# Graduated classification <a id="graduated-classification"></a>


__🔙[Back to Homepage](../intro.md)__

- 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) |



:::{dropdown} Video: Applying a graduated classification to a layer
<video width="100%" controls src="https://github.com/GIScience/gis-training-resource-center/raw/main/fig/graduated_classification.mp4"></video>
:::


__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.
:::{figure} ../../../fig/Graduated_histogram.png
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- *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.

## The number of classes <a id="the-number-of-classes"></a>

- 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.

:::{figure} ../../../fig/classification_method_map.drawio.svg
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name: classification_method_map_wiki
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The impact of different class breaks on maps (Source: HeiGIT, adapted from [Axis Maps](https://www.axismaps.com/guide/data-classification)).
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