Missing elevation data for specific routes

[nrenner]

Reported in nrenner/brouter-web#147 by @MichaelK92.

http://brouter.de/brouter-web/#map=17/44.44707/15.18322/OpenStreetMap&lonlats=15.183293,44.447082|15.181603,44.448656

    "coordinates": [
      [15.183296, 44.447090],
      [15.183191, 44.447111],
      [15.183030, 44.447200],
      [15.182328, 44.447885],
      [15.181645, 44.448615, 37.5],
      [15.181602, 44.448655, 37.5]
    ]

Moving the start point past the previous node in the curve, some elevation data is filled in:
http://brouter.de/brouter-web/#map=17/44.44707/15.18322/OpenStreetMap&lonlats=15.18339,44.447082|15.181603,44.448656

    "coordinates": [
      [15.183390, 44.447079, 27.0],
      [15.183354, 44.447079, 27.5],
      [15.183191, 44.447111, 27.5],
      [15.183030, 44.447200, 27.5],
      [15.182328, 44.447885, 27.5],
      [15.181645, 44.448615, 37.5],
      [15.181602, 44.448655, 37.5]
    ]

Ways:
https://www.openstreetmap.org/way/253261970
https://www.openstreetmap.org/way/253354195

[bagage]

Another example (bridge is the issue): https://brouter.damsy.net/latest/#map=15/45.1781/4.8188/standard&lonlats=4.814286,45.18863|4.811883,45.184122|4.814329,45.180613

[nrenner]

Out of interest I took a closer look at the elevation data for those cases (GeoTIFF from http://srtm.csi.cgiar.org/srtmdata/, converted to BIL format, and to ASCII for debugging with MapcreatorTest.java):

Coastal road, Rudelić-Draga

http://brouter.de/brouter-web/#map=18/44.44725/15.18269/osm-mapnik-german_style&lonlats=15.181603,44.448656;15.183572,44.447143

Screenshot leaflet-raw-dem srtm_40_04_bil.zip

The red cells are voids (no data), the CGIAR CSI SRTM has shorelines cut out (FAQ).

BRouter interpolates height within a block of four cells, if one of these cells has no data, the elevation of the node is also set to no data:

31  54    
11  30    30  56
          nd  27    27  40    40  25
= 37.5              nd  16    16  11
          = n/d          
                    = n/d     = 27.5
                      

brouter/brouter-map-creator/src/main/java/btools/mapcreator/SrtmRaster.java

Lines 49 to 62 in ab2f5e3

	double eval = (1.-wrow)*(1.-wcol)*get(row ,col )
	+ ( wrow)*(1.-wcol)*get(row+1,col )
	+ (1.-wrow)*( wcol)*get(row ,col+1)
	+ ( wrow)*( wcol)*get(row+1,col+1);
	// System.out.println( "eval=" + eval );
	return missingData ? Short.MIN_VALUE : (short)(eval*4);
	}
	private short get( int r, int c )
	{
	short e = eval_array[ (nrows-1-r)*ncols + c ];
	if ( e == Short.MIN_VALUE ) missingData = true;
	return e;
	}

Ferry, Bremerhaven

http://brouter.de/brouter-web/#map=12/53.5235/8.5707/osm-mapnik-german_style&lonlats=8.584442,53.538267;8.565903,53.529595;8.545475,53.520411

Screenshot leaflet-raw-dem srtm_38_02_bil.zip

The shoreline with no data reaches into the river mouth where the ferry crosses.

Bridge, Saint-Vallier

https://brouter.damsy.net/latest/#map=15/45.1781/4.8188/standard&lonlats=4.814286,45.18863;4.811883,45.184122;4.814329,45.180613

The river has elevation data (srtm_37_03_bil.zip), but BRouter is removing elevation from bridges and tunnels, in order to interpolate between start and end elevation:

brouter/brouter-map-creator/src/main/java/btools/mapcreator/OsmNodeP.java

Lines 52 to 56 in c89058e

	public short getSElev()
	{
	// if all bridge or all tunnel, elevation=no-data
	return ( bits & NO_BRIDGE_BIT ) == 0 || ( bits & NO_TUNNEL_BIT ) == 0 ? Short.MIN_VALUE : selev;
	}

This just doesn't work if a route point as start of a segment is on the bridge.

So this is generally working as designed and these exceptional cases are probably hard to improve, therefore closing.

edit: fix interpolation blocks layout, last block no vertical offset