# Distances in {Plate Carrée projection - EPSG:4326 - WGS84}

"4326 is just the EPSG identifier of WGS84" 

Indeed http://spatialreference.org/ref/epsg/4326/html/ says it is a GEOGraphic Coordinate System:

``````GEOGCS["WGS 84",
DATUM["WGS_1984",
SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]]
``````

Citing , [...]

one of the most common ways the round world is displayed on a map is using the simplest projection we have:

``````x = longitude
y = latitude
``````

The name of this projection is “Plate Carrée”, and is widely used because it is so simple. However we often seem to forget that we are talking about a projection. Therefore the spatial reference for this projection is very often (mis)referenced as a spherical coordinate system like for EPSG:4326.

In  then a correct WKT for the Plate Carée projection is reported (a projection is overlaid over WGS84/EPSG:4326):

``````PROJCS["World_Plate_Carree_Degrees",
GEOGCS["GCS_WGS_1984",
DATUM["D_WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],
PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],
PROJECTION["Plate_Carree"],PARAMETER["False_Easting",0],
PARAMETER["False_Northing",0],PARAMETER["Central_Meridian",0],
UNIT["Degree",111319.49079327357264771338267056]]
``````

which is ~EPSG:54001, except that units are not meters but degrees.

I am quite a bit confused for example if, at least in practice, EPSG:4326 is equivalent to Plate Carrée equirectangular projection.

E.g. in a computing environment, projected data is much more convenient since distances are Euclidean and not great-circles: would the distance between two points in EPSG:4326 be different then with respect to the same points in a Plate Carrée?

This is maybe specific to the single application, e.g.  Open Layers uses the term 'EPSG:4326' to mean the Plate Carrée projection. `R` seems to assume a spherical WGS84 instead (-> r-sig-geo).

References:

• To the best of my understanding, Plate Carree is not equivalent to EPSG:4326. The later is an unprojected (Geographic)CRS. whereas 54001 projects by assuming the Lon and Lat lines are equidistant, parallel lines. Plate carree causes severe distortion as you go north and south from the equator. Distances measured in 54001 will be different from distances in 4326. – Micha May 20 '12 at 10:35
• @Micha: projecting a grid with projection 4326 in qgis shows that the meshes are squares, which is a plate carree or cylindrical equal-spaced projection (Porter W. McDonnell: Introduction to Map projections, 2nd ed, pages 23-25) AM I misunderstanding something?? – Kurt May 20 '12 at 11:46
• @Micha: now also pseudo Plate Carée is stacked over the top of my confusion. I'm lost. Any practical clarifying toy example? – Campa May 20 '12 at 17:53