Geoscience Reference
In-Depth Information
common in braided rivers where the slope is not very steep and the length
of each section is small.
An uphill edge happens when an error in the z values of an edge turns its
flow direction uphill. This error, caused by a high tolerance on the z values
accuracy, is troublesome to detect. In an uphill edge,
cpMax
and
cpMin
are
inverted and fathers and children edges are classified as siblings, actually
disconnecting the graph and preventing the generalization algorithm to
work on all the edges down hill from the current edge.
The algorithm tries to detect the uphill edges: if an edge has only siblings,
the connected edges are classified again inverting
cpMax
and
cpMin
and if
this new classification provides at least one father and one child, the edge
is flagged as uphill and the latter classification is kept.
Unfortunately not all the uphill edges can be detected; since flat edges are
much easier to treat, it has been chosen to reduce the number of possible
uphill edges using a z threshold
zT
: if
cpMax - cpMin < zT
then the edge is
classified as flat. This results in many edges, both uphill and correct, being
treated as flat: we somehow traded many good edges and some bad ones
for plenty of flat edges, but this is definitely worthy, as flat edges will not
usually block the generalization process.
Even though it is not possible to set the flow direction on a flat edge, it is
still possible to classify the connected edges following the first six of the
rules above. One more rule is used to deal with flat edges:
•
if
c
is connected to a flat edge
f
, all the edges connected to
f
should be
considered connected also to
c
This rule virtually collapses each flat edge to a point, connecting together
its fathers and children. In a group of connected flat edges like those inside
a lake, this rule means that the inlets of the lake are directly connected to
its outlet.
With the eight rules listed above and no uphill edges above the z threshold
zT
it is possible to find in the river network sources and drains and to find,
for each edge, its fathers and children.
