Map Projections. 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.

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Presentation transcript:

Map Projections

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:

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:

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:

Projection Comparison Equal Area Projection Equal Angle Projection

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:

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

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)

MapWindow Shapefile Projections Go to the Toolbox tab (or GIS plug-in) to reproject, assign projections, etc. to shapefiles, grids, and/or images

Add the reprojected shapefile as a map layer to compare projections Select “Ignore Mismatch” if prompted Add the reprojected shapefile as a map layer to compare projections Select “Ignore Mismatch” if prompted

PZ-90 Reprojected Shapefile

Calculating Area to Compare Projections Go to the Toolbox > Standard > Calculate Polygon Area

Layer or shapefile Name of field in table Units

Calculated area of each shape (country) in km^2 using our original projection (WGS84)

Calculate the area of each country using our reprojected shapefile

Compare areas (in km^2) of Zaire from two different projections: WGS84 (1) Africa Albers Equal Area Conic (2)

Add a column to the table to display the area in 1000 km^2 (kkm^2)

New column Equation

Compare areas (in kkm^2) of Greenland from two different projections: WGS84 (1) Africa Albers Equal Area Conic (2)

Different projections calculate different areas for the same country