How to compute area in acres in QGIS?

Question

I am new to QGIS. I am creating a shapefile that will have several polygons. I need to have a column in the attribute table that will show the acres of each polygon. I have successfully done this but when I draw new polygons they don't get their acres calculated. What is the simplest way to overcome this? I am under the impression that this could perhaps be accomplished through GRASS or Python. I don't know anything about these programs though. Ultimately, I need to have a simple system for drawing new polygons labeling them with a field ID and having the ares calculated and labeled. This needs to be simple enough that interns who have never used gis can pick it up relatively quickly.

Answer 1

Activate edit mode, and open field calculator. There, use the Geometry -> $area function on existing field, or create a new field. Note that the units are the same as your CRS, so if it's in square meters, multiply by 0.00024711

Answer 2

You should use spatial database for this. Using database you will be able to create triggers that will automatically populate or refresh columns in your table. Here is an example of two triggers that I use for the goal that you described in one of my spatialite databases.

Trigger for creation a new polygon:

CREATE TRIGGER calculate_area  -- calculate_area is the trigger name
AFTER 
INSERT ON my_table             -- my_table is the table name
BEGIN
UPDATE my_table
SET AREA = Area(Geometry)      -- AREA is the name of column that stores area data and Area is the name of function to calculate it
WHERE Geometry = new.Geometry; -- Geometry is the name of column where geometry is stored
END

Trigger for modification of existing polygon:

CREATE TRIGGER calculate_area_on_update
AFTER 
UPDATE OF Geometry ON my_table
BEGIN
UPDATE my_table
SET AREA = Area(Geometry)
WHERE Geometry = new.Geometry;
END

You should adjust calculation of the area (Area(Geometry)) for your needs to get acres or whatever.