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Calculating Polygon Areas in GRASS GIS

While playing around with the ever useful GRASS GIS, it occured to me that I often do all of my ‘simple’ tasks in QGIS and only break out GRASS for the heavy analysis. This means that I don’t always know how to do some basic tasks using GRASS directly. This became apparently while trying to calculate areas on a polygon layer using GRASS. It turns out that it’s a simple task, once you know how, but—as with most things GRASS—can be a bit tricky at first. So, how do we do it?

Basically, the idea is to create a new column where the area data will live:

v.db.addcol mapname col=”area_sqkm DOUBLE PRECISION”

Then we’ll fill the new column with the area using v.to.db. Note that v.to.db will also let you calculate a lot more than just the area; see the full list.

v.to.db map=mapname option=area col=area_sqkm unit=k

Other options for units are:

  • Mi: miles
  • F: feet
  • Me: meters
  • K: kilometers
  • A: acres
  • H: hectares
  • R: radians
  • D: degrees

Enjoy!

Comments

Comment from Wilber
Time July 13, 2015 at 5:35 pm

It is not needed to use a column, if you want to see on the fly, just can add -p option; it will only print in cmdline

Comment from Toto
Time September 6, 2016 at 7:48 am

> v.db.addcol mapname col=”area DOUBLE PRECISION”
v.db.addcol: command not found

Comment from darrencope
Time September 6, 2016 at 7:55 pm

Sounds like your install isn’t correct, or your path isn’t set up to include your GRASS install.

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