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Figure 4: sections after harmonization.
In braided rivers the watercourses are segmented in many sections due to
the big number of edge intersections and this results in sections that may
be too small to be correctly classified: it might happen then that a small
section of a broad river could be classified as a minor river 1 , and then
collapsed to a line, leaving an empty space (see Figure 3c ). In this case the
classification of this small section is overridden and the section “up-
graded” to “big” river, returning it to its original representation of area
river section (see Figure 4c ).
Also in normal long streams, the loss of an area due to the collapse to line
of an area section could result in a confluence having an “odd” shape: this
is the case of a minor river leaving or entering a “big” river or a minor
river becoming a “big” river as it flows downhill. In such cases the har-
monization algorithm will change the shape of the area river sections
touching the minor river in order to assure a smoother representation of the
class change (e.g. see Figure 4d ) .
In some other cases, finally, the generalized river could result in a se-
quence of line-area-line-area sections; in this case the algorithm iteratively
changes the classification of the shortest section to that of the contiguous
ones if they are below a length threshold that has been set to 500 meters.
2.3 Pruning
The IGMI specifications for the DB25 require that watercourses shorter
than 250 meters should not be acquired. This threshold can not be applied
directly to the data because deleting all the edges shorter than 250 meters
would disconnect the graph of the river network. This constraint could
1 We use the term minor river for rivers classified as "very small", "small" or "medium"; i.e.
represented only by a line and not also by an area.
 
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