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I have a polygon shapefile showing mining areas across all of Australia. I want to calculate the area for each of the polygons however they have been provided in a Geographic Coordinate System (GDA_94).

Can anyone tell me what projected coordinate system I could use that would enable me to calculate the areas of each polygon?

Will I need to transform the data and how might this affect the calculations given the size of Australia (i.e. it covers multiple GDA or UTM zones)?

marked as duplicate by PolyGeo Nov 13 '15 at 18:41

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There are several projected coordinate systems that are valid for the whole Australia. I suggest that you use Australia Albers equal area because global projection often induce more distortions locally. For a global projection suitable for Australia, Hobo-Dyer (a cylindrical equal area projection) could be good too. As you can see on wikipedia, the Tissot indicatrice is close to a circle in Australia, which means that there little shape distortion (in addition to being equal-area).

Remark (if you need accurate values): Before you project, make sure that your polygons are delineated with enough vertices (e.g. using some densify function). Indeed only the vertices are projected when you change the coordinate system. So if you have large polygon this could cause distortions (vertices are connected by straight lines before and after the projection).

  • Thanks very much. To confirm, I can do this in ArcGIS for GDA_94 and WGS_84 (another Aus dataset I have) using the 'Project' tool yes? Will I need to input a Geographic Transformation or is it fine to just chose output coordinate system as GDA 1994 Australia Albers projection? Thanks again – jess_n1 Nov 13 '15 at 14:33
  • according to this site (geoproject.com.au/gda.faq.html), GDA_94 and WGS_84 lie within 1 m. For small scale (in you case whole Australia), there is thus no need for geographic transformations: you can consider that WGS84 and GDA_94 are identical. So you just need to "project" (after "densify") – radouxju Nov 14 '15 at 19:14

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