The left pane shows an example retrieved from an ENC where the long sides of
a dredged-area feature (visualized with a dotted pattern) trigger
the creation of triangles (dashed magenta lines) that overlap with the same feature edges.
After the application of the proposed interpolation criterion,
the resulting triangulation shown in the right pane does not present the overlap issue.
To avoid the creation of triangles crossing feature edges (and, thus, the corresponding
misrepresentation in the reconstructed bathymetric model),
additional nodes are inserted along the feature edges that are longer than
the interpolation length of one centimeter at the ENC compilation scale.
The one-centimeter interpolation value was established heuristically as
a trade-off value between the requirement of avoiding the triangle-crossing-edge condition and
the performance cost paid for a larger point cloud to triangulate.
Chart Comparison
An example of DtoN detection. In the left pane, a 10.1-meter survey sounding (in blue)
is surrounded by two charted soundings (in black) and two depth contours of ten meters
and twenty meters.
The right pane shows the result of the triangulation (dashed lines in magenta) and
a DtoN candidate (grey circle with a cross simple).
This latter has a vertical distance of about five meters (red value overlaying the circle)
from the underlying tilted triangle.
The algorithm uses the vertical distance from the generated TIN to evaluate
whether flagging the soundings as DtoN candidates or potential chart discrepancies.
The default logic of the applied thresholding is derived from
the NOAA Office of Coast Survey (OCS) Hydrographic Surveys Specifications and Deliverables (HSSD) manual.
In deep waters, the DtoN concept does not apply to surface navigation.
However, based on the consideration that the presence of a very large discrepancy
in comparison to the charted water depth may represent an issue
for specific marine operations (e.g., towing a side scan sonar),
the algorithm reports – when using default parameters – potential DtoNs
for vertical distance values larger than the ten percent of the water depth.
However, since those values should follow the specific cartographic organization policy
(or other types of specific needs) of the adopting cartographic organization,
the algorithm was made is flexible to using customized threshold values as input parameters.
Chart Comparison
Example of Chart Comparison output.
The outcomes of the algorithm execution are plotted.
ENC-derived depth values are colored by depth, while algorithm-flagged input survey soundings
are colored by their discrepancy.
If present, the subset of untested input soundings that require human evaluation are displayed
with a magenta diamond.
The default output is provided in S-57 format.
For easy sorting and identification of discrepancies, the magnitude of the discrepancy
against the chart is stored both as cartographic symbol features (S-57 acronym: $CSYMB)
and soundings (i.e., storing the discrepancy value as the depth coordinate).
Output in the popular Shapefile and KML formats may be also generated.
