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Clydesdales/Athenas (200+ lb / 91+ kg) Looking to lose that spare tire? Ideal weight 200+? Frustrated being a large cyclist in a sport geared for the ultra-light? Learn about the bikes and parts that can take the abuse of a heavier cyclist, how to keep your body going while losing the weight, and get support from others who've been successful.

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Old 10-11-13, 01:51 PM   #1
LabRat2k3
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Must be wearing the hills down

My goal for this month has been to work more on my climbing which brings me to a question. Just how is elevation gain calculated? I have ridden the same route three days this week and the elevation gain on strava has went down every day. Started at 597 ft, 534 ft, and today 498 ft. The only solution I can figure is my fat ass must be flatting the hills out by riding over them. At this rate in a few years West Virginia will be as flat as Kansas and bigger than Texas. I suspect the problem is the precision, or lack there of, that my phone measures everything with, because the grade of the hills is never the same from day to day. Heck, the grade isn't even the same going up as it is going down on the same ride. I guess I'm going to have to get an actual GPS for my bike. Garmin seems to be the only real option right? When I started riding I had no idea that a significant amount of my weight loss would be from my wallet.
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Old 10-11-13, 02:29 PM   #2
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When we use GPS for timber cruising, we set the machine to require 3 satellites to give us a fix. This increases the accuracy, but we can't always get our coordinates.
The way phone/GPS seems to work all the time, I suspect they provide a fix based on only one satellite
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Old 10-11-13, 02:44 PM   #3
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I will agree with Strava's elevation numbers being a bit wonky. I previously used Runkeeper and would see 2-3x more elevation on that app than what Strava shows. I also know of people who run Strava and a Garmin unit simultaneously and get very different readings.
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Old 10-11-13, 02:52 PM   #4
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It's not an uncommon problem. I've got a bare bones GPS computer and export the data to SportTracks software and the same exact ride produces various results like the OP gave. What's funny is that I make a round trip and the rise and fall never match. Wouldn't they have to if I start and stop at the same place? There is sometimes 40-100 foot difference.
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Old 10-11-13, 03:50 PM   #5
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So there are really 2 ways to calculate elevation.

The first is what you'll find on devices like the Garmin Edge computers: Barometric Pressure. These measure the air pressure and calculate the altitude based on that. It's not deadly accurate, but it's usually accurate enough. Changes in the weather will affect it, so typically the longer you ride, the more inaccurate it gets.

Then there's GPS-based elevation. GPS doesn't actually calculate elevation, the signal merely provides a 2D polar coordinate that tells you your latitude and longitude. This value is then interpretted by online mapping software, mixing it in with the National Geographic Survey data to calculate your elevation changes. The problem with this method is that GPS is not as accurate as it looks. Typically it'll be off by up to 20-30 feet, but most mapping software is smart enough to show you on a road when it renders the final graphic. However, most mapping software calculates your elevation before it smooths out the lines. If you ride on a road that follows a ledge, for example, the GPS location can show you as being at the bottom of that ledge in one instant, 30 feet to the right, and then up on the road again a few seconds later. The elevation calculations will say "ok he just descended 20 feet and then climbed 20 feet in the last 20 seconds", thus adding 20 feet to your elevation, and causing your numbers to fluctuate.

The bottom line is; the numbers are "noisy". The equipment required to calculate your precise elevation at all times is far too expensive to put into a cellphone so it merely relies on approximations, and you'll never get a consistent number.
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Old 10-11-13, 09:23 PM   #6
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Quote:
Originally Posted by Mithrandir View Post
So there are really 2 ways to calculate elevation.

The first is what you'll find on devices like the Garmin Edge computers: Barometric Pressure. These measure the air pressure and calculate the altitude based on that. It's not deadly accurate, but it's usually accurate enough. Changes in the weather will affect it, so typically the longer you ride, the more inaccurate it gets.

Then there's GPS-based elevation. GPS doesn't actually calculate elevation, the signal merely provides a 2D polar coordinate that tells you your latitude and longitude. This value is then interpretted by online mapping software, mixing it in with the National Geological Survey data to calculate your elevation changes. The problem with this method is that GPS is not as accurate as it looks. Typically it'll be off by up to 20-30 feet, but most mapping software is smart enough to show you on a road when it renders the final graphic. However, most mapping software calculates your elevation before it smooths out the lines. If you ride on a road that follows a ledge, for example, the GPS location can show you as being at the bottom of that ledge in one instant, 30 feet to the right, and then up on the road again a few seconds later. The elevation calculations will say "ok he just descended 20 feet and then climbed 20 feet in the last 20 seconds", thus adding 20 feet to your elevation, and causing your numbers to fluctuate.

The bottom line is; the numbers are "noisy". The equipment required to calculate your precise elevation at all times is far too expensive to put into a cellphone so it merely relies on approximations, and you'll never get a consistent number.
Excellent information.

Fixed one word of it for you.
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Old 10-11-13, 09:49 PM   #7
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Originally Posted by skilsaw View Post
When we use GPS for timber cruising, we set the machine to require 3 satellites to give us a fix. This increases the accuracy, but we can't always get our coordinates.
The way phone/GPS seems to work all the time, I suspect they provide a fix based on only one satellite
This right here. I'm a reseller for Delorme and the more sats that can be locked onto the more accurate it is. Additionally there is a "corrective" data source that is used to correct various anomalies but one would only care about that if precision was a concern.
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Old 10-12-13, 04:35 AM   #8
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I have used a few different apps the latest being map my ride...I dont have elevation gain im in Fl lol however the average speed and time was wrong.Data told me i averaged 23.5 mph and did it in 1hr 40 min. 30 miles.However i actually did it in 2 hours and that puts me at 15 mph and that would be where i normally ride.The calories were correct i use a calculator after each ride and it was close so just the speed and time is way off.
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Old 10-12-13, 05:48 AM   #9
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Thank you. I never even thought about the "wobble" in the GPS signal. If it were to put me 30 feet either way in some places I could be 50-60 ft lower than the road, down in the river, or 100+ft higher than the road, up on the cliffs. That makes a lot of sense now. I guess I will just try not to worry so much about numbers and just push myself to climb more and faster.
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