The New Zealand “ghost” location, and how L2 QC now catches it
A tiger shark tagged in Shark Bay, WA appeared near New Zealand in cleaned L2 data. The point was an Argos error over open ocean. Land filters cannot see that. We added a distance-from-deployment check (and tightened speed filtering) so those ghosts drop out.
What went wrong
Zhenyu flagged an impossible location on PTT 226172 (tiger shark, deployed Shark Bay):
| Field | Value |
|---|---|
| Time | 2022-05-26T08:26:16Z |
| Ghost location | about −35.1°, 160.0° (Tasman Sea, near NZ) |
| Distance from deploy | ~4,500 km |
| Next kept L2 point | ~95 minutes later, back near Shark Bay |
| Implied speed | ~800 m/s (limit for sharks is 4.5 m/s) |
Real kept L2 points for this animal were typically within a few hundred kilometres of deployment. The Tasman point was not swimming. It was a bad Argos fix that still passed QC.
Sketch of the problem
Not to scale. Red = bad Argos location that was previously kept in L2. Green dashed ring = new distance limit.
Why the old filters missed it
Land filter
Removes locations that fall on land. The ghost was over the ocean, so it looked “wet” and stayed in.
trip::speedfilter
Checks consecutive points against a max speed. In this case it still kept the first far-away ghost (a known awkward case for first/last jumps and short segments), so the Tasman point survived into L2.
What we changed
In curate_l1_to_l2() (MegaMoveLevels):
- Distance-from-deployment filter — optional max_dist_km. For GAT sharks we set 1500 km. That is wide enough for real coastal/regional movement, but drops NZ-scale ghosts (~4500 km).
- Explicit consecutive-speed pass — after trip::speedfilter, re-check consecutive pairs; if a pair exceeds the speed limit, drop the point farther from the deployment site.
- Short tracks — skip trip::speedfilter when a deployment has fewer than 3 points (it was crashing on a 1-point track).
QC plots now show distance removals in purple. Shark workflow: max_dist_from_deploy_km <- 1500.
After the fix (July 2026 SPOT batch)
For PTT 226172: L2 now has 22 points, farthest longitude about 113.8° (Western Australia). The Tasman ghost is gone. Delivered to the RSB share under …/Dahlia Foo/GAT data cleaned for MegaMoveDB/gat_sharks_july2026/.
Takeaway
Argos can invent ocean locations that are not on land and not always caught by a consecutive-speed filter. For animals that stay regional, measuring distance from the known deployment site is a simple, effective safety net.