waterWayChecks.md

WaterWay Checks

Description

This check identifies waterways that are closed (i.e., would have a circular water flow), waterways that do not have a place for the water to go (a "sink"), and waterways crossing waterways. It also looks for ways that may be going uphill (requires elevation data, see ElevationUtilities)

Live Example

1. This canal (osm id: 46115760 version 5) is a closed waterway, which makes no semantic sense without a pump somewhere in the waterway. In this case, it should be tagged natural=water + water=canal.

2. The waterways at (osm id: 500672157 on 2020-08-20) cross each other, when they should be connected.

Useful configuration variables:

{
    "WaterWayCheck": {
        "waterway.sink.tags.filters": "natural->sinkhole|waterway->tidal_channel,drain|manhole->drain",
        "waterway.tags.filters": "waterway->river,stream,tidal_channel,canal,drain,ditch,pressurised",
        "ocean.valid": "natural->strait,channel,fjord,sound,bay|harbour->*&harbour->!no|estuary->*&estuary->!no|bay->*&bay->!no|place->sea|seamark:type->harbour,harbour_basin,sea_area|water->bay,cove,harbour|waterway->artificial,dock",
        "ocean.boundary": "natural->coastline",
        "waterway.elevation.resolution.min.uphill": "1" (meter),
        "waterway.elevation.resolution.min.start.end": "457.2" (meters)
    }
}

If you are using the NASA SRTM elevation files, their resolution is either 30m or 90m. This means that the elevation checks will ignore uphill waterways by default when using the NASA SRTM files.

You may also want to look at ElevationUtilities.

Code Review

In Atlas, OSM elements are represented as Edges, Points, Lines, Nodes & Relations; in our case, we’re are looking at LineItems, which Lines and Edges are specific subtypes of. This is due to the fact that a waterway may either be a navigable way or a non-navigable way (rivers are generally considered navigable, while streams may or may not be navigable).

Our first goal is to validate the incoming Atlas object. Valid features for this check will satisfy the following conditions:

• Must be an LineItem with one of the following tags:
  • waterway=RIVER
  • waterway=STREAM
  • waterway=TIDAL_CHANNEL
  • waterway=CANAL
  • waterway=DRAIN
  • waterway=DITCH
  • waterway=PRESSURISED

    @Override
    public boolean validCheckForObject(final AtlasObject object)
    {
        return !isFlagged(object.getOsmIdentifier()) && object instanceof LineItem
                && this.waterwayTagFilter.test(object);
    }

After the preliminary filtering, each object goes through a series of if statements. The first checks if the line is closed. The second checks if the waterway is going uphill (requires elevation data), and if the resolution of the elevation data is good enough to determine that the waterway goes uphill, the object is flagged. At this point, we check to see if the waterway ends in a sink, for this check, we attempt ensure that the waterway ends inside the boundaries, and not in a neighboring area. Furthermore, we reuse the check for uphill ways to help improve the error message. Once all of those checks have finished, we check for waterway intersections. If more than one check flags the object, instructions and offending objects are appended to the CheckFlag.

To learn more about the code, please look at the comments in the source code for the check. WaterWayCheck.java