In a previous post, I described how we’ve carried out a first foray into trail curation on OpenStreetMap to deal with unauthorized trails appearing on the map. Rather than deleting them, we’ve changed their tags so they’re easily filtered out of the map but still present, so later mappers understand why those paths visible in satellite imagery aren’t showing up on their favorite OSM-based map.
Ok, so far so good. but how many trails did we find and change? And how did we find them?
Building on work we did with Trailhead Labs for the Department of Interior’s Dev4Outdoors hackathon (more on that here), we created a functional if not so pretty layered map to highlight the differences between OpenStreetMap data and official data from OneTam, the cosortium of agencies that manage these public lands.
We found 19 trail segments in OSM that did not appear in the offical data and appeared to be social trails. We tagged them as such, and you can see the edits in these viewers:
- http://nrenner.github.io/achavi/?changeset=34599982
- http://nrenner.github.io/achavi/?changeset=33656715
- http://nrenner.github.io/achavi/?changeset=33655914
- http://nrenner.github.io/achavi/?changeset=33655830
It’s important to note that we didn’t delete these trails. Instead we tagged them as “social_path”, which allows us to filter them out of our maps, and also allows anyone else to remove them from display, while leaving them in the source data. That’s important so that future volunteer mappers know why trails they see on the ground or in satellite imagery are not showing on public-facing maps derived from OSM data.
The idea is to acknowledge that the paths exist but flag them so that it’s clear that they are not authorized and should not be used. Also, in some cases, we split a trail up to show that one part is authorized and the other is not.
The method we used to create the “visual diff” of official vs. OSM trails is rather involved and relies on CartoDB.com, Stamen’s Sandwich Maker utility, and a desktop OSM editor called JOSM. Not for the faint of heart and best set up when you need to survey a large area, since you can see differences and make edits right in the same window.
But any time you see an errant trail on an OSM-derived map, like the one we use on CaliParks.org, you can head over to openstreetmap.org, create an account, log in, and edit the trail right in your browser. And you can do some pretty good comparing in your browser over at TrailEditor.org’s fusion view.
Note that we also added one trail (the Music Stand Trail) that was missing from OSM, which is great. What we didn’t do was try to judge whether actual trail routes were better on OSM vs. the official data. There are some really promising ways to do that, including using Strava’s heatmaps to see where people are actually walking and biking.