cmglaciology

Colormaps for glaciology, packaged for Matplotlib.

Name	Type	Intended use
speed	Sequential	Ice surface speed
akbathtopo	Topographic	Bathymetry and surface elevation, Alaska
aktopo	Topographic	Surface elevation (land only), Alaska
dem_gris	Topographic	Surface/bed elevation, Greenland

Install

python -m pip install git+https://github.com/pism/cmglaciology.git

or, from a clone:

python -m pip install -e .

Usage example

import cmglaciology.cm as cmg
import matplotlib.pyplot as plt
import numpy as np

x = np.linspace(0, 1, 100)[np.newaxis, :]

plt.imshow(x, aspect='auto', cmap=cmg.speed)
plt.axis('off')
plt.show()

For a reversed colormap, append _r to the colormap name:

plt.imshow(x, aspect='auto', cmap=cmg.speed_r)

For a discretized colormap, use the .resampled method on any of the colormaps:

plt.imshow(x, aspect='auto', cmap=cmg.speed.resampled(25))

Alternatively, the registered name string can be used. Names are prefixed with cmg.:

import cmglaciology  # required in order to register the colormaps with Matplotlib
...
plt.imshow(x, aspect='auto', cmap='cmg.speed')
plt.imshow(x, aspect='auto', cmap='cmg.speed_r')

For an image of all the available colormaps:

from cmglaciology import show_cmaps

show_cmaps()

Notes on the colormaps

The colormaps were designed in QGIS against data values, so their color stops are not evenly spaced. The stops are (data value → position in the colormap is linear between the first and last value):

• speed: 10, 30, 100, 250, 750 (m/yr)
• akbathtopo: -2000, 0, 1, 1250, 2000, 2500, 3000, 3500 (m)
• aktopo: 0, 1, 1250, 2000, 2500, 3000, 3500 (m)
• dem_gris: -500, 0, 1, 1500, 2000 (m)

To reproduce the QGIS rendering, use vmin/vmax equal to the first and last stop. The topographic maps have a sharp break at sea level (0–1 m).

Adding a colormap

1. Export the color ramp from QGIS (Layer Properties → Symbology → Save color map to file) and put the file in qgis/.
2. Run python scripts/qgis2txt.py to regenerate the 256-level RGB tables in cmglaciology/cmaps/.
3. Add the name to one of the groups at the top of cmglaciology/cm.py.
4. Run python cm.py from the cmglaciology/ directory to refresh colormaps.png, and pytest to check everything.

Acknowledgements

The structure of this package — one text file of RGB values per colormap, loaded and registered with Matplotlib on import, with reversed _r versions and a show_cmaps() overview — is taken from cmcrameri by Callum Rollo and contributors, which packages Fabio Crameri's Scientific colour maps. cmglaciology/cm.py and the tests are adapted from cmcrameri's code, which is distributed under the MIT License, Copyright (c) 2020 Callum Rollo. Many thanks to Callum and the cmcrameri contributors for making this so easy to reuse. Any errors in this adaptation are our own.

No colormap data from cmcrameri or the Scientific colour maps is included here. If you need perceptually uniform colormaps, use cmcrameri directly.

License

MIT, see LICENSE.txt.