employed in central Europe

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

- 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 Universal interoperability is only feasible within globally unequivocal reference systems DO NOT USE LOCAL SYSTEMS!

- 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

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

- 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 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

#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

- 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

- 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

- 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

- 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

- Sphere
- simple, for small scale work

- Ellipsoid
- improved adjustment to ‚real‘ shape

- Geoid
- not a geometrically, but physically (gravity) defined body.

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 ‚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

- 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

- 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

- 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

- 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

- 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

- 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

Additional information regarding spatial reference systems can be found in:

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

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

- 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

- 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

- International Standards Organisation
- ISO TC211

- European Standards Organisation
- CEN TC287

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

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

- 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

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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