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geo_northarrow

Simple North arrow geometry for GIS plotting in Python. Compatible with GeoPlot, Geopandas, and libraries built on Matplotlib.

image

Requirements:

  • shapely (via pip) <=2.0.1 or >2.0.1. (only requires Polygon class)
  • geopandas (often requires install via conda)
  • numpy
  • matplotlib

Installation:

(Use !pip for notebook installations, Jupyter, Colab, etc.)

pip install git+https://github.com/pmdscully/geo_northarrow.git

(See example install & use Google Colab notebook here)

Example 1 Geopandas Plotting code:

See original example: https://geopandas.org/en/stable/docs/user_guide/mapping.html

import geopandas
import geodatasets
from geo_northarrow import add_north_arrow

chicago = geopandas.read_file(geodatasets.get_path("geoda.chicago_commpop"))
groceries = geopandas.read_file(geodatasets.get_path("geoda.groceries"))
display(chicago.head())
ax = chicago.plot(column="POP2010");

add_north_arrow(ax, scale=.75, xlim_pos=.9025, ylim_pos=.965, color='#000', text_scaler=4, text_yT=-1.25)
Output:

image

Example 2 code:

import matplotlib.pyplot as plt
import geopandas

from geo_northarrow import add_north_arrow

fig, ax = plt.subplots(figsize=(2,3))
_ = ax.set_xlim(0,100) #Define axis limits.   Either manually or by plotting data - e.g. ax = gdf.plot(ax=ax)
_ = ax.set_ylim(0,100) # NB: add_north_arrow() will use the axis limits to define its default relative position and scale.

#...
#... your mapping code...     
#...
add_north_arrow(ax, scale=1.75, xlim_pos=.9025, ylim_pos=.165, color='#000', text_scaler=4, text_yT=-1.25)

# 
# -- Just for the sample plots.
ax.set_facecolor((1.0, 0.47, 0.42))
Output:

image

add_north_arrow Function Arguments:

- ax,           - your Matplotlib/ Geopandas axes object.
- scale=1,      - Scale of the Arrow and 'N' character.
- xlim_pos=.9,  - Relative position on the x-axis of the plot. (left:0 to right:1.0)
- ylim_pos=.8,  - Relative position on the y-axis of the plot. (bottom:0 to top:1.0)
- xT=None,      - Absolute transform position on x-axis (Alternative to xlim_pos). Probably will not require.
- yT=None,      - Absolute transform position on y-axis (Alternative to ylim_pos). Probably will not require.
- color='k',    - Color of the polygon (arrow and 'N' character), any matplotlib color scheme (i.e. #000 to #fff and matplotlib color naming).
- text_scaler=1.0,   - Secondary scale multiplier for the 'N' character, for customized sizing.
- text_yT=1.0        - Fine tuning of the y-axis position for the 'N' character, for customized positioning.

How it works:

The function just plots a polygon into the matplotlib axes. The north arrow is composed of 3 sub-polygons (left triangle, right triangle and a serif 'N' character as a polygon) and default layer-ordering (z-order) is used.

  • To make things simpler, the function attempts to scale and transform the north arrow (with N character) according to a benchmarked-relative size, relative to the axis lengths of your GIS data plot. Therefore, call add_north_arrow() after plotting your main map data content (or after defining the axis limits - ax.set_xlim(0,1)/ ax.set_ylim(0,1)).
  • However, the north_arrow is only suitable for maps with north-south axis that is identical to the matplotlib yaxis, as the orientation for north is not extracted from map data.
  • It is isolated from other plot artefacts, so easy to add alongside scalebar (e.g. see https://geopandas.org/en/stable/gallery/matplotlib_scalebar.html)

LIMITATIONS / FUTURE WORK:

  1. No attempt is made to determine the north orientation from your map or data. Therefore this north_arrow is only suitable for maps with north-south axis that is identical to the matplotlib yaxis. In future, add an angle input to rotate the arrow in terms of north orientation.
  2. Replace shapely and geopandas depedencies with pure matplotlib Polygon plotting.