We need to be able to identify sinks

[timlinux]

Problem

Given a line layer, identify by selection all sink (points with no downstream nodes)

Proposed solution

The implementation should take the following steps:

• extract nodes with basic metadata (upstream count, related line id, related nodes etc.) as per #8
• add an attribute to the node table that indicates if a node is a sink
• Implement logic that determines sinks on the network based on the logic (in pseudocode):

IF upstream_node_count > 0 AND downstream_node_count == 0
THEN
    sink = True
ENDIF

The logic here being that if there is more than zero upstream node and there no downstream node, so this node must be the end of the water for this river segment.

• select all line features in the river layer whose starting node is a sink
• overlay the nodes layer on the river layer with sinks clearly symbolised (client to provide appropriate symbology, in the mean time we can just make up our own visual vocabulary).

  Expected outcome

• The extracted nodes layer should have a column added called sink and populated with 0 if the node is not a sink and 1 if it is.
• The line layer should have all segments selected whose origin node is a sink selected
• The user should be able to easily visually identify all segments whose origin node is a sink

Example nodes attribute table (excludes other extracted feature types):

id	upstream_node_count	downstream_node_count	sink
1	3	0	1
2	2	3	0