- Open image file in Photoshop (should be in the correct projection)
- Use magic wand or quick selection tool to select desired feature
- If the selection edges are also containing undesired features, choose from the menu Image > Adjustments > Black and white, and set the level for the desired colour at minimum to sharpen the edges
- Open the paths window (menu>window>paths)
- In the top right of the path window, click the drop down menu
- Click 'Make work path'
- Tolerance: 1px for low res imagery
- Menu > File
- Export > paths to illustrator > Save as *.ai
- Open the file you exported
- File > Save As > Save as SVG
- Get the ImportVec plugin if you haven't already
- Drag the SVG to JOSM window
- It may be stuck at 'opening files', simply close that dialog
- In the import vector graphics dialog set the scale. For example, if your source image has a resolution of 10m per pixel, and you saved paths with a resolution of 1px, then when converting back, you should again choose 10m per unit.
- Curve steps is at 4 by default, this should be ok, but you can simplify later.
- Click OK in the dialog
- Move around the imported ways to align them. Depending on the projection, you may have to split and align parts individually.
- Simplify if needed
- Add appropriate tags
- Validate and upload