• How does Garmin calculate percent grade?

    On a recent mountain bike ride, a couple of us with Garmin devices began to wonder how percent grade is determined by the device. Through a local contact, I was able to go directly to the design source, Jared Bancroft, PhD – Sensors & Algorithms Team Leader.

    Here is what Jared told me:

    Grade is responsive to the slope of the trail or road.  It relies on the distance as determined from GPS and the barometric altimeter sensor.  Basically, it’s a ratio of the change in altitude over distance traveled. The longer the distance, the more averaged the grade is.  We try to use the smallest distance, but if the distance is too small it looks really noisy and the grade bounces all over the place.  Also, anytime GPS isn’t available or sufficiently accurate enough (e.g. heavy tree coverage in the mountains) grade also suffers.  That said, if you have a speed sensor, percent grade comes in more accurately since we can use the speed sensors distance rather than the GPS based distance that may or may not be correct.   

    Grade typically responds within several seconds. It won’t react to large rocks, wheelies, or popping up your front tire.  Its more trying to emulate the slope of the terrain you take.  We have to filter the results so that delays it a bit. 

    Accuracy of percent grade is more based on GPS reception quality than time on the hill. Small dips get averaged out but longer hills are usually pretty good.

    Grade should be consistent on rides.  If you take the same path, it should be the same (both by definition and seen on the device). That said, the distance grade is calculated over, the smoothing filter and the GPS quality all play a role in the final number you see on your device.

    Now you know!


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