Originally Posted by
rachel120
I want fairly accurate distance so I can know calories burnt. Calorie counting ended up being more realistic if I tossed time spent doing what activity out the window and instead focused on miles, since cycling 20 minutes at 12 miles an hour were the same calories as walking 1 hour at 3 miles an hour.
Speed is important to me since I've been non active for a month and wearing layers now makes me feel slower. I want to make sure I'm not unreasonably slow when part of traffic.
Calories
Calorie estimates based on GPS or even on a heart rate monitor are quite inaccurate (and often seem to estimate high). And it's likely to be even more inaccurate if the app doesn't know when you are biking or walking.
You might look into a heart rate monitor that can work with your phone. My HRM lets me maintain a hard, but sustainable effort.
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googling for numbers:
walking: something like 90 or 100 calories per mile.
biking: between 20 and 25 calories per mile.
My faster, moderately hard effort group rides are often around 30 cal/mile. I've been as low as 15 cal/mile on a 40 mile easy paced ride. An all-out 5 minute hill climb was about 90 cal/mile.
Your biking 1/3 hour at 12 mph = 4 miles = approx
80 to 100 cal. (Bikes are extremely efficient!)
walking 3 miles = approx
280 to 300 cal
Of course, these are averages for a whole session. Lots of hills or intense efforts will bump up the numbers.
Power meters on bikes measure calories quite directly, and fairly accurately. (basically: There's about 4 calories per joule, and people waste 3/4 of their calorie burn in heat, 1/4 going to useful effort. So the number of joules just about matches the number of calories burned. ) Many riders report somewhere in the low 20 calories per mile from this data.)
Averages
All GPS apps and devices have to do some data smoothing. They all use slightly different methods, so there's always variation. My ride recording will report slightly different distances, and somewhat different elevation gain and average speed if I download the same data file to 3 different apps. But they are all within 10% or 15% usually, and that's good enough.
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I have a wheel sensor with my Garmin GPS, for accurate distances and speeds. It's usually quite similar to other rider's Strava phone distances. The elevations can be off a little, maybe 10-20% variance between different devices and apps.
GPS accuracy:
My older Garmin 705 records once a second. Here's a ride I did, heading uphill north outbound, then downhill southbound and turning right. You can even see which side of the road I was on. The one-per-second dots are color coded by speed.
Newer phones are faster and more accurate with GPS than my old, slow Garmin. But trees and buildings can affect the accuracy.
