Graduated classification

Graduated classification#

Graduated classification in GIS involves categorizing spatial data into classes or ranges based on a progression of values.
This method is particularly useful for visualizing quantitative data, allowing for the differentiation of intensity, density, or magnitude across a spectrum, facilitating a nuanced representation of geographic phenomena.
Graduated classification is used for quantitative data, usually interval or ratio scaled.

Data Scale	Definition	Example	Typical Data Format
Interval Scale	Equal intervals between values, no true zero point	Temperature (Celsius)	Float (44.5 Degree)
Ratio Scale	Equal intervals with a true zero point	Population, Length, Number of trees	Integer (5 Trees) or Float (12.5 km of Road)

To classify data in classes…

Right-click on your layer
Click on Symbology
Click on Graduated
In the Value dropdown menu select the column based on which you want to classify your data.
Downright select the number of classes you want to use.
Under Mode select the classification method you want to use e.g. Equal count (Quantile).
Click on Classify. Now you should see all classes and the distribution of values. To add or delete single classes use the - and + buttons.
Optional: Click on Histogram -> Load Values. Now you can see the exact distribution of values over the classes. This is very practical to decide on a classification method. You can also check the mean value and standard deviation.

Optional: In the Symbol dropdown menu you can select the colours and symbols you want to use.
Optional: In the Color ramp dropdown menu you can specify the range of colours you want to use. To see all color ramps click on the down arrow of the Color ramp -> All Color Ramps.
Optional: Under Legend Format you can adjust how precise the range of the classes will be displayed in the legend. Usually, it is practical to not use too complicated numbers in the legend.
Optional: You can open the panel Layer Rendering on the button of the window. Here you can adjust the opacity/ transparency of the layer.
Click Apply to put your adjustment into effect.
Click OK to close the window.

Deciding on the number of classes, and where the ranges for the different classes lie, has a profound impact on the resulting map.
There are seven ways in QGIS to split quantitative data into classes.
The four most important ones are: Equal intervals, Quantile, Natural breaks, Manual.
In general, you should limit the number of classes between 3 to 9.