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google versus rwgps elevation

Old 04-21-21, 05:15 PM
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IPassGas
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google versus rwgps elevation

The total amount of climbing over a route is often very different between google and RWGPS. Which is more accurate in your experience?
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Old 04-22-21, 07:26 AM
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I don't have an answer for you, but I can explain the problem. The routes drawn on maps consist of small straight lines. The elevation differences are calculated on the ends of these lines. On curvy (think switchbacks) and/or mountainous areas, these lines will hit different parts of the road/terrain. Depending on how a particular site creates its lines, its elevation numbers will be different. So will their distance totals.

That said, the totals for any particular route (assuming it isn't too long) shouldn't all that different. How much difference have you seen?
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Old 04-22-21, 07:30 AM
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I can tell you that sometimes google is 1000% wrong! Sometimes a route we takes seems relatively normal then at the end the graph shows a 90 degree angle! Perhaps the total elevation is accurate but the graph is not to be trusted.
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Old 04-22-21, 08:39 AM
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Originally Posted by raybo View Post
How much difference have you seen?
I am planning a 3 week loop through Iowa. I export various gpx tracks from rwgps to google, so they are the exact same route. For various single days taken from the entire tour, I often see that the total climbing in rwgps are more than 500 feet greater than in google maps. I find this strange since the map data that rwgps uses is from goggle. Apparently goggle's algorithrim for elevation changes does more averaging over the track. Here is one such example, total climbing: rwgps=2200', goggle=1400'.

https://ridewithgps.com/routes/35599491
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Old 04-22-21, 08:43 AM
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Maybe e-mail RWGPS Tech Support and ask ?
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Old 04-22-21, 10:52 AM
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Originally Posted by IPassGas View Post
I am planning a 3 week loop through Iowa. I export various gpx tracks from rwgps to google, so they are the exact same route. For various single days taken from the entire tour, I often see that the total climbing in rwgps are more than 500 feet greater than in google maps. I find this strange since the map data that rwgps uses is from goggle. Apparently goggle's algorithrim for elevation changes does more averaging over the track. Here is one such example, total climbing: rwgps=2200', goggle=1400'.

https://ridewithgps.com/routes/35599491
I've seen anomalies with routes I've created on various mapping sites. On one, I saw a very steep spike on a road that looked flat to me. I think it was a steep cliff next to a narrow road.

You might check the elevation profiles closely to see if there are some places where it doesn't quite match the lay of the road.
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Old 04-22-21, 11:31 AM
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Originally Posted by IPassGas View Post
The total amount of climbing over a route is often very different between google and RWGPS. Which is more accurate in your experience?
They are probably using the same base elevation data. (I suppose Google could be measuring elevation while driving around but I don't think it's likely.)

So, the difference may be in how it's being calculated from the base elevation.

It's fairly common that the elevation gain measured by a Garmin device is larger than the RWGPS estimate.
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Old 04-22-21, 12:03 PM
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I haven't used RWGPS too much for elevation, but every once in a while, I've found Google Maps to have an interesting discontinuity.
For example, can't reproduce it when I try now, but last summer I plotted a route on the Great Plains between Lubbock and Amarillo. The initial routes has two discontinuities where the road appeared to go from ~3700ft elevation to sea level and then back to ~3700ft in a very short distance. Clearly there isn't such a deep chasm in that area. That by itself was ~7500ft of climb that added considerably to the total for a mostly flat route.
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Old 04-22-21, 02:05 PM
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RWGPS elevation gain is consistently short compared to my GPS computer.
Strava elevation gain is consistently short compared to my GPS computer.

I swear that most of the climbing in Iowa(except for the northeast driftless area) doesnt get calculated accurately because its short punchy hills over and over between creeks.
The GPS unit has a barometer.
So either the barometer is wrong, or the RWGPS and Strava guesstimates are wrong.
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Old 04-22-21, 05:23 PM
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Originally Posted by mstateglfr View Post
So either the barometer is wrong, or the RWGPS and Strava guesstimates are wrong.
I have observed this climbing discrepancy in routes from other places in North America. I would think since rwgps uses google map data, the values would not differ so much for the exact same route. As mentioned above it is rather significant (>500 feet) over approximately 50 miles. I emailed rwgps but they are not sure. I suppose I could fire off an email to the black hole of goggle.

