static_maps.qmd

---
title: "Static Maps"
filters:
  - whitphx/stlite
---

## Static maps

In the HSMA course, we have generally used the geopandas plot method - which is using matplotlib behind the scenes - to produce static maps.

This means that maps are simple to display as we can just use the `st.pyplot()` function that we used for standard matplotlib charts.

```{python}
#| eval: false
import geopandas # <1>
import matplotlib.pyplot as plt # <2>
import pandas as pd
import streamlit as st

st.title("Crime in Cornwall by Type")

#########################################################
# Read the geopandas file and create a matplotlib figure
#########################################################

lsoa_2011_crime_figures_df = geopandas.read_file(
  "https://files.catbox.moe/o94mgi.gpkg" # <3>
  )

fig, ax = plt.subplots() # <4>

lsoa_2011_crime_figures_df.plot( # <5>
  column="sw_5forces_street_by_lsoa_Other crime", # <6>
  legend=True, # <7>
  ax=ax # <8>
  )

st.pyplot(fig) # <9>
```

1. We will need to import the geopandas library to load in and plot a geographic dataset.
2. We also will need to import the `matplotlib.pyplot` module, which we give the standard alias `plt`. When working with geopandas in a .py script or a jupyter notebook, this is only necessary when further modifying the plot; however, we need to structure our plot in a slightly different way to get it to display in a streamlit fileapp.
3. We then import a geopackage file from a URL and save it to a variable. As it's a geopackage rather than a csv or similar, we don't need to define the CRS: it's already specified as part of the geopackage file. The same would be true of a geojson.
4. We use the `plt.subplots()` function, unpacking it to the objects `fig` and `ax` by using the syntax `fig, ax` on the left hand side of our `=` sign. By leaving this blank, we create a single axis - but the value of this step is it gives us access to these objects so we can pass it to the streamlit function for displaying this sort of plot.
5. We then use the `plot` method on the dataframe. We don't need to save the output of this to a variable,
6. We pass in a column to colour the plot by, though this is optional; if we did not, it would just plot the points or polygons in the dataset all in the same colour.
7. We also specify that we want a legend in our plot to give some indication of the range and significance of the clours in the plot.
8. We tell geopandas/matplotlib to plot on the axis we just created using `plt.subplots`, which we do by passing in `ax=ax` to our `.plot` method.
9. The output of using the `.plot()` method on the geopandas dataframe is a matplotlib.pyplot object. Therefore, we need to use `st.pyplot()` to display it.


```{stlite-python}
import micropip
await micropip.install("geopandas")
await micropip.install("matplotlib")
await micropip.install("fiona")
import geopandas
from matplotlib import pyplot as plt
import pandas as pd
import streamlit as st

st.title("Crime in Cornwall by Type")

#########################################################
# Read the geopandas file and create a matplotlib figure
#########################################################

from pyodide.http import pyfetch
import geopandas as gpd

res = await pyfetch("https://files.catbox.moe/a0lcsn.geojson")
data = await res.json()
lsoa_2011_crime_figures_df = gpd.GeoDataFrame.from_features(data)

fig, ax = plt.subplots()

lsoa_2011_crime_figures_df.plot(
  column="sw_5forces_street_by_lsoa_Other crime",
  legend=True,
  ax=ax
  )

st.pyplot(fig)
```

:::{.callout-tip}
Take a look at the [HSMA geographic modelling and visualisation book](https://hsma-programme.github.io/hsma6_geographic_optimisation_and_visualisation_book/python_geopandas_matplotlib.html) to find out more about creating and modifying static maps in python with geopandas and matplotlib.

Note that you will need to adapt the code in those examples slightly to use the same layout as the code above: the main difference is using `fig, ax = plt.subplots()` and the argument `ax=ax` in our plot function, but most other aspects will be unchanged.
:::