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Old 02-18-14, 06:16 PM   #1
FedericoMena
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Any OpenStreetMap contributors / which GPS for logging slope grade?

I bike around the city, and have a few apps for OpenStreetMap on my phone.

Things like OpenCycleMap, as far as I can tell, have a relatively coarse elevation map of the whole world, and use that to interpolate the actual grade of sloped streets.

However, OpenStreetMap lets you actually specify the grade for a specific street segment. I'd like to compute this reasonably accurately, and to specify those numbers by hand on streets with more than an N% climb.

Has anyone done something similar? What kind of GPS would I need - I guess one with a good altimeter?

Thanks in advance! A lowly phone GPS doesn't seem to cut it for this
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Old 02-18-14, 07:52 PM   #2
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What kind of GPS would I need - I guess one with a good altimeter? Thanks in advance! A lowly phone GPS doesn't seem to cut it for this
AFAIK, altitude data from our recreational GPSes is not real accurate. If you're going to go to this extra effort, I'd make sure I had better data.

A method I'm aware of is using SportTracks training software to download the GPS track from your ride. Use the elevation correction plugin. This will download elevation data for your ride track from SRTM (Shuttle Radar Topography Mission). Then you can examine your track in detail in SportTracks to see the change in elevation between different points on your ride.

I'm sure there are other applications to accomplish the same thing: overlaying your two-dimensional ride track with elevation data from far more accurate measurements than those provided by the phone or bike GPSes we're using today.
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Old 02-19-14, 03:24 PM   #3
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A method I'm aware of is using SportTracks training software to download the GPS track from your ride. Use the elevation correction plugin. This will download elevation data for your ride track from SRTM (Shuttle Radar Topography Mission). Then you can examine your track in detail in SportTracks to see the change in elevation between different points on your ride.
Oooooh, that's a really good idea! Thanks! I can totally use the existing street grid in OpenStreetMap and compute accurate elevations from SRTM.
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Old 02-19-14, 05:53 PM   #4
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AFAIK, altitude data from our recreational GPSes is not real accurate.
Altitude data from any GPS is not particularly accurate, due to the way that the GPS system works. I'm not sure if WAAS and similar things help with this or not.

Many GPSs now have built in barometric altimeters that are quite precise but not accurate (the two words mean different things in scientific terms) -- they can detect that you've gone up a foot, but changes in the weather will make them be off by quite a bit over periods like an hour or more.

The Garmin Edge 305 and 705 have this barometer. The 205 and 605 do not. I imagine the newer models have it as well, but the cheaper ones may not.

I imagine that the GPS "calibrates" it's internal altitude via the GPS when it gets its initial fix, and then uses the barometer to detect changes. This is ideal for capturing the slope of a hill, as the high precision works perfectly for this, but it can also mean that by the time your ride is over and you're back to your starting point that it thinks you've gone up 100 feet because the weather has changed.

In any event, if you have one of these GPSs, as long as you have a dozen or more feet of altitude difference between the points where you want to calculate the grade of that region and the data points were taken close to each other (seconds, minutes apart -- not hours) ... it should be pretty accurate.
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