pygrts is a library for geospatial sampling

Use pygrts to generate spatially balanced random samples using the Generalized Random Tessellation Stratified (GRTS) method.

What is GRTS?

A sampling approach that maps 2-dimensional samples onto a 1-dimensional plane, sorted by base 4 hierarchical grid ids. See Stevens and Olsen (2004) for details on the method. Slides outlining the method can be found here and here. The grts R library provides a more in-depth GRTS framework.

@article{stevens_olsen_2004,
  title={Spatially balanced sampling of natural resources},
  author={Stevens Jr, Don L and Olsen, Anthony R},
  journal={Journal of the American statistical Association},
  volume={99},
  number={465},
  pages={262--278},
  year={2004},
  publisher={Taylor \& Francis}
}

Basic example

>>> import pygrts
>>> import geopandas as gpd
>>>
>>> samples = gpd.read_file('samples.gpkg')
>>>
>>> qt = pygrts.QuadTree(samples)
>>>
>>> # Split until the quadrants are less than 5,000 meters
>>> qt.split_recursive(max_length=5000)
>>>
>>> # Get the actual quadrant length
>>> qt.qlen
>>>
>>> # Get the quadrants as a GeoDataFrame
>>> df = qt.to_frame()
>>>
>>> # Get 5 random points using the Generalized Random Tessellation Stratified (GRTS) method
>>> dfs = qt.sample(n=5)
>>>
>>> # Query the k-nearest points to other samples
>>> # lon, lat =
>>> other_samples = np.array([[lon, lat]])
>>> knearest_samples_df = dfs.grts.query_points(points=other_samples, k=1)
>>> assert len(knearest_samples_df.index) == other_samples.shape[0]

Examples

Start with random samples

Split the tree recursively

>>> qt = pygrts.QuadTree(df)
>>>
>>> for i in range(0, 4):
>>>     qt.split()

Split until maximum number of points in each quadrant

>>> qt = pygrts.QuadTree(df)
>>> qt.split_recursive(max_samples=100)

>>> qt = pygrts.QuadTree(df)
>>> qt.split_recursive(max_samples=50)

Split until maximum quadrant length

>>> qt = pygrts.QuadTree(df)
>>> qt.split_recursive(max_length=5000)

Spatially balanced sampling

Generalized Random Tessellation Stratified (GRTS)

>>> qt = pygrts.QuadTree(df)
>>> qt.split_recursive(max_length=10000)
>>> n_samples = 20
>>>
>>> df.sample(n=n_samples, replace=False).plot(
>>>   markersize=20,
>>>   color='orange',
>>>   edgecolor='k',
>>>   lw=0.5,
>>>   label='Random sample with no balancing'
>>> )
>>>
>>> qt.sample(n=n_samples).plot(
>>>   markersize=20, color='#34d800', edgecolor='k', lw=0.5, label='GRTS'
>>> )

Generalized Random Tessellation Stratified (GRTS) with cluster center weights

>>> qt = pygrts.QuadTree(df)
>>> qt.split_recursive(max_length=10000)
>>> n_samples = 20
>>>
>>> df.sample(n=n_samples, replace=False).plot(
>>>   markersize=20,
>>>   color='orange',
>>>   edgecolor='k',
>>>   lw=0.5,
>>>   label='Random sample with no balancing'
>>> )
>>>
>>> qt.sample(
>>>   n=n_samples,
>>>   weight_by_clusters=True
>>> ).plot(
>>>   markersize=20, color='#34d800', edgecolor='k', lw=0.5, label='GRTS'
>>> )