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Filling Holes
Radar scans of the ground (or of planetary bodies) often have "void" areas
of missing elevation data due to various observation interferences.
Here's how to fill such holes with Leveller, using the basic idea of
interpolating elevations between the hole's edges.
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This is the terrain as it should be. We will use it
in our example to extract "hint pixels". In real life, you
would either estimate such hints or copy them from another file.
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This is the hole (void region) we need to fill.
Select it by choosing the Magic Wand tool, set a
tolerance of zero, and then click anywhere inside
the hole.
If you are going to use a smoothing process
to fill the hole, it helps to use Filter, Elevate...
to raise the hole's floor to be as close as possible
to the lowest part of the hole's rim.
If for some reason the hole's floor is higher than
the terrain's lowest point (which could confuse
the Magic Wand tool into selecting outside areas)
you can prune mis-selected areas with the
Arbitrary Select tool and the Ctrl key.
You can also hold off selecting until after
creating any hints (see the next step).
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The narrow ridges are elevations we've placed
inside the hole to help the upcoming interpolation.
Any data you can provide (or reasonably guess at)
is useful. An even distribution of islands
or a cross section (like the one here) is good.
You could even brush in your best manual approxmation
to the missing data; just make sure that you leave
some void areas and that you can easily select them.
Make sure that none of the hints are selected.
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Next, we do a 5x5 Multismooth of several thousand iterations.
If the Level connect option is not used, however, it tends to
interpolate to the hole's rim and ignores the hint pixels.
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With the same hints, however, using the Level connect option
does a better job. The Level Connect plug-in can also be
used, and may require fewer iterations and run faster.
If you are using Multismooth, make sure the hints are
fully unselected.
All that's left is some noise filtering (and probably some brushing)
to make the smooth patch more closely resemble the surrounding terrain.
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If you have really large holes, you can draw elevated contour lines
over the hole, select the hole with the Magic Wand, and then choose
the "Shapes, Heightfield from Points..." command, which will build a
convex hull from a point cloud suggested by the lines. The selection mask
will limit the hull to replacing only the hole.
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Daylon Leveller is powered in part by the OpenGL® API.