How Do You Measure Ascent?

The OMATA One measures ASCENT — change in elevation. It uses a barometric pressure sensor, specifically the ST Microelectronics LPS25HB. This component falls in the category of what are referred to as MEMS — for "Microelectromechanical Systems". MEMS is a remarkable system of, effectively, ultra-small mechanical systems — actual moving mechanical parts — that translate movement into electrical signals. The same technology is used in accelerometers that measure whether your phone is upside down, spinning or has been dropped, or your car suddenly decelerates.

MEMS when used for the LPS25HB literally measures the pressure of air against a small membrane. But air pressure doesn't typically have much meaning to normal humans, unless you're a weather tracker, pilot or nautical sort.

In order to get an accurate measure of the current elevation, you need to know the baseline pressure for where you happen to be, typically the barometric pressure at a local weather station or a nearby airport. This value changes and typically needs to be manually set based on that morning value. Pilots, for example, set the barometric pressure on their altimeter after a call to the tower based on a canonical value at the time they are preparing for takeoff.

As we do not need to know absolute elevation — only change in elevation — we can ignore the "canonical" morning barometric pressure. Instead we just take a sample of the pressure at the beginning of the ride and measure its change over the course of the ride. Then we translate that pressure value into what is known as "pressure altitude". By constantly measuring this value we can create a chain of differences that become a measure of how much you're climbed on your ride — how much the measured pressure has changed as you climb and descend. (Of course, we only measure going up.)

Accuracy with these kinds of measurements is typically quite good. We did some simulations back in 2016 to determine how much a dramatic change in the day's pressure would affect ASCENT accuracy and it was quite small. In one simulation where we tested how far off ASCENT would be if one set off riding on the morning of a significant tropical storm — where pressure drops dramatically over the course of the weather event — we found that the change in pressure altitude would be several hundred feet. On a typical day in Venice Beach, California, for example, the pressure change from morning to evening might represent a change in pressure altitude of 10s of feet.

Email us at [e-mail] if you are curious about these simulations.