the contours exiting the study area. In order to create polygons
representing the contours we have to close the area somehow.
- Manually connecting with a new
polyline the dangling nodes of the dataset created in STEP 2. For
small datasets this might be the quickest solution, but we very
seldom work with such small datasets.
- Creating a bounding rectangle of the
dataset and adding it as a polyline to the dataset created in STEP
- Creating a Convex Hull of the
dataset and adding in to the dataset created in STEP 2.
Depending on the dataset both
methods 2. and 3. will automate the process, but most probably both
will require some additional data processing to fix the topology
STEP 3: Create Convex Hull of the data
STEP 4: Buffer
the Convex Hull with a very small distance. This is to ensure that it
will not touch the contour polylines which will cause splitting of the
original contours during the cleaning process.
STEP 5: Convert the buffered Convex
Hull to it's boundary polyline - Polygon
STEP 6: Merge the contours created in
STEP 2 and the boundary of the buffered Convex Hull created in STEP 5.
STEP 7: Clean the merged dataset - Clean
Dangling Nodes function. Carefully select the dangle tolerance. If
needed manually edit some of the contour ends to be as close as
possible to the convex hull boundary. Evaluate the result of the
function (The Export
Nodes function can be used to find out whether there are still
dangling nodes present).
Note that an automatic procedure might
not be able to fix all topology problems. ET
GeoTools offer a large variety of tools that will help you
productively analyze and fix topology problems.
STEP 8: Build polygons from the dataset
created in STEP 7 - Build
At this stage we have topologically
correct polygons representing our original contours. Each polygon is
constructed by its lower and upper contour. Some polygons have edges
from the dataset boundary, but we are not interested in these edges
because they do not have any elevation information. The polygons
however still do not have any attributes.