Is it correct to call it degrees square?

Will it be the same if calculate the area using Plate Carrée projection?

  • 1
    In Plate Carrée, the areas will be in squared linear units (such as square meters). There's no conceptual or computational problem with this; the main difficulty is that most such areas will be grievously wrong due to distortions away from the Equator. Such distortions of area are, however, also the case with popular projections such as Mercator and Stereographic. The areas in "geographic" coordinates can be converted to square meters (at least to a good approximation) by multiplying them by the square of the length of a degree (approximately 111,300 meters). – whuber Jan 18 '17 at 21:30
  • @whuber How do QGIS or any other desktop GIS implement area calculation for an unprojected polygon data? (How do they get the Value? So I can know the measurements unit myself). Do that ~111,300 meters automatically applied to all latitude? Thanks in advance – Keenan Gebze Jan 21 '17 at 5:12
  • I have rolled back this question because you changed it sufficiently to strand the existing answer. If you have another question to ask then please ask a new question. – PolyGeo Jan 21 '17 at 5:15
  • The area is computed from the coordinates. The calculations are the same, regardless of the units of the coordinates or the projection: to the area algorithm, everything is just numbers. It's up to you to select a coordinate system in which those numbers have meaning and the resulting area is a reasonable approximation to the true area. – whuber Jan 21 '17 at 17:20

Degrees Squared or Square Degrees is a non-sensical term. Consider that lines of longitude converge at the poles. One square degree will be much smaller at the pole than at the equator.

Therefore, when calculating area, you should use a projected coordinate system. Preferably an equal area projection which preserves area. A common example of such a projection is the Albers projection.

Converting to a projected coordinate system where the units are in feet or metres will provide you with correct area units (square feet or square metres).

You will most certainly get a different area calculation than plate carrée which is not equal area.

  • Geodetic area functions are also available with most GIS software. – Vince Jan 18 '17 at 4:18
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    I would qualify "non-sensical", around "oddness of units" here. If it's nonsensical to calculate it should be nonsensical to plot it. Many meaningful sensible things can be done in pure "graphics space" and a lot of data gets delivered in "planar degrees". It won't necessarily be sensible in any particular projection, depending on the location, distribution, extent and topological vagaries of how the shapes are represented. It's just as nonsensical to calculate area in many regions of most projections as it is in longitude-latitude, but hardly any tools will warn or make caveats about that. – mdsumner Jan 18 '17 at 8:04
  • I'm aware that I should use projection. But is there a documentation about on how desktop GIS (eg. my QGIS), implements area calculation for unprojected data? @mdsumner – Keenan Gebze Jan 21 '17 at 5:05

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