OpenStreetMap

How I Map: TIGER Cleanup

Posted by asciiphil on 16 February 2012 in English (English)

This is the first in what will probably be a very occasional series about how I do things in OpenStreetMap. In this post, I'll discuss how I improve the quality of TIGER-imported roads.

In general, I have at least a two-stage process for working on an area. In the first stage, I armchair-map: I use the USGS orthoimagery available for my state (which is 6-inch resolution and excellently rectified and georeferenced) to trace the roads and other major features in the area. I'll generally double-check that tracing with NAIP to make sure I'm not uploading old features. In the second stage, I use Walking Papers to get a printout of the area (usually in separate, page-sized chunks) and drive through it, verifying the road names.

What I'd like to talk about, though, are a couple things that are specific to how I work with TIGER-sourced ways and how they fit into my general workflow.

tiger:reviewed

There are many different approaches to this tag. I use it as a marker as to which stage of my editing a road is in. If its value is "no", either I haven't worked with the full length of the way or I did so before I settled on my current workflow. When I align the way to aerial imagery (using JOSM's excellent Improve Way Accuracy mode), I change the value to "position". Finally, when I've verified the name via ground survey, I remove the tag.

For roads that don't typically have names, like _link roads or service roads, I just delete the tiger:reviewed tag after aligning them to aerial imagery, unless TIGER appears to have given a name to the road anyway. I see the last case most often with personal named driveways.

I've written a couple of things to assist my particular use of the tiger:reviewed tag. For JOSM editing, I wrote a style that highlights "tiger:reviewed=position" in a light green, similar to the way "tiger:reviewed=no" ways are highlighted in yellow. I've made a screenshot of the highlighting and the CSS is in TIGER-aligned.css.

I also have a mapnik stylesheet that makes color-coded overlays for my personal rendering so i can see an area's status at a glance. I use a yellow overlay for tiger:reviewed=no, a green overlay for tiger:reviewed=position, and a red overlay for roads with no tiger:reviewed tag and no name tag. The stylesheet is TIGER.xml and it needs include files dbsettings.inc, extents-us.inc, and mapnik-utils.inc. It's really specific to my rendering, but here's a sample rendering with the overlay composited onto it that shows some of each case.

TIGER 2011

As I work through an area tracing things from aerial imagery, I often find roads that are new enough that they weren't in the TIGER 2005 data. After I've traced such a road, I turn on Ian Dees' TIGER 2011 road tiles to see if I can get the name from there. If I can, I tag the way with "source:name=TIGER 2011" and "tiger:reviewed=position", since I consider any TIGER-sourced data as needing ground verification.

Road Classification

As I'm working on an area, I also change road classifications as needed. In my experience, TIGER data seems to underclassify roads a lot (which is probably better as a default than classifying roads too highly). In my area, contributors before me have generally done a good job (by which I mean I don't usually disagree strongly enough to alter someone else's contribution) assigning classifications from secondary and up. I do a lot of conversions from residential to unclassified or tertiary, though.

Changing a road to unclassified is generally an easy decision. If it's a minor road that's not in a residential area, I make it unclassified. I also use unclassified for roads that are connectors in more rural areas but that I don't feel warrant tertiary classification.

Changing a road to tertiary classification is more of a judgement call. I'll usually do it if: the road has a divider painted down the center, indicating that it's expected to get a fair amount of traffic; the road does not have traffic calming features like speed humps and islands, since those generally indicate that a community wants to limit the amount of through traffic; and there isn't a better road that people should be using instead. (I have to thank mdroads for sharing with me his classification rules of thumb, which I adopted after I saw how well they fit the way I thought about the roads I knew best.) In urban and suburban areas, I also try to keep the ends of tertiary-classified roads on other roads of the same or higher classification, so as to emphasize the road network.

Conclusion

I hope I've been informative with this post. I welcome any comments, questions, or other feedback.

Comment from skorasaurus on 16 February 2012 at 15:39

I meant to send this as a comment instead of a private message...

With road classification, I use the same practices as you for TIGER (here in Cleveland, OH). I also find tertiary relatively ambiguous, with no absolute rules of when it should be applied. I usually decide whether it's tertiary [or unclassified or residential] by a combination of factors: if the speed limit is 35, as opposed to 25 in most residential, the amount of businesses/other uses other than residential on the way, the amount of traffic it receives [relative to roads in the area], the # of lanes, and last, certainly, not least, just a general feel.

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Comment from aude on 18 February 2012 at 01:42

It's helpful to learn your methodology. I'll try the tiger:reviewed tag. :)

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