The goal of sfdct is to provide constrained triangulation of simple features.
Triangulation is performed with respect to the vertices and edges per-feature. This means that each output feature will be composed of triangles that align to all input edges. This will also correspond to the outer hull of any lines or polygons. In a GEOMETRYCOLLECTION the same alignment only applies to geometries individually within the collection, so lines and polygons and point sets are all triangulated as if they were independent.
A future release will triangulate the GEOMETRYCOLLECTION as it it were one set of edges and vertices. This will also allow an entire data set to be triangulated as one.
It's not yet clear to me how to best maintain the original feature identity for the "entire data set" case, or if this even matters. Please get in touch if you are interested!
More general structures for working with grouped simplicial complex data structures are in the works, but aligning with simple features in this package provides a useful illustration of these nuances and how far we can push the standard tools.
This is a basic example which shows you how to decompose a MULTIPOLYGON sf data frame object into a GEOMETRYCOLLECTION sf data frame object made of triangles:
library(sf)
#> Linking to GEOS 3.5.1, GDAL 2.1.2, proj.4 4.9.3
library(sfdct)
nc <- st_read(system.file("shape/nc.shp", package="sf"))
#> Reading layer `nc' from data source `/usr/local/lib/R/site-library/sf/shape/nc.shp' using driver `ESRI Shapefile'
#> converted into: POLYGON
#> Simple feature collection with 100 features and 14 fields
#> geometry type: MULTIPOLYGON
#> dimension: XY
#> bbox: xmin: -84.32385 ymin: 33.88199 xmax: -75.45698 ymax: 36.58965
#> epsg (SRID): 4267
#> proj4string: +proj=longlat +datum=NAD27 +no_defs
(nc_triangles <- ct_triangulate(nc[1:5, c("NAME")]))
#> Simple feature collection with 5 features and 1 field
#> geometry type: GEOMETRYCOLLECTION
#> dimension: XY
#> bbox: xmin: -81.74107 ymin: 36.07282 xmax: -75.77316 ymax: 36.58965
#> epsg (SRID): 4267
#> proj4string: +proj=longlat +datum=NAD27 +no_defs
#> NAME geometry
#> 1 Ashe GEOMETRYCOLLECTION(POLYGON(...
#> 2 Alleghany GEOMETRYCOLLECTION(POLYGON(...
#> 3 Surry GEOMETRYCOLLECTION(POLYGON(...
#> 4 Currituck GEOMETRYCOLLECTION(POLYGON(...
#> 5 Northampton GEOMETRYCOLLECTION(POLYGON(...
(asub <- st_geometry(nc_triangles)[[4]] )
#> GEOMETRYCOLLECTION(POLYGON((-76.091064453125 36.5035667419434, -76.1581497192383 36.4126892089844, -76.095085144043 36.3489151000977, -76.091064453125 36.5035667419434)), POLYGON((-76.1581497192383 36.4126892089844, -76.1609268188477 36.3918991088867, -76.095085144043 36.3489151000977, -76.1581497192383 36.4126892089844)), POLYGON((-76.095085144043 36.3489151000977, -76.0439529418945 36.3535919189453, -76.0016098022461 36.4189147949219, -76.095085144043 36.3489151000977)), POLYGON((-76.0016098022461 36.4189147949219, -76.0439529418945 36.3535919189453, -76.0173492431641 36.3377304077148, -76.0016098022461 36.4189147949219)), POLYGON((-75.9512557983398 36.3654708862305, -76.0016098022461 36.4189147949219, -76.0173492431641 36.3377304077148, -75.9512557983398 36.3654708862305)), POLYGON((-76.0173492431641 36.3377304077148, -76.0439529418945 36.3535919189453, -76.0328750610352 36.3359756469727, -76.0173492431641 36.3377304077148)), POLYGON((-76.091064453125 36.5035667419434, -76.095085144043 36.3489151000977, -76.0016098022461 36.4189147949219, -76.091064453125 36.5035667419434)), POLYGON((-76.1682891845703 36.4270858764648, -76.091064453125 36.5035667419434, -76.1273956298828 36.5571632385254, -76.1682891845703 36.4270858764648)), POLYGON((-76.1682891845703 36.4270858764648, -76.1581497192383 36.4126892089844, -76.091064453125 36.5035667419434, -76.1682891845703 36.4270858764648)), POLYGON((-76.1273956298828 36.5571632385254, -76.3302536010742 36.5560569763184, -76.1682891845703 36.4270858764648, -76.1273956298828 36.5571632385254)))Denser triangles, and optionally D for ensuring Delaunay criterion is met.
st_geometry(ct_triangulate(nc[4, ], a = 0.0007, D = TRUE))
#> Geometry set for 1 feature
#> geometry type: GEOMETRYCOLLECTION
#> dimension: XY
#> bbox: xmin: -76.33025 ymin: 36.07282 xmax: -75.77316 ymax: 36.55716
#> epsg (SRID): 4267
#> proj4string: +proj=longlat +datum=NAD27 +no_defs
#> GEOMETRYCOLLECTION(POLYGON((-76.0277068229108 3...See the vignettes for more examples.
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