I have two similar geodatabase feature class (polygons) layers provided by two different sources in the WGS 1984 system and they have the same angular area (decimal degree). While projecting in World-cylindrical equal area (the projection system I need) they are different by more than 5 sq km.

What is the reason?

I need both of them to have the same area in sq km or ha or miles etc.

I am working in ArcGIS 10.3.1

  • Can you give us more info about the layers in question (source, what they are depicting, etc.)? If they were digitized by different organizations, that could easily explain the difference if the layers cover a large area. – lambertj Jul 20 '18 at 13:19
  • do you have shapefiles, or geodabase – nmtoken Jul 20 '18 at 13:23
  • I think the problem is related to EPSG, can you please check the Coordinate Reference Systems more carefully. I would suggest using this source. Otherwise, the reprojection is needed. – Taras Jul 20 '18 at 13:23
  • Yes, they were digitized by different organizations, but the angular area are exactly same(decimal degree). I have geodatabase – Arun Jul 20 '18 at 13:25
  • We (at Esri) had an intern look at various equal area projections one summer. Results were VERY dependent on the density of vertices in the data. – mkennedy Jul 20 '18 at 17:40

the different area come from the fact that angular area mean nothing in term of actual (real world) area; that's due to the fact that a longitude degree has a real world length of 0m at the pole and increase as you get closer to the equator.

In other word two polygon of 1 square degree (meaning a polygon defined by the intersection of two parallel and two meridian each separated by 1°) may have totally different area on the surface of the earth.

As the projection you choose is an equal area (meaning that projected polygon have an area proportional of their real world area) it is totally normal that you get two different projected area.

Hope this is clear enougth, if not look at a globe and notice how the regular grid define smaller area near the pole and bigger at the equator

  • 1
    Yes, I am totally aware of the differences between angular area and the 2D area. But now I think the problem is come from different digitization – Arun Jul 20 '18 at 13:32
  • What I mean is if your polygon are exactly identical (same coordinate) they should be of the same area, if they have different coordinate they will have different area – J.R Jul 20 '18 at 13:47

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