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Dangles
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GRASS doesn't allow areas/islands with dangles. If an island has dangle, it is not built
and attached to outer area and as a consequence, some functions do not work as expected.
For example Vect_area_area() and Vect_find_area().
Se comments in build_nat.c: Vect_attach_centroids()
Category index
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Category index is created when Vect_build(), i.e. Vect_build_partial(, GV_BUILD_ALL,)
is called. Plus_head.cidx_up_to_date is set to 1 if Category index is created.
Any later call to Vect_write_line() / Vect_rewrite_line() / Vect_delete_line() resets
Plus_head.cidx_up_to_date to 0 and doesn't update category index.
All modules using update on level 2 mus call full build, i.e. Vect_build(), at the end.
All features without category are written to category index of field 0.
In category index is stored also type of feture, that make s possible to find areas
without centroid throug category index.
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