Should river lines be mapped through lakes, estuaries, gulfs, and other large water bodies?

General talk Tagging general discussion
49

NHD and other U.S. federal government datasets such as TIGER and ARNOLD are used for not only cartography but also network analysis, statistical analysis, and other purposes. These use cases aren’t entirely out of scope for OSM, as @amapanda_ᚐᚋᚐᚅᚇᚐ’s river basin map demonstrates.

NHD’s standards describe artificial paths as being useful for hydrologic modeling (i.e., where will my message in a bottle end up):

An Artificial Path is an abstraction to facilitate hydrologic modeling through open waterbodies.
[…]
Artificial paths shall be placed in all polygons except isolated reservoirs, isolated lake/ponds, and isolated basins (reservoirs, lake/ponds, and basins not connected to the stream network). Artificial paths shall represent the shortest path from the inflow to the outflow without crossing through banks or islands.

The draft standards for NHD’s original 1999 edition (pages 2-8–2-9) specifies some limits:

The limit of ARTIFICIAL PATH is:

  • the connection between the inflow and outflow points of an in-line open water body;
  • the line through a head or terminal open water body that connects to the inflow or ouflow point;
  • shorelines for the Atlantic Ocean, Pacific Ocean, Gulf of Mexico, and the Great Lakes

NHD conforms to the Geographic Information Framework Data Content Standard, which also calls for artificial paths:

This part of the standard supports a continuous linear representation of the surface water
network, although it permits the management and exchange of non-networked features. The
network is composed of features that are represented as line strings whose connectivity
relationship is known. The logical network is composed of linear representations of hydrography
features and centerlines (artificially derived flow paths) within area representations such as
reservoirs, rivers, and lakes. A “flows-through” relationship exists between centerlines and the
area feature representations through which they flow. The geometric representation of features
supported by this part of the standard includes point, line, and area representations only
(GM_Point, GM_Curve, GM_Surface).

Network analysis is obviously a motivation here, but artificial paths are also used in cartography. For example, here’s The National Map, a map produced for a general audience by the federal government, overlaid with NHDPlus High Resolution artificial paths and their names, zoomed in on a reservoir in Ohio. The basemap’s labels are clearly based on artificial paths, even if the paths themselves are invisible:

William H. Harsha Lake with the East Fork Little Miami River running through it on The National Map. The inundated river’s path is labeled along NHD artificial paths, which are overlaid in purple.
William H. Harsha Lake with the East Fork Little Miami River running through it on The National Map. The inundated river’s path is labeled along NHD artificial paths, which are overlaid in purple.1920×1148 177 KB

If we compare these artificial paths to old topographic maps, notice how the artificial path doesn’t strictly follow the reservoir’s centerline, but neither does it follow the historical path of the river, which remains as a channel that meanders back and forth along the lakebed. I don’t know exactly what process the USGS used to derive the path from the lake’s geometry, but it is pleasant to look at. As it happens, the topo map also labels the lake along the artificial path, more or less, ignoring the river channel:

William H. Harsha Lake with the East Fork Little Miami River running through it on a mosaic of two USGS topographic maps, with NHD artificial paths again overlaid in purple. The topo map to the left shows the reservoir with the river channel winding through elevation contour lines, while the topo map to the right shows the reservoir in red edited onto an older map from before the reservoir’s completion. In the newer map, a significant meander to the north is bypassed in favor of a shorter artificial path, forcing its tributaries to the north, Slabcamp Run and Elk Lick, to run along straight lines to meet it south of the former natural mouth.
William H. Harsha Lake with the East Fork Little Miami River running through it on a mosaic of two USGS topographic maps, with NHD artificial paths again overlaid in purple. The topo map to the left shows the reservoir with the river channel winding through elevation contour lines, while the topo map to the right shows the reservoir in red edited onto an older map from before the reservoir’s completion. In the newer map, a significant meander to the north is bypassed in favor of a shorter artificial path, forcing its tributaries to the north, Slabcamp Run and Elk Lick, to run along straight lines to meet it south of the former natural mouth.1920×1148 327 KB

I have previously argued that any waterway we draw through a reservoir should ideally hew to the river channel where known. This is perhaps not as convenient for labeling as a hand-drawn artificial path, but it’s consistent with how waterway=river should ideally hew to the thalweg of a river. But even if we decide to keep it simple like the mapmakers above did, it would still be consistent with the guidelines for mapping natural=valley ways without fussing over every meander.

Regardless, to the extent that artificial paths through waterbodies intentionally hew to a “centerline” rather than the thalweg, I would support some kind of tagging to clarify the intent. waterway=artificial seems like a reasonable tag for an artificial path. It’s documented as deprecated out of an assumption that it was just another mistake from the NHD imports, but if the community agrees that the concept has merit, we can simply relegitimize its thousands of occurrences across the country. Alternatively, artificial_path=yes, used mainly in the UK, would enable us to keep the river/stream/canal distinction in waterway for backwards compatibility.

3 Likes