Low-tech method: Drawing smooth tunnel centerlines with constant node spacing in JOSM using a physical circle overlay

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Low-tech method: Drawing smooth tunnel centerlines with constant node spacing in JOSM using a physical circle overlay

Hello everyone,

I would like to share a very simple, low-tech technique I use to manually draw tunnel centerlines (or other strongly curved ways) in JOSM with almost perfectly constant node spacing — usually 20–35 meters — even on tight curves.
This helps Mapillary interpolate the sequence very smoothly without visible bunching or stretching.

The method requires no plugins, no scripts and almost no money — just a transparency sheet, a compass and JOSM’s built-in auto-centering.

How it works – step by step

  1. Create a physical reference circle

    • Take a clear overhead transparency foil (or any thin transparent plastic sheet).
    • Use a drawing compass to draw one or more circles with the radius that corresponds to your desired node spacing at your usual zoom level.
      Example: At my typical zoom I need ~30 m spacing → I draw a circle with ~14–16 cm radius on the foil (you have to calibrate this once for your monitor + JOSM zoom).

  2. Attach the foil to the screen

    • Fix the transparency to your monitor with small pieces of removable tape so it cannot move.

  3. JOSM setup

    • Load OSM Carto (or another background) at ~40–60 % opacity.
    • Zoom to the level where one “step” along the way roughly matches your desired distance (e.g. 30 m).
    • Activate Auto-center on cursor → shortcut: Ctrl + Shift + F (very important!)

  4. Drawing the tunnel way

    • Start a new way (or continue an existing one).
    • Place the center of your drawn circle exactly on the blue follow-cursor crosshair in JOSM.
    • For every new node: move the mouse until the cursor touches the edge of the circle → left-click.
    • After each click JOSM auto-centers → the circle center jumps to the new node → next click again has to be on the circle edge → constant distance.

→ Result: you get a very regular node spacing even on strong bends, while still being able to closely follow the visual tunnel path or the OSM way.

Advantages

  • Geometrically exact spacing (real circle, not algorithm approximation)
  • Complete visual control – you decide where the centerline really belongs
  • Zero software dependencies → works with any future JOSM version
  • Very cheap (~3–8 € for foil + compass)
  • Easy to teach to other contributors

Important note about GPX export for Mapillary

When you export such a hand-drawn way as GPX, the file naturally contains only the coordinates of the nodes you placed — no timestamps are included.
To make the GPX usable for Mapillary (which relies on time information for correct sequencing and speed estimation), you still need to calculate / interpolate reasonable timestamps afterwards.

For one possible approach to fill in the missing times see this older thread:

Limitations

  • Manual clicking → slower than automatic tools
  • Requires initial calibration (circle size + zoom level)

Help

  • Any minor irregularities that appear on long straight parts can easily be smoothed out
  • afterwards using JOSM’s built-in Shift + B function (Align Nodes in Line).

Has anyone tried similar low-tech tricks or even improved variations?
I’m especially interested in:

  • How other people determine / calibrate the correct circle radius for different screens
  • Whether printing transparent radial patterns or protractor-like overlays would be even more convenient
  • Experiences with constant spacing vs. variable spacing for Mapillary interpolation quality

Looking forward to your feedback and possible refinements!

Best regards
osmplius_org

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