Geoscience Reference
In-Depth Information
Figure 2:
The upper panel shows NHDFlowline artificial paths for a portion of the Pomme
de Terre subbasin. The lower panel illustrates the alternative solution created by inter-
secting flowlines with polygonal waterbodies which have been selected on size. Notice
that the lower solution contains fewer gaps in the candidate centerline.
After the intersection, all reaches that are part of the master centerline
delineation are selected to form a master centerline list. The rest of the flow-
lines are relegated to a candidate list to be examined as potential channels
to "fill in" centerline gaps. The last preprocessing step creates a master list
containing the endnodes of each reach in the derived centerline list and
also creates a separate list of endnodes for reaches in the candidate list.
The process of filling in gaps in the centerline is iterative. The candidate
reach list is scanned for reaches that contain at least one endnode coinci-
dent with an endnode from the centerline master list. The algorithm util-
izes a tolerance value to delineate a range of acceptance for coincidence.
This tolerance value is dependent on the stated resolution of the database.
For instance, the stated resolution for the Pomme de Terre database on
download is 0.001 meters, and the algorithm utilizes a slightly larger value
of 0.0015 meters to capture node coordinates that are not precisely
coincident.
