map projections. what is a map projection? a mathematical expression used to represent the 3d...
TRANSCRIPT
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Map Projection
s
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What is a Map Projection?
• A mathematical expression used to represent the 3D surface of the earth on a 2D map
• Always results in distortions
Source: http://support.google.com/earth/bin/answer.py?hl=en&answer=148111
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Projection Surfaces• Cylindrical
– Meridians and parallels are equidistant, straight lines, crossing at right angles
• Conical– Parallel(s) that are touching the
cone are assigned unitary scale• Planar
– Point or circle that intersects tangent plane has no distortion
Source: http://en.wikipedia.org/wiki/Map_projection#Projections_by_surface
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Preservation of a Metric Property
• Conformal- Preserves angles locally (MapWindow uses this projection exclusively)
• Equal-area- Preserves area• Equidistant- Preserves distance from a standard point or
line• Gnomonic- Preserves shortest route between two points
(great circles projected as straight lines)• Azimuthal- Preserves cardinal directions• Compromise- Balancing/minimizing overall distortion
Source: http://en.wikipedia.org/wiki/Map_projection#Projections_by_surface
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Projection Comparison
Equal Area Projection
Equal Angle Projection
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Universal Transverse Mercator• Lat/Lon WGS84 (UTM) used in
Google Earth and MapWindow• Topographical maps, geological
maps • Cylindrical, conformal • Large distortions at the poles• Greenland (blue) vs. Africa
(red) example
Source: http://support.google.com/earth/bin/answer.py?hl=en&answer=148111http://www.pratham.name/mercator-projection-africa-vs-greenland.html
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MapWindow Project Projections• Download and extract Map Projections Sample Projects folder Open
project in MapWindow > Sample Projects > World > World.mwprj• Click “Ignore Mismatch” if prompted• Assigning a projection to a project: Project > Settings
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MapWindow Project Projections
Click on “Project Projection” and choose the projection you wish to apply to the entire project (may require restart of the project)
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MapWindow Shapefile Projections
Go to the Toolbox tab (or GIS plug-in) to reproject, assign projections, etc. to shapefiles, grids, and/or images
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Add the reprojected shapefile as a map layer to compare
projections
Select “Ignore Mismatch” if prompted
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PZ-90 Reprojected Shapefile
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Calculating Area to Compare Projections
Go to the Toolbox > Standard > Calculate Polygon Area
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Layer or shapefile
Name of field in table
Units
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Calculated area of each shape (country) in km^2
using our original projection (WGS84)
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Calculate the area of each country
using our reprojected shapefile
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Compare areas (in km^2) of Zaire from two different projections:
WGS84 (1)Africa Albers Equal Area Conic (2)
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Add a column to the table to display the area in 1000 km^2 (kkm^2)
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New column Equation
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Compare areas (in kkm^2) of Greenland from two different
projections:WGS84 (1)
Africa Albers Equal Area Conic (2)
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Different projections
calculate different areas for the same
country