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Alternative strategies

Select all sidewalks within X meters of the intersection

  1. Select all sidewalks within X meters of the intersection

By studying 'eye test' method

Humans can figure out where to draw crossings pretty easily (even unmarked ones). How do they do it? Here's some guesses.

  1. Locate sidewalk corners. This might be just one sidewalk approaching the street, it may be a legit corner with 2 sidewalks meeting.

  2. If you could reasonably cross from corner to corner, draw a crossing.

  3. The location where the crossing connects to the sidewalk is somewhat subjective and up to the style of the mapper (for now). A natural place is along the line of the incoming sidewalk, if there are 2. Having them meet at just one location is fine for now.

So, the real strategy in code:

  1. Go up the street from the intersection X meters / half way up (whichever comes first). X could be ~10-15 meters. Return this point for each outgoing road.

Original-ish

  1. Go up each street in small increments (~half meter?), generating long orthogonal lines.

  2. Restrict to sidewalks within X meters of the street (something moderately big like 30 meters)

  3. Discard lines that don't intersect sidewalks on both the left and right sides. Can be done by taking the intersection and asking the left/right question

  4. Subset remainder to line segment that intersects the street (trim by the sidewalks)

  5. Remove crossings that intersect any other streets (i.e. crossing multiple streets). This should be done very carefully given that some streets are divided and have multiple intersections. osmnx tries to group these intersections but I don't know what happens to the lines. i.e. we don't want to discard all boulevards. Actual question is more complex? FIXME: revisit this step

  6. The first crossing remaining is probably legit.

FIXME: the problem with this approach is that it will miss the situation where there's only one sidewalk at the corner (one street doesn't have sidewalks, e.g.).

Original-ish 2

  1. Identify sidewalks to the left of street, to the right of street
  2. Find point on the street where an orthogonal line first intersects the sidewalk on the right and does not intersect any other streets.
  3. Repeat for the left side
  4. Whichever is farther down is the primary candidate.
  5. Attempt to extend this orthogonal line such that it spans from right sidewalk to left sidewalk.
  6. If this fails, find the closest point on the 'other' sidewalk to the point on the street. Draw a line from sidewalk to sidewalk.

Original-ish 3: ding ding ding!

  1. Walk down the street in X increments.
  2. Find a line between the street point and the nearest sidewalk to the right that does not intersect another street. One method for doing this is to split the sidewalks up by block beforehand. Alternatively, use the 'block graph' method used in sidewalkify to group sidewalks. Cyclic subgraphs = block polygons, acyclic sugraphs = tricky
  3. Repeat for the left side.
  4. Compare the lines: if they're roughly parallel, keep the crossing endpoints and draw a line between them. This is the crossing
  5. If the crossing is too long (40 meters?), delete it.

Note: rather than incrementing by small amounts and then stopping, this strategy could use a binary search such that an arbitrary parallel-ness could be found.