[Imports-us] Baltimore County Maryland Hydrology Import

Elliott Plack elliott.plack at gmail.com
Fri Sep 5 14:32:42 UTC 2014


Now that the building and address import is complete, I am moving on to
some of the planimetrics that make OSM look good but also contain useful
information. Parts of the county have a very good, dense network of
streams, traced by a few users, but it can be difficult to spot some
streams amidst tree cover on air photos. In these areas where hydrology
(streams, rivers, ponds, storm water management, piers, etc) exist, I am
proposing to import public domain Baltimore County hydrology, while taking
care not to overwrite anyones hard work.

Before I write up a wiki, here are the basic details:


   1. I made a model that simplifies the polyline hydrology feature class
   with a simplification factor of two feet to prevent "overnoding". In
   experimenting, two feet between nodes keeps the data simple on the map, but
   typically doesn't result in it looking too jagged. The source data is drawn
   with a digital pen and then converted to polyline, so any given 10 foot
   segment might have 500 nodes in the source.
   2. I ran Paul Norman's ogr2osm on the result from step one, with a
   custom translation to convert the various feature types in the source data
   to something compatible with the OSM key/value data model. Here is that
   translation: https://gist.github.com/talllguy/52066d13d323f1535f3a
   3. Here is the resulting OSM file of that translation:
   http://1drv.ms/1lLHSIm
   4. In JOSM I look for a stream system that is missing. I disconnect the
   last segment that would intersect an existing water feature already on the
   map, like a river or coastline. I use the 'select all connecting ways' tool
   to select the entire system and then copy that to a new layer and download
   existing OSM data to that layer. (some tags are purposefully translated
   with no type, so I have to go back and correct them individually and check
   the data)
   5. I run the validator to look for issues. Null names, and nodes that
   don't quite connect are easy to find this way.
   6. Upon solving all validator issues I import the system.

Notes:

   - I'm not going to mess with importing coastline. Our coast is pretty
   good and there are too many ways to introduce bugs when importing coastline.
   - Some of the features like boat docks, ramps, and piers may be too
   complicated for import. Those I'd address individually and may just use a
   node for leisure=slipway.
   - For larger rivers where there is a riverbank in the data, I'll use the
   approved water=* tagging.

Questions:

We have data on where the coastline is comprised of bulkhead, like where
the coastline is unnatural. Is that relevant to the project? I haven't seen
a bulkhead type tag but it could be considered a seawall.

As a test, I uploaded a small stream system that had not been traced at
all, the Sawmill Branch, so you all can see how it'd look on the map. That
system is here: http://www.openstreetmap.org/#map=15/39.5217/-76.5440

This is will be a very manual process and as a steward of open data, I will
take great care not to undo other mappers hard work.

-- 
Elliott Plack
http://about.me/elliottp
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