"invalid" geometries in shape data

Michael,

I'll forward to rgis/toposhop/issues some 'flat' triangles (i.e. co-linear)
that come up in my neuro
data. Even Roualt says that TRIANGLE will be in GDAL 2.2 release in May and
has been
wrestling with it since Nov/Dec last.

PostGIS tests first three points of polygon for co-linearity but doesn't
detail precision,
presumably at level provided. stryk seems generally pragmatic in these
things. Even was
miffed in a Dec tweet about triangle having four points (.."and its called
Simple Features),
but coming where it does in the polygon hierarchy, it is going to be
closed.

geophys::TriangleInfo tests for co-linearity using sign of determinant
through the dt function,
though determinant (proper) would likely be quicker for evaluating lots of
them as it is direct call to
.Internal.

I am still trying to wrap my head around RTriangle. I think it must be
sturdy for triangle(s), but in
some sense its goal of mesh propagation upon/within a polygon system is
seemingly time travel
compared to my 'this thing is a triangle, aren't it?" None of the methods
I've seen so far seem to test
for 'no internal boundary', including CGAL. And none of this addresses
'automatic fixing tool' that
you or Delft suggests.

My data, from the wild, seems amenable to a jitter after a test by either
atan2 or determinant to
establish candidates for jittering. determinant seems like it would work
best with sf approach to
st_geometry(sfc) as they are lists and one can discern, following geophys'
approach building out
(adding 1s to a 3rd col of) a 2x4 triangle/polygon matrix to 3x4 and
evaluating on first 3x3 square.

I've said as much as I know to this point, triangles are coming.

Chris

On Wed, Feb 8, 2017 at 1:47 PM, Barry Rowlingson <

"invalid" geometries in shape data

Thread (6 messages)