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I am trying to calculate area of polygons using a shapefile from the world bank where countries are defined. When however I am trying to calcuate the area, the numbers that emerge are different from the official statistics on area. Morever, whenever I use a different projection (e.g. equal area) and recalculate the area, I always get the same results.

Can someone tell me where the mistake could come from?

Thanks!

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When you say "use" a different projection, do you mean that you've reprojected the data using the "project" tool or that you've just defined the projection as something else? Common mistake that I see around the office and on here. How different are your numbers from the official statistics to which you are comparing them? 10%? 50%? – Jason Mar 20 at 20:16
I use ArcGIS 10.1. To change projection I go to Data Management tools->Projections and Transformations->Raster-Define Projection. Is this the right procedure? The differences lie in the range of 10-30%, thus not in a systematic way. – An_Li Mar 21 at 0:45
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The "Define Projection" tool does not reproject the data. Use the "Data Management tools->Projections and Transformations->Feature->Project" tool instead. – Luke Mar 21 at 0:59
Is your shapefile downloadable from somewhere (URL) for others to test? What is the source (URL) of your official statistics? – PolyGeo Mar 21 at 23:04
My source is thematicmapping.org/downloads/world_borders.php. I project it at an equal area projection. – An_Li Mar 29 at 12:15

1 Answer

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Firstly you are using the wrong tool to project the data.

Secondly, the area and lengths calculated from geometry usually don't match the official statistics. This is because of a variety of reasons including :

  • The actual area on the ground is measured taking the terrain onto account.

  • The way the official area is calculated, introduces many systematic error.

  • The area is usually calculated in a local projection.

  • The boundaries in your shapefile may not match the official boundary of the country.

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