Posts Tagged ‘crs’

Common Map Projection Definitions

Sunday, April 3rd, 2011

Just finished teaching my second intro to GIS workshop using open source software (QGIS). Coordinate systems and map projections are always one of the toughest hurdles for novices. It’s hard enough just teaching the basic concepts using the existing CRS libraries in QGIS; having to custom define a number of common projected coordinate systems makes the process more daunting. For example, when we’re producing a thematic map of the US I want to use Lambert Conformal Conic for North America, but I have to give the students a proj4 definition in a text file and explain how you have to comb through the Spatial Reference site to find it.

For reference purposes and to make things a bit simpler, I’m providing some codes and definitions for some common coordinate reference systems (common for the participants in class) in this post. You can look up projection definitions at Spatial Reference and use the map projection resources at Radical Cartography and the USGS to see depictions and explanations of different systems. I created the projection images in this post using NASA’s G.Projector tool; a lightweight cross-platform tool for experimenting with projections.

The following CRS are pretty common and are included in the EPSG library used by QGIS – no need to custom define them, just search by name and code (the EPSG codes are ID codes for each CRS):

Geographic Coordinate Systems:

WGS84 (EPSG 4326): World Geodetic System of 1984, commonly used by organizations that provide GIS data for the entire globe or many countries and used by most web-based mapping engines (Google Maps)

NAD83 (EPSG 4269): North American Datum of 1983, commonly used by most US and Canadian federal government agencies (the US Census Bureau in particular) that provide GIS data

Since WGS84, NAD83, and all geographic coordinate systems are unprojected they will all look like Equirectangular or “Plate Caree” projections, which preserve distances:

Local Projected Coordinate Systems:

NAD 83 / New York Long Island (ft US) (EPSG 2263): The State Plane zone that covers Long Island and New York City is used by all NYC agencies that produce GIS data. Many city and state agencies produce data in their specific state plane zone. An alternate projection, EPSG 32118, represents the same zone but uses meters instead of feet.

NAD 83 / UTM Zone 18N (EPSG 26918): An alternative to State Plane that is better for larger regions; satellite or ortho imagery is often provided based on the UTM zone where the tile is. UTM Zone 18N covers much of the east coast of the US. An alternate projection, EPSG 32618, uses WGS 84 as a datum instead of NAD 83.

The following CRS are common continental and global projected coordinate systems that are NOT included in the EPSG library that is part of QGIS; you have to custom define them using the proj4 definitions.

North America Lambert Conformal Conic: Perhaps the most common map projection for North America, a conformal map preserves angles. LCC can be modified for optimally displaying specific countries (i.e. USA and Canada) or other continents (i.e. South America, Asia, etc.)

+proj=lcc +lat_1=20 +lat_2=60 +lat_0=40 +lon_0=-96 +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs

North America Albers Equal Area Conic: an alternative to LCC, all areas in an AEAC map are proportional to the same areas on the Earth. Can also be modified for specific countries or other continents. Visually it look more “compact” east to west versus LCC.

+proj=aea +lat_1=20 +lat_2=60 +lat_0=40 +lon_0=-96 +x_0=0 +y_0=0 +ellps=GRS80 +datum=NAD83 +units=m +no_defs

Robinson: a global map projection used by National Geographic for many decades. The Robinson map is a compromise projection; it doesn’t preserve any aspect of the earth precisely but makes the earth “look right” visually based on our common perceptions.

+proj=robin +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs

Mollweide: a global map projection that preserves areas, often used in the sciences for depicting global distributions on small maps.

+proj=moll +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs

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