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Do you believe elevation data?

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Road Cycling “It is by riding a bicycle that you learn the contours of a country best, since you have to sweat up the hills and coast down them. Thus you remember them as they actually are, while in a motor car only a high hill impresses you, and you have no such accurate remembrance of country you have driven through as you gain by riding a bicycle.” -- Ernest Hemingway

Do you believe elevation data?

Old 12-01-07, 11:38 AM
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qw1a
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Do you believe elevation data?

Recently, I assaulted one of the members for claiming to have climbed 37% grade. Mea culpa. However, this prompted me to finally map my ride on MapMyRide.com.

Now, I am a bit puzzled. According to the elevation data, my ride includes bits of 35%+ grade climbing. Very short bits, about a couple hundreed meters. Still, I am positive that it's not possible (well, at least not possible for me) to climb anything above grade 20% without falling over and I always thought of that stretch as being about grade 15%.

I, in general, tend to trust my intuition more then the data, so for now I will maintain that it's 15% grade. My first hypothesis was that the website gives you horisontal displacement rather then the distance traveled. This still would not produce 15% grade for that stretch (simple conversion to travel distance) but it would be a bit more realistic.

How is this elevation data determined and how reliable is it? Do you trust it? Would you trust your alitemeter for short hills like that one? I would imagine for 70 meters of vertical travel the pressure change might be overpowered by noise.
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Old 12-01-07, 12:11 PM
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If you'll look at the hybrid view and zoom in enough, you'll see that, assuming you've checked 'follow roads' option, that the road shown only in the map view doesn't in fact perfectly align with the road that can be seen in the hybrid view. That results in massive errors. For instance in one ride I mapped, the super-imposed road cut a corner, just a wee bit but that wee bit happened to fall off the edge of the canyon the road was clinging to and voila, a rather impressive change of elevation that didn't exist on the actual ride. In the end I've pretty much given up on on having accurate elevations shown but I still love the site and the mapping option. And if you're attempting to arrive at percentages for the grades you ride, you're going to be permanently frustrated. Luckily I have an attractive option, actually two options but the second is in a book format, since I live in France. Here's the link to it - https://ciclismo.sitiasp.it/motore.as...feca3cbe&da=az . On the assumption that you're in the states, I don't have a clue if what you ride will be found since I've never bother looking at any grades in the states. Anyway, a cool site that I use often. Note that the language changes according to what country you click on. Or at least it does between France and Italy. I should also add that the percentages for various passes shown on that site and in my books - Atlas des Cols - don't always agree. Sometimes not at all. Solution? Heck if I know. Just ride, sweat, and enjoy the information afterwards.
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Old 12-01-07, 12:11 PM
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i've climbed a 37 percent grade....here in pittsburgh you reguarly get 20+ grades but they are all short....
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Old 12-01-07, 12:52 PM
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Well, no canyons or anything else around, a very straight-forward street going uphill.

I was curious enough to call the village hall and ask (just 5 min ago) - apparently, the engeneering data for this block confirms 36% grade. This is very bizarre, very - will ride that block tomorrow again and check it out. I am still a wuss, but now I owe a beer to that dude I did not believe
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Old 12-01-07, 01:19 PM
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The google map elevations are usually good enough, but there can be spikes in the middle of flat roads, and hills that are actually steeper than the map shows.

Here's a Google Earth example: Cincinnati's Eden Park overlook. The dark pond and the parking lot are both flat, at the edge of a steep hill.
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Old 12-01-07, 04:36 PM
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Originally Posted by rm -rf View Post
The google map elevations are usually good enough, but there can be spikes in the middle of flat roads, and hills that are actually steeper than the map shows.

Here's a Google Earth example: Cincinnati's Eden Park overlook. The dark pond and the parking lot are both flat, at the edge of a steep hill.
Wow! From that image, those cars should have some serious parking brakes and a rock solid Parking gear (unless they are standard), and the ponds should be be spilling over. However, as you said, the image is off.
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Old 12-01-07, 04:43 PM
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I generally don't believe any numbers on bikeforums.
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Old 12-01-07, 04:47 PM
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This distortion is caused by the need to use "point data" to create a 3D map. Triangles are overlaid using the point data and then given a surface which will corrospond to the new map projection. Viewed with the proper interface it can be quite realistic and acurate if the points are close together.