The resolution of barometers in handheld devices is approximately 70 feet. The cumulative up/down can also have significant error depending on if a given 0-70 foot elevation change registers a single bit change in the digital converter of the device. If you go up 10 feet and it happens to change one bit, then that would be added to the cumulative as 70 feet. Perhaps the averaging calculation is more sophisticated, but still cumulative error.

Yes, agreed that Iowa has lots of rollers. Rollers are a kind of averaging (best kind) in that the energy climbing/descending is not mostly lost (no braking). Perhaps the averaging that google does, giving lower overall climbing values, is closer to actual biking effort.

Elevation noise aside, looking forward to touring Iowa in June.
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Old 04-22-21, 09:12 PM
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Originally Posted by IPassGas View Post
The resolution of barometers in handheld devices is approximately 70 feet. The cumulative up/down can also have significant error depending on if a given 0-70 foot elevation change registers a single bit change in the digital converter of the device. If you go up 10 feet and it happens to change one bit, then that would be added to the cumulative as 70 feet. Perhaps the averaging calculation is more sophisticated, but still cumulative error.
I think you're confusing GPS resolution with barometers. A GPS without a barometric altitude sensor is just about worst case for calculating climbing in mountainous terrain. Picking up and dropping satellites with abandon, you can probably count on the top and bottom of a climb both being accurate to within about 50' elevation (if there's clear line of site to most of the sky in those locations), and everything else in between is a crap shoot. Depending on the software, it may give you a good guess on total climbing in between those two points, or ... not. If you had a 250' "measured" jump followed by a 150' dropoff, and it counted both of those, the result can be completely unrealistic.

You can measure barometric pressure with much better resolution than 70 feet. There's still some diversity between device manufacturers as to how much you'll filter out as noise. I've ridden rollers with a Casio watch (which doesn't accumulate anything less than 60 feet climb) and a Garmin with barometric sensor (which starts accumulating climbing distance around six feet). You can climb and descend an awful lot of 45' rollers, ending up with 350' of climbing on the Casio but over 800' on the Garmin.

The same thing happens with mapping software. Do you only count when a route crosses a contour line, and then count that as the distance between contour lines? Or do you try to interpolate between contours? Get into the mountains where USGS mapped terrain with 100' contours, and the first wears you out for almost no counted climbs; while the latter, especially on converted rail line in a narrow gorge, doubles the actual climb.

Google does have some elevation data, presumably from driving its vehicles over some roads and correlating barometric pressure with surveyed datum points. If that's where you're riding, that's likely the second best source (behind highway department survey data) for climbing. On most smaller roads where they've never driven their fancy vans, a GPS with barometric altimeter from a reputable company would be my preferred source of information.
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Old 04-22-21, 09:52 PM
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I find RWGPS elevation gain on a route is very close to what one of my Garmin 800 devices shows. My other 800 shows a much higher gain. After a group ride we sometimes have an elevation question time. Every one has a different number, most of them showing lots more gain than RWGPS. IMO it's like calories on a sports watch: the manufacturers set up the algorithm to make the user happy. More calories and more gain = happier rider. Simple.

I don't trust Google maps with anything. RWGPS is so much more reliable w/r to routing.
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Old 04-23-21, 05:36 AM
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Originally Posted by pdlamb View Post
I think you're confusing GPS resolution with barometers.
I realize there is a difference. For example, here is an engineering analysis which discusses the barometric precision of hand held devices.
https://www.amsys-sensor.com/downloa...amsys-509e.pdf

Perhaps Garmin-like devices do a little better, don't know. My interest is in using rwgps, because it is a useful route planner and provides an elevation profile. But the elevation discrepancy between it and goggle is significant even though rwgps gets their map data from google.

