# Spatial Processing
__🔙[Back to Homepage](../intro.md)__
## Buffer
- Calculate a  `buffer` with a defined distance.
- Dissolve: if 2 or more buffer areas overlap, they can be combined.
:::{Attention}
The unit of distance depends on the projection of the layer. For buffering you usually need a __metric coordinate system__ such as UTM (Universal Transverse Mercator).
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:::{dropdown} Example: Which regions fall within a 100-kilometer radius of major cities?
:open:
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:::{tip}
If you're dealing with really large "megabuffers" or if you can only choose buffer distances in degrees instead of meters, it might mean there's __no metric coordinate system__, or it was __incorrectly reprojected__. The unit of the buffer __depends__ on the __coordinate reference systems of the layer__.
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## Clip
- With the  `Clip` tool you can extract and retain the spatial extent of one vector layer based on the boundaries of another layer.
- `Input Layer`: refers to the specific __layer to be clipped__, for instance, a road network.
- `Overlay Layer`: e.g. a polygon layer of the region (e.g. borders of Heidelberg).
:::{dropdown} Example: Extract the rail network of Germany
:open:
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## Dissolve
- The  `dissolve` tool aggregates geometries with the same attribute values.
- When 2 or more buffer areas overlap, they can be combined using dissolve.
:::{dropdown} Example: Extract the rail network of Germany
:open:
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:::{Attention}
In QGIS, only the attributes selected for the dissolve operation will receive the accurate attribute, while the remaining attributes remain unaggregated; hence, in this example, the name is not accurately represented (e.g. the name for Western Europe might be mistakenly assigned as "Netherlands").
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## Intersection
The  `intersection` tool extracts the part of the layers which overlap.
1. In the top bar, navigate to `Vector` → `Geoprocessing Tool` → `Intersection` OR `Toolbox` → Search for `Intersection`.
2. `Input layer`: select layer one
3. `Overlay layer`: select layer two
4. `Intersection`: Specify where you want to save the results and give it a good name
:::{Note}
* Order of input layer and overlay layer does not matter here
* Possibility to keep only certain fields of the output layer
* ⚠️ Attention: Attribute values that refer to output areas (e.g. population) are no longer meaningful after the intersect
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:::{figure} ../../../fig/Intersect_concept_2.png
---
width: 500px
name: Intersect_concept_2
---
`Intersection` operation between a two-features input layer and a single feature overlay layer (left) - resulting features are moved for clarity (right). Source: GISGeography.com
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:::{dropdown} Example: Intersect countries with timezones
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## Centroids
With the  `Centroids`-tool, you can create a new layer with points at the centre of each polygon.
1. In the top bar, navigate to `Vector` → `Geometry Tools` → `Centroids`. Alternatively, search for `Centroids` in the [Processing Toolbox]. Open the tool by Double-clicking on it.
2. `Input layer`: select the polygon layer
3. Click `Run`.
4. The new layer will be added to your project.
:::{dropdown} Example: Create a centroid for each district in madagascar
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