Projections
Dr. Hans van der Kwast
OpenCourseWare
ocw.unesco-ihe.org
Learning objectives
• After this course you will be able to
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Understand why we use projections
Know the advantages and disadvantages of projections
Choose the right projection for your purpose
Understand the difference between on-the-fly projection and the
projection of layers
Why projections?
• Map projections portray the surface of the earth or portion of the
earth (3D) on a flat peace of paper or on a screen (2D)
• A Coordinate Reference System (CRS) defines, with the help of
coordinates, how the 2D projected map in a GIS is related to real
places on the earth
By Globcal International (http://globcal.net/globcal.png)
By Crates (Own work)
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Latitude and longitude
• Latitude and longitude in
degrees
• Geographic Coordinate
Reference Systems
• WGS-84
• Location of UNESCO-IHE:
52° 0' 32” North
4° 21' 24” East
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Converting Lat/Lon to decimal degrees
• 52°0’32” (52 degrees, 0 minutes, 32 seconds)
• 52 + 0’/60 + 32”/3600 = 52.00889 degrees North
• How much is 4°21’24” East in decimal degrees?
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Map projections
• Problem: from a 3D world to a 2D
map
• Have you ever peeled an orange?
• Properties of geographic objects
that are distorted:
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Area
Scale
Shape
Direction
Source: Carol
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Three families of map projections
a) Cylindrical projections
b) Conical projections
c) Planar projections
• All projections have
advantages and
disadvantages
• Distortions of angular
conformity, distance and area
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Projections that compromise distortions
• Robinson projection:
• Compromises
distortions of area,
angular conformity
and distance
• Winkel Tripel
projection
Robinson projection
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Projections with angular conformity
• Conformal or orthomorphic
projections:
• Mercator projection
• Lamber Conformal Conic
• Results in distortion of
areas
• Larger the area the larger
the distortion
• Used by USGS
topographical maps
• Used for: navigation,
meteorology
Mercator projection
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Projections with equal distance
• Equidistant projection
• Constant scale
• Maintains accurate
distances from the centre
of the projection or along
given lines
• Examples: Plate Carree
Equidistant Cylindrical,
Equirectangular,
Azimuthal Equidistant
projection
• Use: radio and seismic
mapping, navigation
Plate Carree Equidistant Cylindrical projection
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Projections with equal distance
United Nations logo uses the Azimuthal Equidistant projection
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Projections with equal areas
• Equal area projection
• Preserves proportions
of areas
• Results in distortions in
angular conformity
• Examples: Alber’s
equal area, Lambert’s
equal area, Mollweide
Equal Area Cylindrical
projection
• Use: general
reference, education
Mollweide Equal Area Cylindrical projection
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Universal Transverse Mercator (UTM)
• UTM is a global map projection
• Divided in 60 equal zones, 6 degrees wide in longitude from
East to West
• UTM zones numbered 1-60 starting at the international date line
• Origin on the equator at a specific longitude
• N or S are used to distinguish between Northern and Southern
hemisphere
• E.g. Uganda:
• UTM Zone 36N
• Kampala: 452611 Easting, 36127 Northing
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Universal Transverse Mercator (UTM)
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Some terminology
• Datum
• Spheroid
• Geoid
• False Northing, False Easting
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Datum, spheroid, geoid
1. Ocean
2. Reference ellipsoid
3. Local plumb line
4. Continent
5. Geoid
By MesserWoland (Own work)
• Datum: localised
approximation of earth’s
ellipsoid. Global: e.g. WGS-84
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Example False Northing, False Easting
Dutch projection:
Rijksdriehoekstelsel
• Origin originally in
Amersfoort (O.L.Vrouwetoren)
• Since 1970 moved to:
False Northing 155000 m,
False Easting 463000 m
"RDbounds" by Hans Erren
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Which projection to use?
• Depends on:
• Regional extent
• Type of analysis
• Availability of data (national data, global data)
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GIS and Projections
• Decide on the projection of your model data before you
start preprocessing!
• You need a common reference system (per project or for
your organisation):
• Local coordinate system (e.g. Amersfoort/RD new)
• Global coordinate system (e.g. UTM Zone 31N/WGS-84)
• Geographic Coordinate Reference System (Lat/Lon, WGS-84)
Coordinates, more practical
• Use EPSG codes to standardise projections within a project!
• Supported by most open source GIS desktop and server applications, incl. QGIS,
GDAL
• EPSG codes (European Petroleum Survey Group), examples:
• Amersfoort RD/New: 28992
• UTM Zone 31 North, datum WGS-84: 32631
• Google Earth (Lat/Lon WGS-84): 4326
• Online reference:
• http://spatialreference.org
• http://epsg.io
• http://prj2epsg.org
On-the-fly reprojection (OTF)
• All layers visualised in a
GIS application need to be
in the same projection
• Instead of reprojecting all
layers to the same
projection, GIS applications
use On-the-Fly
reprojection.
Beware! OTF reprojection does
not change the projection of layers!
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3 Cases with projections
1. Projection is known AND projection is assigned  No action
needed
2. Projection is known BUT NOT assigned  Assign projection to
layer
3. Projection is unknown  Georeference layer (register/rectify)
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