It seems people find rwgps aligns best with their actual experiences.
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Old 04-23-21, 06:03 AM
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what device?

I noticed older phones had terrible GPS accuracy in any sort of difficult terrain (mountains, forest etc.). Sometimes it would place me a few ft next to a path i was on, sometimes more. Sometimes it would miss a piece of switchbacks and just add a straight line over some hundred yards where it should have been zig-zag and curves. Newer phones (I have a 3 or 4 year old flag ship phone) are MUCH better in the very same locations. Probably not 100% perfect, but i don't notice the obvious problems anymore.

I don't pay attention to elevation, but what I observed also must apply to elevation.

Any barometric elevation measurement with a phone probably is useless without scientific instruments. That also needs to be calibrated to weather since that changes pressure as well.
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Old 04-23-21, 07:55 AM
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First..everything discussed here is an estimate. Estimates are made on the resolution(& accuracy) of the base data and how it's processed. Accuracy costs money. Highly accurate data(on the scale of global terrain, or even just the USA) can be very expensive to generate. High accuracy files (datasets) are huge and are pricey to manage and process. At the end of the day it comes down to what level of accuracy is adequate for a consumer-level (free) mapping app at a modest cost.

Maps we see and use are very much composite images that pull data from many different sources, depending on the intended use of the "map".

While RWGPS may use google maps as a source, it may pull elevation data from a different source(elevation data is more of a core interest to RWGPS than generalized elevation info is to a consumer-level google maps user), or may use (process) a higher resolution elevation dataset than google uses. Given the volume of data google maps needs to process they may compress some source files(or processing) in the interest of increasing throughput/lowering costs. The accuracy of the output they generate is probably perfectly acceptable to most of their users, however an engineer building a road would be using much better datasets than what google uses.

It may well be that the focus of the OP's interest approaches the resolution of the base data..error increases as you approach the limits of the dataset.

We have knowledge where we have data. Actually measuring elevation of a ride using a GPS device during the ride (assuming the device determines elevation with reasonable accuracy) would probably be the best estimate of the actual elevation experienced as the sampling rate is very high (as it records the ride) compared to what google maps is using..which is probably generated by satellite(?).

On the grand scale..knowing elevation is somewhere around 2200 ft or 1400 ft over a 50 mile ride is better than no info at all, or an estimate of 4500 ft.

As mentioned above..ask RWGPS. Some analyst there can give you a very good explanation..if you can get access to him/her.
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Old 04-23-21, 12:57 PM
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Google maps uses SRTM data with a 90-m horizontal resolution. That is a grid of points on the earth where the elevation is known every 90meters apart. All points in between are interpolated between the nodes to calculate an elevation.

Most high end gps use either a better resolution data set or barometric pressure sensor to calculate elevation.
Therefore, the data sets will result in different elevations profiles.
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Old 04-26-21, 10:00 PM
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I wonder if RWGPS processes the millions of rides they collect, and uses the tracks generated by barometric altimeter-equipped devices to improve the base maps' elevation gain estimates. Seems like it'd be a neat thing to do, and if they're running their own servers, it could be done with in a low priority background job. I'm sure every device would start with a different estimate of elevation, even from a fixed starting point. But take a few hundred rides, process the elevation changes from all the devices, perhaps do a weighted average based on how good a Garmin's altimeter is compared to a Bryton's. I'd think they could substantially improve on most base maps' elevation gain estimates for a given route -- if that route were sufficiently popular.
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Old 04-27-21, 06:10 AM
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Apparently, google and rwgps use the same 90m interpolation for route planning.
https://ridewithgps.com/help/grade-and-elevation
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Old 04-27-21, 02:52 PM
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Interesting read.

As I mentioned above, estimates, estimates, and more estimates...and lots of data processing. The shuttle flying overhead and gathering data gets the job done over a large area, but it has its limitations. Undoubtedly a gross improvement over our broad-scale understanding of elevation prior to this work. In the end, accurate or fast, pick one.

What we need is the (I assume) google car that drives around and records the "street view" images to also gather accurate elevation data.
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