[GIS] Calculating percentage using QGIS Field Calculator

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I have a shapefile containing counties and their respective population for a given area. I woould like to use the Field Calculator to create a new field that contained the % of the total population in each county. I was looking for a way of doing it in one step inside the Field Calculator, i.e., how to build an expression that states something like "divide the population of each county by the sum of the population of all counties in this same column and multiply it by 100". I'm actually stuck in the "sum of all values" part…

Not sure if I'm demanding too much of a simple solution or if I'm just being blindsided and not seeing some obvious answer. Anyone?

Best Answer

If you dont have to iterate through to solve multiple fields then you are right on track, but just use statistics to get your total sum (in the attribute table right click on the pop field and choose statistics). Then set up your expression with the exact logic you stated above

[New_Field] =

([POP_COUNTY_FIELD] / Total Population) * 100

Total Population will be the value you found from statistics

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[GIS] Batch Update Fields in Field Calculator or Model

You should be able to write this in a python scripting using an update cursor fairly easily.

import arcpy
tablename = "c:\temp\*NAMEOFYOURTABLE*"
fieldvalues = ['fieldname1','fieldname2','fieldname3'......'fieldnameX']

cur = arcpy.UpdateCursor(tablename)
for row in cur:
    sourcevalue = row.getValue('*name of field where source value coming from*')
    for f in fieldvalues:
        row.setValue(f, sourcevalue)
    cur.updateRow(row)

[GIS] Summarizing column in QGIS field calculator

Update: QGIS 2.18 (07/07/2017):

In the Field Calculator, you can use the Aggregate function sum which allows you to sum the values of a column based on an expression. You can also, optionally, group and filter features for summation. So in your field calculator, you could simply use:

 (sum( "column_B" ) / "column_B") * 100

Original post: 23/04/2015

You can't sum columns using the Field Calculator (although it would be useful in certain situations). To be able to sum columns and obtain a value, you can use:

QGIS function - Basic Statistics (Vector > Analysis Tools > Basic Statistics)
QGIS Plugin - GroupStats (Plugins > Manage and Install Plugins)
QGIS Plugin - Statist (Plugins > Manage and Install Plugins)

So what I would do is create a new integer or real column (eg. "Sum") and add the value you obtain from any of the methods into the Expression window. Then create another column (eg. "Percentile") and edit the following expression to suit your column names:

("Sum" / "column_B") * 100

Best Answer

Related Solutions

[GIS] Batch Update Fields in Field Calculator or Model

[GIS] Summarizing column in QGIS field calculator

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