A few days ago I hiked and GPS mapped with Motion-X several trails around Mt. Chocorua in New Hampshire. My track matches some of the existing trails well, some not, but parts of the Chocorua River end up on the wrong side of the Weetamoo Trail according to where the river is currently mapped. How can I check to see if the current river mapping is correct or not. If the river is not in the correct location, can I reposition it in the area where I have data ? My knowledge level is of a novice having only mapped a few dozen jogging trails so far on OSM.

asked 28 Aug '15, 22:12

williamp's gravatar image

accept rate: 0%

Hi Williamp, Read the beginners pages http://wiki.openstreetmap.org/wiki/Recording_GPS_tracks, as well as http://wiki.openstreetmap.org/wiki/Accuracy_of_GPS_data and walk the trail another time with a close look at the river valley to collect more traces.

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answered 29 Aug '15, 00:25

Hendrikklaas's gravatar image

accept rate: 5%

edited 29 Aug '15, 22:38

It's very frustrating to realize that the only GPS instrument you can afford to buy can be off by so much that your recorded track runs through a river or lake. Unfortunately, that happens all too often, especially in hilly terrain or areas of heavy tree cover. That's why we upload many traces in cities for example. It's to help us position the aerial imagery better to get around the inherent inaccuracy of our consumer-grade GPS receivers.

I reckon you could reposition the river but how would you know exactly where to place it? Unless there are several GPS traces done either by walking its banks or floating it you cannot know if the river is out of place or your trace.

I'm afraid you're stuck.

(29 Aug '15, 00:48) AlaskaDave

To do it right with consumer grade GPS you really need to get tracks from different times to average out the errors due to satellite position and atmospheric conditions (nothing you can do about systemic errors from signal reflection off of landscape or buildings). So the more GPX traces the better.

In my area there a pretty large number of people using Strava to track their trail running and/or mountain biking. In the JOSM editor at least, there are layers you can turn on that shows a heat map the aggregated GPX data uploaded by those thousands of people over a very long period of time. I've taken to using that, where available, to verify and align trails. Sure beats the few GPX tracks that I can collect in an area or that others have uploaded to OSM.

Strava has a modified version of iD that can actually align selected portions of ways to the heat map(s). Maybe they've integrated that into the standard iD editor by now but since I mostly use JOSM I can't say.

(29 Aug '15, 01:16) stf

I usually use an average of all traces to fine tune paths and roads. This is simple to do with Potlatch2.

(29 Aug '15, 19:30) andy mackey

Hi Willi i noticed that you uploaded a trace several months back, do you still have and could you upload the trace for your Mt. Chocorua hikes so everyone could use it to decide using an average of several traces to check trails. When I fired up the Potlatch2 editor for the area in question with Bing background selected it looked as if the two together could work well. If you could then pin point some of the possible errors for us to consider? Bing looks quite good and it may be possible to see trails and stream beds with it. It is also possible that streams erode different routes over time and may have been imported or mapped a while ago. Could the streams when dry have been hiked along and gpxed then mapped as trails in some places, it that possible? Yes you can edit the line of the streams using all the information you have available. Geo located jpegs, notes either written and audio which can also be geo-located by way-marks or the time, gpx traces, aerial images, and your memory can be used.

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answered 30 Aug '15, 07:53

andy%20mackey's gravatar image

andy mackey
accept rate: 4%

edited 01 Sep '15, 09:18

Perhaps a good way to map a dry steam is to hike it when it is dry, the erosion may make the line obvious. See also https://help.openstreetmap.org/questions/4437/how-do-you-tag-a-dry-creek

(01 Sep '15, 09:22) andy mackey
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question asked: 28 Aug '15, 22:12

question was seen: 1,840 times

last updated: 01 Sep '15, 09:22

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