User:Ff5722/JOSM road tracing
Road tracing using JOSM
When mapping unmapped areas from satellite imagery, one frequently needs to trace roads from satellite imagery. JOSM is the best editor for this, as it offers several methods to trace roads, with each method having its advantages. All in all, this can speed up mapping dramatically, and also give more accurate and smoother end results.
1 standard drawing mode
This mode draws straight lines between the points you create with a mouseclick. The advantage is that you can place each node very accurate, and the line in between will be totally straight. The downside is that you have to click for every node, which, for bendy roads, is time consuming and RSI inducing.
There are some ways to enhance this however:
Useful for when you are drawing streets in a grid. CTRL+L to straighten the way.
Draw a way with two nodes. Press CTRL+O to make a circle out of this way. Must have for drawing roundabouts. Disadvantage: the number of nodes of the circle is the same, no matter it's radius, so huge roundabouts or circular roads look rough.
Alternatively, draw a closed loop way, then press O to make it circular. If you have a large rough circular way, you can add a handful of nodes distributed anywhere on the circle, then press O to make it round and smooth.
Another trick for this circle tool is to draw symmetrical closed polygons. Three nodes + O makes an equilateral triangle, 4 nodes a perfect square, 5 nodes a pentagon, etc...
JOSM/Plugins/Curves (is now part of JOSM/Plugins/utilsplugin2, which is a must-have anyway). If you press SHIFT+C while drawing a way, JOSM will make a semi-circle out of the last three points of the way. As with the circle tool, the number of nodes only depends on the curvature, so wide bends look rough.
You can also utilize this tool for bends that are not perfectly semi-circular. Most bends are a combination or can be approximated by more than one semi-circular parts. Experiment what looks best, pressing CTRL+C after one, two or three additional nodes.
CTRL+F enters FastDraw mode. You click once with the mouse to set the start node, then hold in space and drag your mouse to draw the way. If you come close enough to an existing node, a connection will be added. Press backspace to remove the last node. When you want to save the way, press ENTER once to simplify, and again to make the way permanent. Only after the first ENTER you can still undo the simplification by pressing backspace.
The simplification in general removes redundant nodes on straight parts of the way, but you also lose some detail in gentle curves. In FastDraw mode, you can also add nodes that will always be kept after simplification, by using mouse clicks like in standard drawing mode. A trick to adjust the simplification level without digging into the settings, is to zoom in or out before you simplify, so that more or less nodes will be removed.
Another useful trick is that you can exit FastDraw mode to add features in the standard drawing mode, and then go back in FastDraw mode to continue where you left off.
Advantages: very little mouse clicking, (even for bendy roads), simplification means that you don't have to correct every error (they will be smoothed out)
Disadvantages: need to zoom in (and thus pan often) to get nice detail level, simplification algorithm can be hit or miss and you risk losing carefully added detail if you press ENTER twice carelessly. Depending on how steady your hand is, you will need to do some offset adjustments later.