...
Of course, most standards include an ‘unknown’ value option for both the flow direction and the inspection direction, and in the data there could be situations where the definition simply does not exist. Both of these situations are considered to require will give the same results if they exist in the data.
If the flow direction is unknown or NULL, then:
For a downstream inspection:
Flow direction is A – B (notice that there is no arrow and we simply show the manhole that is defined in the upstream field on the left and the manhole that is defined in the downstream field on the right)
Start and end manholes are A → B (i.e. starts at A and ends at B)
Inspection direction is A – B (there is no arrow because the inspection direction cannot be defined without the flow direction being defined or vica versa)
For an upstream inspection:
Flow direction is A – B (as above)
Start and end manholes are B → A (as above)
Inspection direction is B – A (as above)
...
The only thing we can define clearly with any confidence in these cases is the ‘start’ manhole and the ‘end’ manhole.