I believe the problem comes from file size. An accurate map with points 3 meters apart would look very realistic and be huge in "byte" size. To serve over the internet this would require too much bandwidth. The larger the area being displayed the farther apart these points end up giving the distortion you are concerned about.

sign me an ex digital map tech
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Old 12-01-07, 07:09 PM
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1) I don't believe the elevation data on MapMyRide (or any online mapping program).

2) I don't believe there is a paved 37% grade in mainland USA. I think that's wishful thinking.
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Old 12-01-07, 07:23 PM
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All the mapping web sites are OK for elevation at specific points, but terrible for gradient and cumulative elevation gain. If you want to calculate cumulative elevation gain, plot an elevation profile and add up the differences between the big peaks and valleys. Ignore all the ripples in between, since they often do not exist in real life.
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Old 12-01-07, 08:28 PM
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Originally Posted by Machka View Post
1) I don't believe the elevation data on MapMyRide (or any online mapping program).
Well, barring an actual geodesic study, satellite data is as good as it gets. In this case, however, the city has done a survey when they were issuing the building permit. According to them, this particular street block has an elevation gain of 32.8 feet over the length of the street. The length of the block, according to their record is 92 feet.

Originally Posted by Machka View Post
2) I don't believe there is a paved 37% grade in mainland USA. I think that's wishful thinking.
Well, it would strongly depend on the length of the that incline - when you are jumping the curb, you are climbing an infinite grade (a finite dElevation over a very very small distance).

I am still maintaining that it's not correct and will check it out tomorrow - I have a lazer range-finder with a built-in level which should resolve this particular question.
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Old 12-01-07, 08:34 PM
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I trust only official USGS bronze survey markers. Even the contour lines that MapQuest and others interpolate between are approximations. If I really, really care about some slope I borrow my wifes' pocket transit (Brunton, Riverton, Wyoming) and measure it directly.
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Old 12-01-07, 09:35 PM
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Originally Posted by qw1a View Post
Well, barring an actual geodesic study, satellite data is as good as it gets.
That's really, really sad! The so-called satellite data from which these online mapping programs get their data can't even map the roads and land features correctly. There's an area near where I live that used to be a river with no roads in the general vicinity. In the early 1980s, a dam was built turning the river into a large lake. A number of recreational areas were created around the lake, and roads were built, including one over the dam. But according to Google maps, and several others, that area is still the river with no roads. And that's not the only mistake in my area. They're getting their information from 30 years ago!

If I can't trust them to map the roads correctly, how can I trust them to map elevation?
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Old 12-01-07, 10:00 PM
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Originally Posted by Machka View Post
If I can't trust them to map the roads correctly, how can I trust them to map elevation?
Roads change a lot more frequently than elevations. Also, roads are usually mapped by commercial companies who focus on area where there is the most profit (i.e. cities).
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Old 12-01-07, 10:27 PM
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https://www.gmap-pedometer.com/?r=1480970 This shows the last part of the climb to Summit Circle in Westmount. The elevation tracking shows the end is at the same elevation as the start of the plot. According to the altimeter function on my bike computer it is about 140 ft higher, so some of the readings on the mapping software are off by about 100 ft.
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Old 12-01-07, 10:27 PM
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Originally Posted by johnny99 View Post
Roads change a lot more frequently than elevations. Also, roads are usually mapped by commercial companies who focus on area where there is the most profit (i.e. cities).
True, but one of my routes uses these roads that don't exist in Google maps and the others ... if I were to try to map the elevation of that route, I'd get incorrect data. And that would be the case to some degree or another with just about all my routes.
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Old 12-01-07, 10:37 PM
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Most mapping sites use elevation data from the USGS: https://gisdata.usgs.gov/XMLWebServic...ce_Methods.php

Since each of the parameters (latitude/longitude, elevation) have a certain amount of gaps or noise in them, you certainly have to take these calculations with a grain of salt. And the formulas for calculating the distance between a latitude & longitude are also approximations.

