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WGS84 is a reference ellipsoid that determines coordinates of every point on Earth using latitude, longitude and height about the its surface. This ellipsoid is often used as a base to make maps. Though, to make a map on a flat paper or a flat digital screen you need a projection too. An example of a popular projection would be UTM.

Mainstream GIS software give plethora of projection alternatives to display your data layers. Among them, they also let you display your data using WGS84 only.

I think they are using a certain projection but what projection is that?

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    I'm not quite sure what you are asking. Do you want to understand the Plate Carree projection?
    – Devdatta Tengshe
    Commented Oct 7, 2013 at 9:09
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    @DevdattaTengshe No I just wanted to know what projection is used when displaying a layer using WGS84 (which normally stands for an ellipsoid) on a GIS software and from the answers it came out that when WGS84 is used, a Plate Caree projection takes place.
    – multigoodverse
    Commented Oct 7, 2013 at 9:18
  • That is not true for all GIS software. MapInfo uses a more advanced projection than just treating geographic coordinates as a simple rectangle. Very easy to see, if you compare screen dumps for the same area. Mercator based, if I am not mistaken.
    – Uffe Kousgaard
    Commented Oct 8, 2013 at 9:56
  • @UffeKousgaard: You're right, MapInfo doesn't simply use a plate carée: For geographic coordinates (like the standard "EPSG 4326: WGS84") MapInfo uses an equirectangular projection with the standard parallel at the center of the map. If the map is centered on the equator, that's equivalent to a plate carèe projection, but at other latitudes φ, the map will be stretched by a factor of 1/cos(φ).
    – Jake
    Commented Oct 8, 2013 at 15:21
  • @ArditSulce: Since you have unaccepted my answer, am I right to assume you feel your question hasn't been fully addressed yet? Could you clarify what other information you're looking for?
    – Jake
    Commented Oct 9, 2013 at 7:27

5 Answers 5

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WGS84 doesn't define a projection, so it's up to the GIS software to decide which projection to use for displaying the data on the screen (unless you manually pick a projection, of course).

In the simplest case, a plate carée projection (i.e. equidistant cylindrical with standard parallel 0°) is used, which in essence just interprets the angular units of the geographic coordinates as linear units of a cartesian coordinate system. This is the projection used by proj4 based systems, e.g. QGIS or GRASS, for all coordinate reference systems that use +proj=latlong or +proj=longlat.

MapInfo also uses an equidistant cylindrical projection for displaying geographic coordinates, with the twist that the parallel at the center of the map is used as the standard parallel. This means that if the map is centered on the equator, a plate carée projection results, but at other latitudes φ, the map will be stretched by a factor of 1/cos(φ), which helps reduce distortion.

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By "using WGS84 only" it sounds like you are referring to a Geographic Coordinate System based on the WGS 1984 datum.

A Geographic Coordinate System does NOT have a projection.

Only a Projected Coordinate System has a projection.

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    The geographic coordinates need to be displayed on the screen though, which requires a projection. I believe that's what the OP is asking about.
    – Jake
    Commented Oct 7, 2013 at 7:47
  • Jake is right. You always need a projection when making a map on a plane. In this case, the plane is the screen of your computer.
    – multigoodverse
    Commented Oct 7, 2013 at 7:49
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    @Jake Why would a Geographic Coordinate System which is effectively a rectangle going from -180,-90 to 180,90 need to be projected for display on the screen?
    – PolyGeo ♦
    Commented Oct 7, 2013 at 7:52
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    @Polygeo: Because the coordinates -180°,-90° in WGS84 are in angular units, and they need to be mapped to cartesian coordinates. In this case, that's done using the trivial equirectangular projection, which simply exchanges the angular units with linear units, but that process is still a projection.
    – Jake
    Commented Oct 7, 2013 at 7:57
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    @Gery Part of this discussion is carried out at gis.stackexchange.com/questions/664/….
    – whuber
    Commented Oct 7, 2013 at 15:12
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The term "WGS84" refers both to a reference ellipsoid and to a geographic coordinate system (aka EPSG:4326) - hence your confusion.

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  • Yes, but this "WGS84" is also behaving like a projection because it displays the data on a plane.
    – multigoodverse
    Commented Oct 7, 2013 at 7:51
  • Exactly! See EPSG:4326
    – MappaGnosis
    Commented Oct 7, 2013 at 7:53
  • I got your point. In the GIS software vocabulary, WGS84 is just a geographical coordinate system not an ellipsoid.
    – multigoodverse
    Commented Oct 7, 2013 at 8:00
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    No - In GIS vocabulary it is still BOTH. It is however most commonly used as a shorthand to mean EPSG:4326.
    – MappaGnosis
    Commented Oct 7, 2013 at 8:24
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    It would be "to a geographic coordinate system" instead of " to a projected coordinate system".
    – Kadir Şahbaz
    Commented Jun 4, 2020 at 11:26
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WGS84 is a "horizontal datum" - a mathematical representation of the Earth used to reference points.

Both geographic coordinate system's and projected coordinate systems can use WGS84 as the datum they reference. The difference is geographic coordinate systems are not projected, they are in latitude and longitude. In other words, not all measures of latitude and longitude are equivalent - they need to be referenced to a datum - most likely WGS84. Projected coordinate systems on the other hand are "flat" - but still need a point of reference (datum) to define locations in space.

In other words, the datum is used to determine the origin of points on Earth by referencing a central point inside the Earth model.

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  • This is a good description of a datum, but it is not what I was asking. By the way, why do you say "horizontal datum"? WGS84 is 3D and you can measure height from it.
    – multigoodverse
    Commented Oct 9, 2013 at 0:27
  • Ah touche.. I should have read more carefully. WGS84 is considered a horizontal datum. For more information refer to this link: tinyurl.com/2g7tnlq
    – derelict
    Commented Oct 9, 2013 at 1:20
  • In other words... horizontal datums are modeled after an ellipsoid which models the earths "surface". As opposed to a geoid - used in vertical datums which are based off mean sea levels. Since an ellipsoid is a simplified model of the earth it does not have a physical meaning and would therefore not be useful to be used for vertical measurements - although it is possible.
    – derelict
    Commented Oct 9, 2013 at 2:57
  • But you can still determine a full 3D position with a "horizontal" datum (i.e. lat,long,h or X,Y,Z). How can you call "horizontal" such a 3D tool? The fact that you have a pure vertical datum, does not mean you have to call remaining the other one. I just started a discussion here about this: gis.stackexchange.com/questions/73818/…
    – multigoodverse
    Commented Oct 9, 2013 at 6:54
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WGS is required to see/define where your map is on a globe/world, and UTM defines where your place/map is on a flat-land/screen

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