From Ocean Teacher Library
Map Projections for CartographyContents |
Background
Because the earth is roughly spherical, it is necessary to use mathematical methods to portray all of it or portions of it on flat media, i.e. charts. These mathematical methods are call "projections," and there is a large technical literature surrounding their construction, their advantages and disadvantages, and their physical appearance. The excellent Wikipedia reference below provides a good basic overview of the technical issues involved in creating projections. Note also the textbook references.
Quick Overview of Projections
- US Geological Survey Map Projections Poster - "A map projection is used to portray all or part of the round Earth on a flat surface. This cannot be done without some distortion. Every projection has its own set of advantages and disadvantages. There is no "best" projection. The mapmaker must select the one best suited to the needs, reducing distortion of the most important features. Mapmakers and mathematicians have devised almost limitless ways to project the image of the globe onto paper. Scientists at the U. S. Geological Survey have designed projections for their specific needs—such as the Space Oblique Mercator, which allows mapping from satellites with little or no distortion. This document gives the key properties, characteristics, and preferred uses of many historically important projections and of those frequently used by mapmakers today." [From USGS website]
- Projections describe at the USGS website:
- The Globe, Mercator, Transverse Mercator, Oblique Mercator, Space Oblique Mercator, Miller Cylindrical, Robinson, Sinusoidal Equal Area, Orthographic, Stereographic, Gnomonic, Azimuthal Equidistant, Lambert Azimuthal Equal Area, Albers Equal Area Conic, Lambert Conformal Conic, Equidistant Conic (Simple Conic), Polyconic, Bipolar Oblique Conic Conformal
- The Globe, Mercator, Transverse Mercator, Oblique Mercator, Space Oblique Mercator, Miller Cylindrical, Robinson, Sinusoidal Equal Area, Orthographic, Stereographic, Gnomonic, Azimuthal Equidistant, Lambert Azimuthal Equal Area, Albers Equal Area Conic, Lambert Conformal Conic, Equidistant Conic (Simple Conic), Polyconic, Bipolar Oblique Conic Conformal
Equirectangular, the Non-Projection
Earth scientists frequently use raw latitudes and (slightly linearly adjusted) longitudes as the unprojected coordinates for charts of ocean data. This "projection" (really not a projection at all) is called by many names:
- Equirectangular projection
- Unprojected
- Geographic projection
- Lat/lon
- Simple cylindrical
- Equidistant cylindrical projection
- Plate carree
- The case when the longitudes are not linearly adjusted, and the grid cells of the graticule are exact equilateral squares; this is the specification for the ESRI ASCII grid format
- Equatorial cylindrical equidistant (ECE) projection - A more formal name for Plate caree
This "projection" has no use in navigation or practical geodesy, but it is in nearly universal use for Geographic Information Systems and other mapping of thematic resources. Higher-level satellite data products are almost always ECE.
Additional Resources
- Wikipedia: Map projection
- USGS Map Projections: A Working Manual - A book-length monograph from the USGS
- Understanding Map Projections - Well-illustrated ESRI textbook
Subsections of this Article
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Information about this article
Short title: Map Projections
Description: none
Expertise level: beginner
Author: Murray.Brown
Approval status: approved
Approved by: Murray.Brown
Last change: 2010-5-20
Subsection of: Geospatial Data Concepts
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