The function can be used to estimate the amount of transport 'flow' at the
route segment level based on input datasets from routing services, for
example linestring geometries created with the route()
function.
The overline()
function breaks each line into many straight
segments and then looks for duplicated segments. Attributes are summed for
all duplicated segments, and if simplify is TRUE the segments with identical
attributes are recombined into linestrings.
The following arguments only apply to the sf
implementation of overline()
:
ncores
, the number of cores to use in parallel processing
simplify
, should the final segments be converted back into longer lines? The default
setting is TRUE
. simplify = FALSE
results in straight line segments consisting
of only 2 vertices (the start and end point),
resulting in a data frame with many more rows than the simplified results (see examples).
regionalise
the threshold number of rows above which
regionalisation is used (see details).
For sf
objects Regionalisation breaks the dataset into a 10 x 10 grid and
then performed the simplification across each grid. This significantly
reduces computation time for large datasets, but slightly increases the final
file size. For smaller datasets it increases computation time slightly but
reduces memory usage and so may also be useful.
A known limitation of this method is that overlapping segments of different
lengths are not aggregated. This can occur when lines stop halfway down a
road. Typically these errors are small, but some artefacts may remain within
the resulting data.
For very large datasets nrow(x) > 1000000, memory usage can be significant.
In these cases is is possible to overline subsets of the dataset, rbind the
results together, and then overline again, to produce a final result.
Multicore support is only enabled for the regionalised simplification stage
as it does not help with other stages.