dual Altimeter

Barry Hansen

I’ve been making new software for Griduino, the grid square navigation assistant.

Here’s a new feature: Altimeter shows readings from its GPS and barometer side by side. It makes a good cross-check to help decide which reading is trustworthy. The internal barometer is really sensitive to even small changes in altitude. After all, Bosche made the BMP388 sensor with flying drones in mind. You can see it change altitude if you blow on it or close a door.

The problem is that both readings can be off, each for different reasons:

  • Since air pressure will naturally change with weather, the absolute altitude reading from the barometer depends the user to input current air pressure. To get a good reading, you’ll need to periodically adjust it to a known altitude or known sea level pressure. The altimeter screen therefore offers + and - buttons to calibrate it. You can get the correct pressure by searching the web for a local weather details, or you can adjust the altimeter to a known altitude.
  • Although the GPS reading can be good, it depends on having GPS satellites in view and almost directly overhead. Also, and by design, the service’s vertical accuracy is less than its horizontal positional accuracy. Note that reflected signals from nearby objects can also throw it off. For example, at my home with poor reception in a small valley, my GPS reports will vary by 300 feet or more over a day. Since I live nearly at sea level, this often thinks I’m underwater. I know Seattle is wet but it’s not that wet.


One of the goals of the Griduino project is to build in features for SOTA (summits on the air) operators. This altimeter is a good start. A SOTA requirement is to operate from within the ‘activation zone’ of 25 meters, vertically, from the summit. Here’s where the barometric pressure sensor shines – it’s exceptionally good at small changes in altitude, within a foot or so. If an operator can reach the summit, he could take a reading and then confidently move around within the required 25 meter zone.

Here’s an example screen where I live at 30’ altitude and the GPS is off by 58’ even though it sees 5 satellites.

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If you have a Griduino, get the pre-compiled binary from  as version 0.34 in the downloads folder. This has all the new layout changes for the altimeter.

Unrelated and just for fun, I changed the countdown screen to show how many days until the June VHF contest.

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I’d like to get suggestions for more SOTA features, so let me know, thanks.

Hope everyone is surviving the freezing cold, snow, pandemic, insurrection, earthquakes and probably a locust swarm next week.


Barry Hansen K7BWH


Seattle, WA  CN87us


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