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thunderhead
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PostMon Jul 11, 2022 8:54 am 
I could have sworn caltopo has better than 100ft samlple rate. But ya, its going to miss some of the smaller wiggles.

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Randito
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PostMon Jul 11, 2022 9:14 am 
Variability in GPS track measurements is an example of "the coastline paradox"

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dkemp
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PostMon Jul 11, 2022 10:47 am 
I used the MapBuilder Topo open source mapping inputs and added the mileages. From Suiattle TH to Holden I got 28.9 miles. I then used the Add Line method fjoro taught me and got 28.3. Considering what I've learned in this thread, that sounds like a 30 miler. smile.gif

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Kim Brown
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PostMon Jul 11, 2022 10:56 am 
Going around washouts and finding creek crossings will make up for any deficiency in map tool mileage, so you can be confident it's a 50 miler . up.gif

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neek
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PostMon Jul 11, 2022 11:42 am 
rossb wrote:
I think if "undo" simply deleted the last point, it would be fine. Still a bit finicky, but folks would manage to work with it.
Yes, just press ESC and the last point is undone. Very handy.
rossb wrote:
It would require a fair amount of work to assemble these, but it could be based on existing data (or data people add). Once you have this sort of thing, it wouldn't be that hard to build tools from it. The "points" would show up on the map, and you could select two (or more) to figure out the distance (similar to how Google figures out driving distance). But the weighted graph would be valuable just as another source of information -- it would be another way to look at things.
I suspect caltopo uses exactly this weighted graph to calculate shortest paths. In fact you'll notice distance labels between junctions on the MapBuilder layer. Elevation, being directional, would be more awkward to display, but all the data is there, no need for manual work.

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neek
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PostMon Jul 11, 2022 11:57 am 
thunderhead wrote:
Any map is subject to error. But caltopo is almost certainly more accurate than either aggregated gps points or even worse, steps. Now dont get me wrong, there are gps units out there that will do this very well, but the cheap one in our pocket isnt one of them, especially on north slopes, in thick trees, or worst of all, in cell-tower "augmented" mode. A gps at least has a chance though. Counting steps is so atrociously innacurate its not even worth glancing at.
I would have thought so too a few years ago, but lately have been getting better numbers directly from tracking software and a good GNSS unit. The smoothing algorithms have gotten quite good. And with a barometric altimeter you probably get better elevation numbers than what's in the old DEM files. Caltopo relies on OSM, with often sloppy user-added data and often horrible agency data. (My method for measuring distance accurately is manually tracing over a track to effectively remove jitter.) Also note when talking about distance in hilly terrain you need to specify planar or topographical ("3D distance" in Garmin speak)...for example if you're going up and down 45 degree slopes your actual distance is the straight-line distance times sqrt(2) (if I'm remembering my middle school geometry correctly).

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rossb
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PostTue Jul 12, 2022 3:44 pm 
neek wrote:
Yes, just press ESC and the last point is undone. Very handy.
Ha! Thanks. I never tried that. I kept trying Ctrl-Z (undo) or right clicking (to see if there was an undo option). Never thought of escape.
neek wrote:
Elevation, being directional, would be more awkward to display, but all the data is there, no need for manual work.
Good point. It would be a matter of accessing the data somehow (they've got it somewhere) and displaying it in a different (more simplified) format.

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Skookum Bill
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PostTue Jul 12, 2022 11:13 pm 
The particular route described by dkemp in his post: "to Miners Ridge, Suiattle & Cloudy Passes to Lyman Lakes, and out RR Creek trail to Holden" was mapped with a GPS for Green Trails Maps about nine years ago. The most current Green Trail maps of the route will have those mileages between significant points and it will be as accurate if not more so as the other techniques described.

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thunderhead
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PostWed Jul 13, 2022 2:26 pm 
neek wrote:
And with a barometric altimeter you probably get better elevation numbers than what's in the old DEM files.
Ya, you can make a pressure altimeter pretty precise(within a few tenths of a percent) if you correct for nonstandard temperature. This is beyond the casual user, most pressure sensors or altimeter apps dont even have that option. Or luck into a standard day. But it will still suffer the coastline paradox if you want to measure your total vertail gain. If you read your altitude at the start and finish, you gloss over all the bumps... except for those few glorious trails that are purely up. If you try to sample at a high rate... your very act of stepping will create significant error.

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Cyclopath
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PostWed Jul 13, 2022 3:20 pm 
thunderhead wrote:
Counting steps is so atrociously innacurate its not even worth glancing at.
Back in the day, before GPS, before paper maps, before paper, a mile used to literally be two thousand steps. Mile, from mil for thousand. A mile was a thousand paces and a pace is a step with each foot. Professional measurements were done by walking and counting steps. eek.gif Eratosthenes hired a professional step counter to measure the distance between Syene, Egypt, where the sun didn't cast a shadow on the summer solstice, and Alexandria where he was and it did. Using math to work out the angles and the distance between the two cities as worked out by step counting, he estimated the size of the earth with an error of about 5%. Which is pretty good for 240 BCE.

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Cyclopath
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PostWed Jul 13, 2022 3:25 pm 
neek wrote:
And with a barometric altimeter you probably get better elevation numbers than what's in the old DEM files
Absolutely yes. Keep the barometer calibrated!

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