I think users of online apps that give % grade (veloroutes.org, mapmyride.com) should look at it in relative terms: if it says 4% you know you could go up it faster than if it says 25%. If your city has a grade that's 20% or above, they probably have an official 'steepest roads' page like Seattle's.

All I know is that pedaling a 39x23 gear on a 20% grade is HARD.
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Old 12-01-07, 10:44 PM
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IslandBoy pretty much nailed it-- there's too much interpolation to be able to trust the elevation data for routes following roads along slopes, particularly steep ones with a lot of switchbacks. Unless you're zoomed way, way in, a single pixel to one side or the other can come out as a quite large elevation difference if you're traversing a steep slope. Even if you're zoomed way in, the dataset probably isn't high enough resolution to give you accurate elevations for a road or trail cut into the side of a mountain-- the pic above shows that very clearly.

Try plotting a route along a steep road with a a lot of tight turns and compare the elevation change over the route with the elevation change just by taking the endpoints-- in really bad cases it can be a factor of 2 or more difference.

If you just want to get elevations of start and end points of a ride, or a long road that goes relatively straight up a moderate grade it will be reasonably accurate.
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Old 12-01-07, 10:58 PM
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Originally Posted by mattm View Post
Most mapping sites use elevation data from the USGS: https://gisdata.usgs.gov/XMLWebServic...ce_Methods.php

Since each of the parameters (latitude/longitude, elevation) have a certain amount of gaps or noise in them, you certainly have to take these calculations with a grain of salt. And the formulas for calculating the distance between a latitude & longitude are also approximations.

I think users of online apps that give % grade (veloroutes.org, mapmyride.com) should look at it in relative terms: if it says 4% you know you could go up it faster than if it says 25%. If your city has a grade that's 20% or above, they probably have an official 'steepest roads' page like Seattle's.

All I know is that pedaling a 39x23 gear on a 20% grade is HARD.
Isn't that site just for the US though? Would it apply to the rest of the world? I would imagine that Canadian data comes from here: https://gsc.nrcan.gc.ca/index_e.php or somewhere similar. But yes, the numbers would very likely be approximations. I've had a look at several elevation maps for an area just west of here in my geography class, and they're good, but they aren't exact. From one map, I tried to calculate the elevation gain (and thus the % grade) of one particular road which was approx 2 kms. First, it was hard to tell the exact, precise length of the road from the map, and then it was difficult to figure out the exact elevation gains because the road curves around a lot, and each bar is a range. As near as I could figure it was approx. a 10% grade.

In all the years I've been cycling I have ridden everything from a 0% grade to a 25% grade, and from my experience:

-- 10% is a fairly strenuous climb, but I can make it up without getting off and walking, and without going into my granny gear.

-- 15% is a really strenuous climb, but I can make it up in my granny gear.

Anything over that, I'm walking ... and ...

-- 25% is a really strenuous walk, involving many stops to catch my breath and get my heart rate back down!

BTW - I knew I was on a 25% grade because the sign at the top told me so. They label the roads with the % grade in the UK.
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Old 12-01-07, 11:15 PM
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the USGS's service covers most of the world, for instance this route in vancouver has elevation data: https://veloroutes.org/bikemaps/?route=6055 (veloroutes uses the USGS webservice)
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Old 12-01-07, 11:24 PM
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Originally Posted by mattm View Post
the USGS's service covers most of the world, for instance this route in vancouver has elevation data: https://veloroutes.org/bikemaps/?route=6055 (veloroutes uses the USGS webservice)
If by most of the world you mean anywhere on the North American continent

The only elevation data I really trust is what I get from the pressure-based altimeter in my HR monitor. Rarely are there sufficient pressure changes over the course of a climb for the measured relative elevation to be thrown off.
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Old 12-02-07, 02:05 AM
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Originally Posted by Machka View Post
True, but one of my routes uses these roads that don't exist in Google maps and the others ... if I were to try to map the elevation of that route, I'd get incorrect data. And that would be the case to some degree or another with just about all my routes.
You're going to get incorrect data anyway if you trust the elevation data in the freebie on-line mapping programs. None of them will give you really useful cumulative elevation data.
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