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Platte Carre, Platte Carree, Plate Carrée, Equirectangular, Geographic, Cartesian, Equidistant Cylindrical, Carte Parallelogrammatique…

Those names all refer to the same thing, and it looks like this…


source: NASA composite satellite imagery, Wikipedia.

What Is It?
So many of my page hits come from people googling "Platte Carre" or some spelling variant.  What is it?  It is a geometric method of peeling off the surface of our round earth and flattening it out into a rectangle so that all of the lines of latitude (the horizontals; Y) and longitude (the verticals; X) are straightened and arranged to form an evenly spaced grid (plate carrée is French for "flat square" -a description of the equally spaced lat longs).  One visually obvious byproduct of this projection is increasingly extreme horizontal stretching in the higher latitudes.  Satellite imagery of cities like Helsinki or Anchorage or Sydney end up looking really distorted.

Why Is It?
This is a popular projection in the GIS world because it is able to show the entire world at once.  Also, since the lat long grid is square, it lends itself nicely to the pixel-y row-and-column world of computerized mapping.  As a result, almost all public Web Map Services (WMS) use it.  The reason you are reading this in the first place is probably because you are trying to drape a Platte Carre WMS over a Google Maps or Virtual Earth basemap and things are not lining up (because they use the Mercator projection, not Platte Carre).  Fortunately, Platte Carre maps will line up nicely in the 3D environment (Google Earth, Virtual Earth 3d) because it only requires a linear transformation to be draped over the 3D sphere.

Where Did It Come From?
Marinus of Tyre invented it around 100 ad.  He was a geographer, map maker, and mathematician; pretty much the father of the Latitude Longitude Coordinate System.

More Info…

 

Wikipedia has a short but good entry on this projection:
http://en.wikipedia.org/wiki/Equirectangular_projection

 

Comparing and contrasting Platte Carre and Mercator:
http://idvux.spaces.live.com/blog/cns!2EB6AAF6C3AC1EBE!259.entry

 

What is Mercator?
http://idvux.spaces.live.com/blog/cns!2EB6AAF6C3AC1EBE!244.entry

 

Overcoming the projection mismatch:
http://idvux.spaces.live.com/blog/cns!2EB6AAF6C3AC1EBE!231.entry

 

Map Projections in the 3D environment:
http://idvux.spaces.live.com/blog/cns!2EB6AAF6C3AC1EBE!177.entry

  ******Update******

J Archie sent along this note to me correcting my assertion that the Equidistant Cylindrical projection is synonomous with those listed above.  Here is what Archie says…

Equidistant Cylindrical would only be the same as the other projections listed: Platte Carre, Equirectangular etc, if the Earth were a perfect sphere. Where a Platte Carree wall map has the dimensions 180 cm x 360 cm , the Equidistant Cylindrical would have the dimensions 180 cm x 361 cm, taking into account the relatively larger equatorial circumference.

******Update******

Thanks, Archie!

John Nelson / IDV Solutions / john.nelson@idvsolutions.com

 

32 responses

  1. mboszko

    Thanks for the excellent description of this projection. I\’m working with building 3D animated globe maps for broadcast, and was just trying to clear up this very question.

    January 1, 2008 at 6:13 pm

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