User:Ff5722/Using Photoshop and Illustrator for tracing

In Photoshop

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

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