In my last post, I used some SQL to help identify breaks in the OpenStreetMap's road map. If you recall, I created a table that contained a list of points and descriptions for each of those points. In this post, I'll use this table to create a WMS layer that will be overlaid onto an OSM map. The description field will be used to style the points so that I can easily tell what we need to do to repair them (eg, add nodes, move nodes, investigate further, etc). Then, I'll choose a tool to actually perform the repair and allow the rest of the community to benefit from these findings.
Tools
The defacto tool for this job is the Java OSM Editor. I checked out a few other tools, but this seemed to be "the one." It also supports viewing custom WMS layers, which will be really useful in a bit.
Method
Orienting Thy Self
First thing's first: I needed to orient myself on the map. By clicking Imagry-> OpenStreetMap, I added an OSM layer. From there, I zoomed into my target area. In addition to this, I added satellite imagery. In this particular case, I'm using some imagery provided by the MN DOT.
In order to make things easier on myself, I took the table that I made earlier (which I'll call, DanglingRoads) and made it into a WMS layer. This way, I can easily spot the places on the map that I need to edit. To make editing even easier, I've styled my new layer by the error messages that were logged while I was reviewing the possible errors.
Cool Layer, Bro
The process is pretty simple. First, we can use our favorite WMS server to create a layer. In my case, I use GeoServer. When adding the layer, make sure to make all columns of the DanglingRoads table available. We'll need them in order to style the layer. Next, I applied a style. I'm not going to post mine but I'm willing to supply it to those who ask.
Editing the Roads
In order to edit the roads, I needed to combine an OSM layer with the layer made from the DanglingRoads table. In order to add the new WMS layer, I had to go to Edit-> Preferences and then click on the Imagery Preferences button. From there, a custom layer can be added. In my case, I needed for the layer to have transparency. Because JOSME doesn't have an option for enabling transparency, I had to append this to the WMS query string (thanks to Phil Gold form the OSM Newbies mailing list):
format=image/png&transparent=true
From there, I zoomed in on every point that I had to edit. After zooming in, sometimes it's necessary to right-click on the layer's name and select Change resolution. Once you've zoomed in close enough, you can click on the Download map data... button. This will reach out to the OSM servers and grab a piece of mapping data. From there, you can edit it to your heart's content and then upload your changes (OSM registration required).
I should mention that the tool took some practice. I had to download a few different pieces of data, attempt to edit them, and then throw them away. I also had to make sure that I consulted the OSM documentation to make sure that I was retagging data correctly.
Afterthoughts
JOSM is a powerful tool, but it has its quarks. First and foremost, it has a built in (editable) memory limit. The first time that I used it, I experienced a crash due to the memory limit. Secondly, having to readjust the resolution of a layer each and every time I zoom in and out was a pain. I think that editing would have gone much smoother if JOSME just did this for me. My last and final gripe is that OSM doesn't support secure password authentication. I feel that any site should give me the option of not sending my passwords over plain HTTP.
Overall, I've had a great experience so far. The editing has been slow, but it's been smooth and easy. I still have more edits to commit, so I'm sure I'll discover new tricks along the way.
Thanks to all the folks at OSM for creating such an awesome product and set of tools!
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