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I have a shapefile of counties in the US, and I would like to calculate the area of each polygon so that I can run a population density calculation. However, I am having trouble calculating the areas.

My steps were:

Vector -> Geometry Tools -> Export/Add Geometry Columns

I set the input layer to the layer I wanted to add the geometry columns to, then set Calculate using to "Layer CRS" (which is NAD83, EPSG:4269). My understanding was that it would then calculate the area in meters since NAD83 is in meters, but the values are too low to make sense (i.e. 0.2 area for a whole county).

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  • What version of QGIS are you using? Earlier versions of QGIS have had a variety of issues with area calculation. Many of these issues seem to be resolved in the current version (2.18). – csk Sep 8 '17 at 17:37
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    CRS NAD83 with EPSG:4269 is not in meters. It's in degrees. – xunilk Sep 8 '17 at 17:39
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you have to set your project>project_properties>CRS to utm, lambert or anything projected; otherwise it will give you a "square arcsec" or "square degree" value for area

Edit

function $area in field calculator gives you ellipsoidal area, which is in square meters. Also "identify results" with the option "display feature panel" gives you an ellipsoidal area (or distance) without having to create and populate a field

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    I think you mean a "degree" or "square degree" value rather than "arcsec". – mkennedy Sep 9 '17 at 0:48
  • @Elio Diaz So I see CRS's like NAD83(NSRS2007) / Maine East (ftUS) EPSG:26862, is this what you mean? And these are specific states, is there a projection for the whole country? And can I really just change it this way, I don't have to reproject using GDAL or something? – NaN Sep 11 '17 at 12:17
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    No, that's the wrong way. You have to set it back, or remove and re-add the layer to the project before reprojecting with Save As .... – AndreJ Sep 11 '17 at 13:45
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    Yes, I'm sorry I overlooked that, AndreJ is right, you have a correct CRS NAD83 with EPSG:4269 (lat long) layer, you right-click the layer and "save as" choose the projection. Then it will be a correct, projected layer which you can use to meaasure areas \n For a whole country you may use Lambert Conical Conformal projection – Elio Diaz Sep 11 '17 at 14:32
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    @ElioDiaz These bottom steps worked for me – NaN Sep 11 '17 at 18:23
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You shouldn't need to do your own area calculations. That information is included in shapefiles provided by the US Census Bureau.

As you can see in this excerpt from the technical documentation for the 2016 TIGER shapefiles, the current land area of each county is provided in the attribute table field "ALAND." enter image description here

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  • Thanks for this documentation, the reason that I wanted to do this calculation was because I couldn't find the units for ALAND, and I'm still not sure what they are. – NaN Sep 8 '17 at 19:32
  • I vote for square meters. But these are form Census data, independent from the accuracy of your shapefile. – AndreJ Sep 11 '17 at 14:58
  • All you need to do to figure out the unit is to find out the area of any county, and compare it to the ALAND value. For example, according to Wikipedia, Cuyahoga County, Ohio has a total area of 1,246 square miles (3,230 km2), of which 457 square miles (1,180 km2) is land and 788 square miles (2,040 km2) (63%) is water. – csk Sep 11 '17 at 21:13
  • It is in square meters – zelusp Mar 25 '19 at 17:53

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