.. currentmodule:: geopandas
.. ipython:: python
:suppress:
import geopandas
import matplotlib.pyplot as plt
plt.close('all')
Set operations with overlay
============================
When working with multiple spatial datasets -- especially multiple *polygon* or
*line* datasets -- users often wish to create new shapes based on places where
those datasets overlap (or don't overlap). These manipulations are often
referred using the language of sets -- intersections, unions, and differences.
These types of operations are made available in the GeoPandas library through
the :meth:`~geopandas.GeoDataFrame.overlay` method.
The basic idea is demonstrated by the graphic below but keep in mind that
overlays operate at the DataFrame level, not on individual geometries, and the
properties from both are retained. In effect, for every shape in the left
:class:`~geopandas.GeoDataFrame`, this operation is executed against every other shape in the right
:class:`~geopandas.GeoDataFrame`:
.. image:: ../../_static/overlay_operations.png
**Source: QGIS documentation**
.. note::
Note to users familiar with the *shapely* library: :meth:`~geopandas.GeoDataFrame.overlay` can be thought
of as offering versions of the standard *shapely* set operations that deal with
the complexities of applying set operations to two *GeoSeries*. The standard
*shapely* set operations are also available as :class:`~geopandas.GeoSeries` methods.
The different overlay operations
--------------------------------
First, create some example data:
.. ipython:: python
from shapely.geometry import Polygon
polys1 = geopandas.GeoSeries([Polygon([(0,0), (2,0), (2,2), (0,2)]),
Polygon([(2,2), (4,2), (4,4), (2,4)])])
polys2 = geopandas.GeoSeries([Polygon([(1,1), (3,1), (3,3), (1,3)]),
Polygon([(3,3), (5,3), (5,5), (3,5)])])
df1 = geopandas.GeoDataFrame({'geometry': polys1, 'df1':[1,2]})
df2 = geopandas.GeoDataFrame({'geometry': polys2, 'df2':[1,2]})
These two GeoDataFrames have some overlapping areas:
.. ipython:: python
ax = df1.plot(color='red');
@savefig overlay_example.png width=5in
df2.plot(ax=ax, color='green', alpha=0.5);
The above example illustrates the different overlay modes.
The :meth:`~geopandas.GeoDataFrame.overlay` method will determine the set of all individual geometries
from overlaying the two input GeoDataFrames. This result covers the area covered
by the two input GeoDataFrames, and also preserves all unique regions defined by
the combined boundaries of the two GeoDataFrames.
.. note::
For historical reasons, the overlay method is also available as a top-level function :func:`overlay`.
It is recommended to use the method as the function may be deprecated in the future.
When using ``how='union'``, all those possible geometries are returned:
.. ipython:: python
res_union = df1.overlay(df2, how='union')
res_union
ax = res_union.plot(alpha=0.5, cmap='tab10')
df1.plot(ax=ax, facecolor='none', edgecolor='k');
@savefig overlay_example_union.png width=5in
df2.plot(ax=ax, facecolor='none', edgecolor='k');
The other ``how`` operations will return different subsets of those geometries.
With ``how='intersection'``, it returns only those geometries that are contained
by both GeoDataFrames:
.. ipython:: python
res_intersection = df1.overlay(df2, how='intersection')
res_intersection
ax = res_intersection.plot(cmap='tab10')
df1.plot(ax=ax, facecolor='none', edgecolor='k');
@savefig overlay_example_intersection.png width=5in
df2.plot(ax=ax, facecolor='none', edgecolor='k');
``how='symmetric_difference'`` is the opposite of ``'intersection'`` and returns
the geometries that are only part of one of the GeoDataFrames but not of both:
.. ipython:: python
res_symdiff = df1.overlay(df2, how='symmetric_difference')
res_symdiff
ax = res_symdiff.plot(cmap='tab10')
df1.plot(ax=ax, facecolor='none', edgecolor='k');
@savefig overlay_example_symdiff.png width=5in
df2.plot(ax=ax, facecolor='none', edgecolor='k');
To obtain the geometries that are part of ``df1`` but are not contained in
``df2``, you can use ``how='difference'``:
.. ipython:: python
res_difference = df1.overlay(df2, how='difference')
res_difference
ax = res_difference.plot(cmap='tab10')
df1.plot(ax=ax, facecolor='none', edgecolor='k');
@savefig overlay_example_difference.png width=5in
df2.plot(ax=ax, facecolor='none', edgecolor='k');
Finally, with ``how='identity'``, the result consists of the surface of ``df1``,
but with the geometries obtained from overlaying ``df1`` with ``df2``:
.. ipython:: python
res_identity = df1.overlay(df2, how='identity')
res_identity
ax = res_identity.plot(cmap='tab10')
df1.plot(ax=ax, facecolor='none', edgecolor='k');
@savefig overlay_example_identity.png width=5in
df2.plot(ax=ax, facecolor='none', edgecolor='k');
Overlay groceries example
-------------------------
First, load the Chicago community areas and groceries example datasets and select :
.. ipython:: python
import geodatasets
chicago = geopandas.read_file(geodatasets.get_path("geoda.chicago_commpop"))
groceries = geopandas.read_file(geodatasets.get_path("geoda.groceries"))
# Project to crs that uses meters as distance measure
chicago = chicago.to_crs("ESRI:102003")
groceries = groceries.to_crs("ESRI:102003")
To illustrate the :meth:`~geopandas.GeoDataFrame.overlay` method, consider the following case in which one
wishes to identify the "served" portion of each area -- defined as areas within
1km of a grocery store -- using a ``GeoDataFrame`` of community areas and a
``GeoDataFrame`` of groceries.
.. ipython:: python
# Look at Chicago:
@savefig chicago_basic.png width=5in
chicago.plot();
# Now buffer groceries to find area within 1km.
# Check CRS -- USA Contiguous Albers Equal Area, units of meters.
groceries.crs
# make 1km buffer
groceries['geometry']= groceries.buffer(1000)
@savefig groceries_buffers.png width=5in
groceries.plot();
To select only the portion of community areas within 1km of a grocery, specify the ``how`` option to be "intersect", which creates a new set of polygons where these two layers overlap:
.. ipython:: python
chicago_cores = chicago.overlay(groceries, how='intersection')
@savefig chicago_cores.png width=5in
chicago_cores.plot(alpha=0.5, edgecolor='k', cmap='tab10');
Changing the ``how`` option allows for different types of overlay operations. For example, if you were interested in the portions of Chicago *far* from groceries (the peripheries), you would compute the difference of the two.
.. ipython:: python
chicago_peripheries = chicago.overlay(groceries, how='difference')
@savefig chicago_peripheries.png width=5in
chicago_peripheries.plot(alpha=0.5, edgecolor='k', cmap='tab10');
.. ipython:: python
:suppress:
import matplotlib.pyplot as plt
plt.close('all')
keep_geom_type keyword
----------------------
In default settings, :meth:`~geopandas.GeoDataFrame.overlay` returns only geometries of the same geometry type as GeoDataFrame
(left one) has, where Polygon and MultiPolygon is considered as a same type (other types likewise).
You can control this behavior using ``keep_geom_type`` option, which is set to
True by default. Once set to False, ``overlay`` will return all geometry types resulting from
selected set-operation. Different types can result for example from intersection of touching geometries,
where two polygons intersects in a line or a point.
More examples
-------------
A larger set of examples of the use of :meth:`~geopandas.GeoDataFrame.overlay` can be found `here `_
.. toctree::
:maxdepth: 2