Satellite imagery of the Hunga Tonga-Hunga Ha'apai volcano on December 24, 2021.

How the volcanic eruption in Tonga was seen by cycling GPS – 4,000 km away

It looks like there's a glitch in the ride data, but there isn't.

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Late last week the Pacific nation of Tonga experienced what is likely the largest volcanic eruption of the 21st century so far.

Over several days the Hunga Tonga-Hunga Ha’apai volcano spewed clouds of ash more than 30 km into the sky. The eruption was so loud it was heard in Anchorage, Alaska – nearly 10,000 km away.

Tsunamis radiated out from the Tongan archipelago, with two people drowning as far away as Peru. A pressure wave from the eruption was measured all around the globe, having seemingly circled the planet as much as four times.

As one eagle-eyed CyclingTips reader noted, that pressure wave was even detectable via cycling GPS units in Australia.

Michael Brown is a keen amateur rider and an associate professor in Monash University’s School of Physics and Astronomy in Melbourne. In an email to CyclingTips this week, Brown explained that the effects of the Tongan eruption could be seen in the data from a stationary trainer ride he did last Saturday evening.

“I started my Garmin Edge 500 going last night on my stationary [trainer] and the altimeter (barometer-based) picked up the change in pressure from the Tongan volcano blast,” Brown wrote. “The big dip in altitude caused by the increase in pressure, and then the pressure / altitude doesn’t settle down for an hour or so.”

Like many cycling GPS units, the venerable Garmin Edge 500 – the cycling GPS of choice for many years – uses an “internal barometric altimeter” to work out a rider’s elevation based on the air pressure where they are.

While this method is generally very good at calculating altitude, it is prone to some errors, most often caused by changes in the weather, and more specifically changes in air pressure. Which is what happened during Brown’s ride.

When the shockwave from the volcanic eruption swept across Melbourne last Saturday evening (at more than 1,000 km/h) the atmospheric pressure rose, which Brown’s Garmin interpreted as him suddenly heading to lower altitude. Again, he was on his stationary trainer at the time.

Brown’s ride started at 6:41pm. You can see the effects of the pressure wave from around 7pm.

So how do we know these fluctuations were caused by the volcanic eruption? How can we be sure it wasn’t some random anomaly? It wouldn’t be the first time a Garmin Edge 500 has provided erroneous elevation data.

“The timing, amplitude and shape of what I measured on the Garmin Edge lines up very nicely with what BOM [Bureau of Meteorology] measured,” Brown explained.

“The change in altitude I measured was roughly 20 metres downwards, which corresponds to 2.4 hPa (240 Pa) which is similar to BOM’s measurements.”

See below for the BOM data. Note that the spikes and troughs are reversed compared to Brown’s ride data. That is, where Brown’s data shows a decrease then increase in his apparent altitude, the BOM data shows an increase then decrease in atmospheric pressure. This makes sense – again, an increase in atmospheric pressure is interpreted by a barometric altimeter as a decrease in elevation.

The times at the bottom of this graph are in UTC, 11 hours behind Brown in Melbourne.

Brown almost certainly isn’t the only one to have recorded the shockwave from the Tongan eruption on Saturday night. Others just might not have realised they’d done so.

“I suspect anyone riding a fairly flat ride that evening may have picked up the sudden 20 metre dip in their altitude too,” Brown said. “[It’s] an instance where one does not want to correct the altitude in Strava.”

Did you note something similar during a ride on Saturday evening? Let us know in the comments below.

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