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Concatenating drifter datasets#

import numpy as np
import lzma
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import xarray as xr
import pandas as pd
import trajan as ta

Importing a dataset with two drifters in the Barents Sea

with lzma.open('barents.nc.xz') as barents:
    ds = xr.open_dataset(barents)
    ds.load()

assert 'obs' in ds.dims

2025-06-24 07:11:27 fv-az1279-468 h5py._conv[2392] DEBUG Creating converter from 7 to 5
2025-06-24 07:11:27 fv-az1279-468 h5py._conv[2392] DEBUG Creating converter from 5 to 7
2025-06-24 07:11:27 fv-az1279-468 h5py._conv[2392] DEBUG Creating converter from 7 to 5
2025-06-24 07:11:27 fv-az1279-468 h5py._conv[2392] DEBUG Creating converter from 5 to 7
2025-06-24 07:11:27 fv-az1279-468 h5py._conv[2392] DEBUG Creating converter from 3 to 5

Split into two observational datasets for this example

ds = ds.rename(drifter_names='trajectory').traj.condense_obs()
print(ds)

d1 = ds.isel(trajectory=0).traj.to_1d().traj.to_2d()
d2 = ds.isel(trajectory=1).traj.to_1d().traj.to_2d()
print("d1=", d1)
print("d2=", d2)

2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.traj2d[2392] DEBUG Condensing 2287 observations.
2025-06-24 07:11:27 fv-az1279-468 trajan.traj2d[2392] DEBUG Condensed observations from: 2287 to 2287
<xarray.Dataset> Size: 128kB
Dimensions:     (trajectory: 2, obs: 2287)
Coordinates:
    trajectory  (trajectory) <U16 128B 'UIB-2022-TILL-01' 'UIB-2022-TILL-02'
  * obs         (obs) int64 18kB 0 1 2 3 4 5 6 ... 2281 2282 2283 2284 2285 2286
Data variables:
    lon         (trajectory, obs) float64 37kB 29.85 29.83 29.82 ... 21.14 21.15
    lat         (trajectory, obs) float64 37kB 77.3 77.31 77.31 ... 74.58 74.58
    time        (trajectory, obs) datetime64[ns] 37kB 2022-10-07T00:00:38 ......
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Single trajectory, a trajectory dimension will be added
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Using trajectory_id variable name (trajectory) as trajectory dimension name
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: time, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected structured (1D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Single trajectory, a trajectory dimension will be added
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Using trajectory_id variable name (trajectory) as trajectory dimension name
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: time, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected structured (1D) trajectory dataset
d1= <xarray.Dataset> Size: 33kB
Dimensions:     (trajectory: 1, obs: 1027)
Coordinates:
  * trajectory  (trajectory) <U16 64B 'UIB-2022-TILL-01'
  * obs         (obs) int64 8kB 0 1 2 3 4 5 6 ... 1021 1022 1023 1024 1025 1026
Data variables:
    lon         (trajectory, obs) float64 8kB 29.85 29.83 29.82 ... 25.11 25.11
    lat         (trajectory, obs) float64 8kB 77.3 77.31 77.31 ... 76.57 76.57
    time        (trajectory, obs) datetime64[ns] 8kB 2022-10-07T00:00:38 ... ...
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters
d2= <xarray.Dataset> Size: 73kB
Dimensions:     (trajectory: 1, obs: 2287)
Coordinates:
  * trajectory  (trajectory) <U16 64B 'UIB-2022-TILL-02'
  * obs         (obs) int64 18kB 0 1 2 3 4 5 6 ... 2281 2282 2283 2284 2285 2286
Data variables:
    lon         (trajectory, obs) float64 18kB 27.82 27.8 27.78 ... 21.14 21.15
    lat         (trajectory, obs) float64 18kB 77.11 77.11 77.11 ... 74.58 74.58
    time        (trajectory, obs) datetime64[ns] 18kB 2022-10-07T00:00:40 ......
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters

Concatenate two 2D datasets (with observation dimension).

dc = xr.concat((d1, d2), dim='trajectory')
dc = dc.traj.condense_obs()
print(dc)

assert np.all(ds.lat.values[~np.isnan(ds.lat.values)] ==
              dc.lat.values[~np.isnan(dc.lat.values)])

