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It's quite fun trying to work out where an actual walking track is based on a printed topographic map (has good coverage of advertised official tracks but misse

by strken 3y ago

It's quite fun trying to work out where an actual walking track is based on a printed topographic map (has good coverage of advertised official tracks but misses some), OSM (has nearly everything but tends to hallucinate non-existent tracks), the output of a local map-maker (compiles OSM and other sources but sometimes has the same track twice in slightly different places), and Google etc. (doesn't show anything a hatchback can't traverse, but has good satellite imagery that can show some tracks). This weekend I'm going to go explore one of the OSM tracks and see where it's a total hallucination, old and overgrown, or real.

RicoElectrico 3y ago
What do you mean by "hallucinate"? FYI most of the time it's a human drawing what they're seeing on the aerial imagery. I know many instances in which someone mapped a track through a field which subsequently got plowed by the field owner, but otherwise I have no idea. Could you tell any specifics?
- mike_d 3y ago
  Imagine you draw a line following a trail on a map. Looks good, ship it! Now someone else loads that same line on their computer and it looks like a jagged mess because they are twice as zoomed in on the map as you were. Oops. What about the wide parts vs narrow parts of the trail? Do you represent that as two lines or just keep one down the middle? You send your trail to a friend who overlays it on their map that uses a different datum? How were the photos from the airplane assembled, and how did they represent a round object on a flat image? Did they have a good GPS lock when they took the photos, or did they even bother to check? What if the line you created is to be used 100 years in the future when the center of mass of the earth or magnetic north has shifted, how does your coordinate system account for that? I know just about enough GIS stuff to fit on a few index cards, but I quickly realized it is way more complex than how simple Google Maps makes it out to be.
- strken 3y ago
  Sorry, hallucinate in the sense that the map says there's a track but there isn't anything there, whether or not it was drawn by a human. I meant it as an allusion to the map tripping balls but forgot that the overlap with AI terms would confuse HN. With OSM I think a lot of walking tracks are from GPS traces uploaded by volunteers, so maybe sometimes stuff sneaks in. Or maybe the map shows old tracks that have since been erased by rangers. Or maybe there's a nearby track that has been marked down in the wrong place.

by strken 3y ago

RicoElectrico 3y ago

What do you mean by "hallucinate"? FYI most of the time it's a human drawing what they're seeing on the aerial imagery. I know many instances in which someone mapped a track through a field which subsequently got plowed by the field owner, but otherwise I have no idea. Could you tell any specifics?

mike_d 3y ago
Imagine you draw a line following a trail on a map. Looks good, ship it! Now someone else loads that same line on their computer and it looks like a jagged mess because they are twice as zoomed in on the map as you were. Oops. What about the wide parts vs narrow parts of the trail? Do you represent that as two lines or just keep one down the middle? You send your trail to a friend who overlays it on their map that uses a different datum? How were the photos from the airplane assembled, and how did they represent a round object on a flat image? Did they have a good GPS lock when they took the photos, or did they even bother to check? What if the line you created is to be used 100 years in the future when the center of mass of the earth or magnetic north has shifted, how does your coordinate system account for that? I know just about enough GIS stuff to fit on a few index cards, but I quickly realized it is way more complex than how simple Google Maps makes it out to be.
strken 3y ago
Sorry, hallucinate in the sense that the map says there's a track but there isn't anything there, whether or not it was drawn by a human. I meant it as an allusion to the map tripping balls but forgot that the overlap with AI terms would confuse HN. With OSM I think a lot of walking tracks are from GPS traces uploaded by volunteers, so maybe sometimes stuff sneaks in. Or maybe the map shows old tracks that have since been erased by rangers. Or maybe there's a nearby track that has been marked down in the wrong place.