Random x, y offset for drifter deployments (#253)

* add small random noise to drifter release locations

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

* add limit to offset which can be added

* update test to account for x,y offsets

* [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: j-atkins

src/virtualship/instruments/drifter.py

@@ -8,7 +8,7 @@
 from virtualship.instruments.base import Instrument
 from virtualship.instruments.types import InstrumentType
 from virtualship.models.spacetime import Spacetime
-from virtualship.utils import register_instrument
+from virtualship.utils import _random_noise, register_instrument
 
 # =====================================================
 # SECTION: Dataclass
@@ -102,11 +102,18 @@ def simulate(self, measurements, out_path) -> None:
         fieldset = self.load_input_data()
 
         # define parcel particles
+        lat_release = [
+            drifter.spacetime.location.lat + _random_noise() for drifter in measurements
+        ]  # with small random noise to get different trajectories for multiple drifters released at same waypoint
+        lon_release = [
+            drifter.spacetime.location.lon + _random_noise() for drifter in measurements
+        ]
+
         drifter_particleset = ParticleSet(
             fieldset=fieldset,
             pclass=_DrifterParticle,
-            lat=[drifter.spacetime.location.lat for drifter in measurements],
-            lon=[drifter.spacetime.location.lon for drifter in measurements],
+            lat=lat_release,
+            lon=lon_release,
             depth=[drifter.depth for drifter in measurements],
             time=[drifter.spacetime.time for drifter in measurements],
             has_lifetime=[

src/virtualship/utils.py

@@ -560,3 +560,9 @@ def _find_files_in_timerange(
         )
 
     return [fname for _, fname in files_with_dates]
+
+
+def _random_noise(scale: float = 0.01, limit: float = 0.03) -> float:
+    """Generate a small random noise value for drifter seeding locations."""
+    value = np.random.normal(loc=0.0, scale=scale)
+    return np.clip(value, -limit, limit)  # ensure noise is within limits

tests/instruments/test_drifter.py

@@ -36,7 +36,7 @@ def test_simulate_drifters(tmpdir) -> None:
     drifters = [
         Drifter(
             spacetime=Spacetime(
-                location=Location(latitude=0, longitude=0),
+                location=Location(latitude=0.05, longitude=0.05),
                 time=base_time + datetime.timedelta(days=0),
             ),
             depth=0.0,