After completing this tutorial, you will be able to:
- Aggregate the geometry of spatial data using
dissolvebased on an attribute in
- Aggregate the quantitative values in your attribute table when you perform a dissolve in
What You Need
You will need a computer with internet access to complete this lesson and the spatial-vector-lidar data subset created for the course.
In this lesson, you will use
Python to aggregate (i.e. dissolve) the spatial boundaries of the United States state boundaries using a region name that is an attribute of the dataset. Then, you will aggregate the values in the attribute table, so that the quantitative values in the attribute table will reflect the new spatial boundaries for regions.
Import Packages and Data
To get started,
import the packages you will need for this lesson into
Python and set the current working directory.
# import necessary packages to work with spatial data in Python import os import numpy as np import geopandas as gpd import matplotlib.pyplot as plt import matplotlib.lines as mlines from matplotlib.colors import ListedColormap import earthpy as et # make figures plot inline plt.ion()
# import United States state boundaries state_boundary_us = gpd.read_file("data/spatial-vector-lidar/usa/usa-states-census-2014.shp")
Dissolve Polygons Based On an Attribute with Geopandas
Dissolving polygons entails combining polygons based upon a unique attribute value and removing the interior geometry.
Below you will dissolve the US states polygons by the region that each state is in. When you dissolve, you will create a new set polygons - one for each region in the United States.
To begin, explore your data. Using
.geom_type you can see that you have a mix of single and multi polygons in your data. Sometimes multi-polygons can cause problems when processing. It’s always good to check your geometry before you begin to better know what you are working with.
# query the first few records of the geom_type column state_boundary_us.geom_type.head()
0 MultiPolygon 1 Polygon 2 MultiPolygon 3 Polygon 4 Polygon dtype: object
Next, select the columns that you with to use for the dissolve and that will be retained.
In this case, we want to retain the columns:
# select the columns that you with to use for the dissolve and that will be retained state_boundary = state_boundary_us[['region', 'geometry']] # dissolve the state boundary by region us_regions = state_boundary.dissolve(by='region') # view the resulting geodataframe us_regions
|Midwest||(POLYGON Z ((-82.863342 41.693693 0, -82.82571...|
|Northeast||(POLYGON Z ((-76.04621299999999 38.025533 0, -...|
|Southeast||(POLYGON Z ((-81.81169299999999 24.568745 0, -...|
|Southwest||POLYGON Z ((-94.48587499999999 33.637867 0, -9...|
|West||(POLYGON Z ((-118.594033 33.035951 0, -118.540...|
And finally, plot the data. Note that when you dissolve, the column used to perform the dissolve becomes an index for the resultant geodataframe. Thus, you will have to use the reset_index() method when you plot, to access the
If you do not reset the index, the following will return and error, as region is no longer a column, it is an index!
us_regions.plot(column = 'region', ax=ax)
You can use
us_regions.reset_index().plot(column = 'region', ax=ax) to reset the index when you plot the data.
# create the plot fig, ax = plt.subplots(figsize = (10,6)) # plot the data us_regions.reset_index().plot(column = 'region', ax=ax) # Set plot axis to equal ratio ax.set_axis_off() plt.axis('equal');
Dissolve and Aggregate Data Attributes
In the example above, you dissolved the state level polygons to a region level. However, you did not aggregate or summarize the attributes associated with each polygon.
Next, you will learn how to aggregate quantitative values in your attribute table when you perform a dissolve. To do this, you will add
aggfunc = 'summaryfunction' to your dissolve call.
You can choice a suite of different summary functions including:
Aggregate the data using the ‘sum’ method on the ALAND and AWATER attributes (total land and water area).
The values for ALAND and AWATER will be added up for all of the states in a region. Those new summed values will be returned in the new dataframe.
# select the columns that you wish to retain in the data state_boundary = state_boundary_us[['region', 'geometry', 'ALAND', 'AWATER']] # then summarize the quantative columns by 'sum' regions_agg = state_boundary.dissolve(by='region', aggfunc = 'sum') # display the new dataframe regions_agg
|Midwest||(POLYGON Z ((-82.863342 41.693693 0, -82.82571...||1943869253244||184383393833|
|Northeast||(POLYGON Z ((-76.04621299999999 38.025533 0, -...||869066138232||108922434345|
|Southeast||(POLYGON Z ((-81.81169299999999 24.568745 0, -...||1364632039655||103876652998|
|Southwest||POLYGON Z ((-94.48587499999999 33.637867 0, -9...||1462631530997||24217682268|
|West||(POLYGON Z ((-118.594033 33.035951 0, -118.540...||2432336444730||57568049509|
# create the plot fig, ax = plt.subplots(figsize = (12,8)) # plot the data using a quantile map of the new ALAND values regions_agg.plot(column = 'ALAND', edgecolor = "black", scheme='quantiles', cmap='YlOrRd', ax=ax, legend = True) # Set plot axis to equal ratio ax.set_axis_off() plt.axis('equal');
Optional Challenge 1
Create a quantile map using the AWATER attribute column.
Optional Challenge 2
Aggregate the data using the ‘mean’ method on the ALAND and AWATER attributes (total land and water area). Then create two maps:
- a map of mean value for ALAND by region and
- a map of mean value for AWATER by region