Isochrones and Transport Accessibility

Isochrones represent areas reachable from a starting point within a given time or distance limit along an iduedu.UrbanGraph transport network. This functionality enables analysis of transport accessibility using pedestrian, automobile, public transport, or intermodal graphs prepared by IduEdu.

The library provides several methods for generating isochrones depending on the required level of detail and visualization.

Baseline Isochrones

Show a single area reachable within a specified time.

objectnat.get_graph_isochrones(urban_graph, *, weight_value_cutoff, gdf_origins=None, origin_nodes=None, graph_node_column='graph_node_id', weight_type='time_min', geometry_type='radius', zone=None, buffer_factor=0.7, road_buffer_size=5.0, max_workers=None)[source]

Calculate accessibility isochrones from origin objects over an urban graph.

An isochrone is the area reachable from an origin within weight_value_cutoff. Unlike coverage, each origin gets its own independent isochrone, but all shortest-path searches run in a single parallel Numba call. The function:

  1. Snaps each origin to its nearest graph node (or uses the provided origin_nodes).

  2. Runs one Dijkstra search per origin in parallel.

  3. Builds a reachability polygon for each origin with the selected geometry_type.

  4. Optionally clips each isochrone to zone.

Parameters:

  • urban_graph (UrbanGraph) – City graph with node and edge tables.

  • weight_value_cutoff (float) – Maximum travel time (minutes) or distance (meters) defining the isochrone extent.

  • gdf_origins (geopandas.GeoDataFrame, optional) – Origin objects. If the table contains graph_node_column those ids are used; otherwise geometries are snapped to nearest nodes. Pass either this or origin_nodes.

  • origin_nodes (Iterable, optional) – Ready-made origin node ids, used instead of gdf_origins.

  • graph_node_column (str) – Name of the graph node id column in gdf_origins.

  • weight_type (Literal['time_min', 'length_meter']) –

    Type of edge weight used for path calculation:

    • "time_min": time-based accessibility in minutes

    • "length_meter": distance-based accessibility in meters

  • geometry_type (Literal['radius', 'ways', 'separate']) –

    Method used to build each isochrone:

    • "radius": union of residual-radius buffers around reachable nodes

    • "ways": buffered reachable road geometry (walk edges only on intermodal/walk graphs)

    • "separate": a single merged buffer-based isochrone

  • zone (gpd.GeoDataFrame | gpd.GeoSeries, optional) – Boundary polygon to clip the resulting isochrones.

  • buffer_factor (float) – Buffer multiplier used by geometry_type="separate" (default 0.7).

  • road_buffer_size (float) – Edge buffer for geometry_type="ways", in meters (default 5.0).

  • max_workers (int, optional) – Number of parallel Numba threads. Defaults to the Numba runtime.

Returns:

One isochrone polygon per origin with a source_node column and geometry, returned in the CRS of gdf_origins (or the graph CRS). The index matches gdf_origins / origin_nodes.

Return type:

gpd.GeoDataFrame

Notes

  • For geometry_type="ways" the search budget is slightly extended so road edges are not clipped short of the boundary.

  • Origins with no reachable nodes are dropped from the result.

isochrone_ways_15_min

Isochrone for road network within 15 minutes.

isochrone_radius_15_min

Isochrone using radius-based method (15 min).

isochrone_3points_radius_8_min

Isochrones for three start points (8 min).

Stepped Isochrones

Show accessibility ranges divided into time intervals (e.g., 5, 10, 15 minutes).

objectnat.get_stepped_graph_isochrones(urban_graph, *, gdf_origins=None, origin_nodes=None, graph_node_column='graph_node_id', weight_type='time_min', geometry_type='radius', weight_value_cutoff=None, zone=None, step=None, buffer_factor=0.7, road_buffer_size=5.0)[source]

Calculate stepped accessibility isochrones from origin objects over an urban graph, splitting each reachable area into concentric bands of width step.

Unlike coverage, the search runs outward from the origins (along the original edge direction on a directed graph). The function:

  1. Snaps each origin to its nearest graph node (or uses the provided origin_nodes).

  2. Runs a multi-source Dijkstra search outward from the origins.

  3. Buckets reachable nodes into steps and builds banded geometry with the selected geometry_type.

  4. Optionally clips the bands to zone.

Parameters:

  • urban_graph (UrbanGraph) – City graph with node and edge tables.

  • gdf_origins (geopandas.GeoDataFrame, optional) – Origin objects. If the table contains graph_node_column those ids are used; otherwise geometries are snapped to nearest nodes. Pass either this or origin_nodes.

  • origin_nodes (Iterable, optional) – Ready-made origin node ids, used instead of gdf_origins.

  • graph_node_column (str) – Name of the graph node id column in gdf_origins.

  • weight_type (Literal['time_min', 'length_meter']) –

    Type of edge weight used for path calculation:

    • "time_min": time-based accessibility in minutes

    • "length_meter": distance-based accessibility in meters

  • geometry_type (Optional[Literal['radius', 'ways', 'separate']]) –

    Method used to build each step’s geometry:

    • None: Voronoi cells around graph nodes

    • "radius": Voronoi cells clipped to residual-radius buffers

    • "ways": Voronoi cells clipped to buffered road geometry (walk edges only on intermodal/walk graphs)

    • "separate": independent circular buffers per step

  • weight_value_cutoff (float, optional) – Maximum travel time or distance. If None, the isochrone is built out to the farthest reachable node.

  • zone (gpd.GeoDataFrame | gpd.GeoSeries, optional) – Boundary polygon to clip the resulting stepped isochrones.

  • step (float, optional) – Width of each step, in units of weight_type. Defaults to 100 meters for length_meter and 1 minute for time_min.

  • buffer_factor (float) – Residual-radius multiplier for geometry construction (default 0.7).

  • road_buffer_size (float) – Edge buffer for geometry_type="ways", in meters (default 5.0).

Returns:

Stepped isochrone polygons with a dist column (the upper bound of each step, in units of weight_type) and geometry, returned in the CRS of gdf_origins (or the graph CRS).

Return type:

gpd.GeoDataFrame

Notes

  • Multiple origins are processed together: the bands describe the combined reachability of all origins (each node keeps the distance to its nearest origin).

  • An empty GeoDataFrame is returned when nothing is reachable.

stepped_isochrone_ways_15_min

Stepped isochrones for road network (5–15 min).

stepped_isochrone_radius_15_min

Stepped radius-based isochrones (5–15 min).

stepped_isochrone_separate_15_min

Separate stepped zones visualized per time interval.

Example notebook

Isochrone Analysis from Points of Interest