2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.traj2d[2392] DEBUG Condensing 2287 observations.
2025-06-24 07:11:27 fv-az1279-468 trajan.traj2d[2392] DEBUG Condensed observations from: 2287 to 2287
<xarray.Dataset> Size: 128kB
Dimensions:     (trajectory: 2, obs: 2287)
Coordinates:
  * trajectory  (trajectory) <U16 128B 'UIB-2022-TILL-01' 'UIB-2022-TILL-02'
  * obs         (obs) int64 18kB 0 1 2 3 4 5 6 ... 2281 2282 2283 2284 2285 2286
Data variables:
    lon         (trajectory, obs) float64 37kB 29.85 29.83 29.82 ... 21.14 21.15
    lat         (trajectory, obs) float64 37kB 77.3 77.31 77.31 ... 74.58 74.58
    time        (trajectory, obs) datetime64[ns] 37kB 2022-10-07T00:00:38 ......
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters

Concatenating two 1D datasets, with observations at different times.

d1 = d1.traj.to_1d(
)  # trivial conversion since `d1` only contains a single trajectory. No need for gridtime.
d2 = d2.traj.to_1d()  # Also trivial for d2.
print(d1)

assert 'obs' not in d1.dims

2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
<xarray.Dataset> Size: 25kB
Dimensions:     (trajectory: 1, time: 1027)
Coordinates:
  * time        (time) datetime64[ns] 8kB 2022-10-07T00:00:38 ... 2022-11-17T...
  * trajectory  (trajectory) <U16 64B 'UIB-2022-TILL-01'
Data variables:
    lon         (trajectory, time) float64 8kB 29.85 29.83 29.82 ... 25.11 25.11
    lat         (trajectory, time) float64 8kB 77.3 77.31 77.31 ... 76.57 76.57
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters

Concatenating two 1D datasets will cause a lot of NaNs to be inserted.

dc = xr.concat((d1, d2), dim='trajectory')
print(dc)

assert np.all(ds.lat.values[~np.isnan(ds.lat.values)] ==
              dc.lat.values[~np.isnan(dc.lat.values)])

<xarray.Dataset> Size: 127kB
Dimensions:     (trajectory: 2, time: 3163)
Coordinates:
  * time        (time) datetime64[ns] 25kB 2022-10-07T00:00:38 ... 2022-11-23...
  * trajectory  (trajectory) <U16 128B 'UIB-2022-TILL-01' 'UIB-2022-TILL-02'
Data variables:
    lon         (trajectory, time) float64 51kB 29.85 nan 29.83 ... 21.14 21.15
    lat         (trajectory, time) float64 51kB 77.3 nan 77.31 ... 74.58 74.58
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters

Converting to 2D and condensing the dataset will give a cleaner result.

dc = xr.concat((d1.traj.to_2d(), d2.traj.to_2d()),
               dim='trajectory').traj.condense_obs()
print(dc)
assert dc.sizes['obs'] == ds.sizes['obs']

2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: time, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected structured (1D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: time, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected structured (1D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting trajectory dimension
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detecting time-variable for "obs"..
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected obs-dim: obs, detected time-variable: time.
2025-06-24 07:11:27 fv-az1279-468 trajan.accessor[2392] DEBUG Detected un-structured (2D) trajectory dataset
2025-06-24 07:11:27 fv-az1279-468 trajan.traj2d[2392] DEBUG Condensing 2287 observations.
2025-06-24 07:11:27 fv-az1279-468 trajan.traj2d[2392] DEBUG Condensed observations from: 2287 to 2287
<xarray.Dataset> Size: 128kB
Dimensions:     (trajectory: 2, obs: 2287)
Coordinates:
  * trajectory  (trajectory) <U16 128B 'UIB-2022-TILL-01' 'UIB-2022-TILL-02'
  * obs         (obs) int64 18kB 0 1 2 3 4 5 6 ... 2281 2282 2283 2284 2285 2286
Data variables:
    lon         (trajectory, obs) float64 37kB 29.85 29.83 29.82 ... 21.14 21.15
    lat         (trajectory, obs) float64 37kB 77.3 77.31 77.31 ... 74.58 74.58
    time        (trajectory, obs) datetime64[ns] 37kB 2022-10-07T00:00:38 ......
Attributes: (12/13)
    Conventions:          CF-1.10
    featureType:          trajectory
    geospatial_lat_min:   74.5454462
    geospatial_lat_max:   77.4774768
    geospatial_lon_min:   17.2058074
    geospatial_lon_max:   29.8523485
    ...                   ...
    time_coverage_end:    2022-11-23T13:30:28
    creator_email:        gauteh@met.no, knutfd@met.no
    creator_name:         Gaute Hope and Knut Frode Dagestad
    creator_url:          https://github.com/OpenDrift/opendrift
    summary:              Two drifters in the Barents Sea. One stranded at Ho...
    title:                Barents Sea drifters

Total running time of the script: (0 minutes 0.154 seconds)

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