iD is good for editing at a small scale. You can add some buildings, some roads, fix invalid data, and everything else.
JOSM is meant for larger scale edits. It isn't as simple to use as iD, but can let you change tons of features at once. For example, it's possible to download an entire neighborhood, and then select buildings that are in a certain size range and not attached to any other features, and then square all of them at once. Or, you could use plugins to quickly trace building footprints faster than with iD using a plugin.
So basically, JOSM doesn't do anything iD can't do, but it does let you do the same things more efficiently at a large scale. I still prefer iD for most of my changesets because it's easier to start up then JOSM.
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iD is good for editing at a small scale. You can add some buildings, some roads, fix invalid data, and everything else.
JOSM is meant for larger scale edits. It isn't as simple to use as iD, but can let you change tons of features at once. For example, it's possible to download an entire neighborhood, and then select buildings that are in a certain size range and not attached to any other features, and then square all of them at once. Or, you could use plugins to quickly trace building footprints faster than with iD using a plugin.
So basically, JOSM doesn't do anything iD can't do, but it does let you do the same things more efficiently at a large scale. I still prefer iD for most of my changesets because it's easier to start up then JOSM.