We had approximately ten people at the Silver Spring mapping party last Saturday. This was the first for the new Mapping DC group, following on from a mapping party in June that was organized by CloudMade (now former) community ambassador Russ Nelson.

We are focusing on areas surrounding DC proper, as the DC government has shown willingness to allow us to import their GIS data. That’s not the case for surrounding jurisdictions such as Montgomery County, Maryland.

My slice of the “cake” was the area between (south) of the railroad tracks and the DC border. I added the buildings along East-West Highway, including the NOAA buildings. I also added the Falkland Chase apartment complex and the Blair apartment complex — both which occupy an entire square block with numerous individual apartment buildings and shops. Behind the Falkland Chase apartments was a small park, with a creek, walking paths, and an off-lease dog area.

Near the Mayorga coffee shop where we met, there is a brand new apartment building (The Veridian). When mapping this, the Yahoo! imagery was uselessly way out of date. Blair Mill Road used to cut through from Georgia Avenue to East-West Highway. The road has been reconfigured as Blair Mill Way, and does not connect with the rest of Blair Mill Road on the south side of East-West Highway. I got this change and the new building into OSM. I notice that Google Maps is still outdated with the old Blair Mill Road.

I’m not done with making edits to my area, and other people are still working on theirs. But, here are images that show before and after.

Before:

After:

To get around the copyright issues, much of the OpenStreetMap data is collected in the field by volunteers, using GPS units. GPS works by triangulating signals from U.S. government (military) satellites, and calculating a location.  This requires a clear line of site to the satellites, thus one needs to be outdoors in order to collect GPS data. GPS clearly will not work in a tunnel. Yet, one feature that would really be useful to have on OpenStreetMap are subway lines.  How do we map underground subway lines when GPS units won’t work there?

I had added the Silver Spring Metro station to OSM, but Metro’s Red Line was missing from OSM.  The Silver Spring station is above ground, as Metro’s Red Line from Silver Spring to Union Station follows along the CSX freight train tracks.  So, it’s possible to map this portion with GPS or via aerial photo mapping (from the Yahoo! imagery).  North of Silver Spring (to Glenmont), the Red Line goes below ground again.  The next metro station (Forest Glen) is so deep below ground that the station has only elevators and no escalators.  The Red Line is also below ground from Union Station to the Grosvenor station.

When building something so deep that carries large numbers of people, it was necessary for the tunnels to have emergency exits located between the stations, along with ventilation shafts and other infrastructure that comes up to the surface.  These are fairly large and can be easily spotted, and often are marked as belonging to Metro.  The surface infrastructure often consists of  large vents, often with a fire standpipe and sometimes with an emergency exit (which is locked from the outside, though can be opened from the inside).  Sometimes, there also are small buildings, such as power substations.

To map the subway tunnel, GPS measurements can be taken for the surface infrastructure, which along with the stations, provide enough nodes to mark the location of the Red Line, even when it is underground. The subway lines do not have any sharp turns, but have gradual turns where needed and these can be interpolated.

Also, in the case of Metro, portions of the Red Line follow beneath major roads (Georgia Avenue from Forest Glen to Glenmont; Connecticut Avenue from Dupont Circle to Van Ness, and Wisconsin Avenue / Rt 355 above Tenleytown).  Between Van Ness (on Connecticut Ave) and Tenleytown (on Wisconsin Ave), the Red Line cuts across, beneath Yuma Street.  This general knowledge can also help in marking the Red Line on OpenStreetMap.

Noyes St substation:

16th St:

Seminary Place:

At the Beltway:

Just south of Forest Glen:

Above Forest Glen station: