mapedit 0.5.0 and Leaflet.pm

(This article was first published on r-spatial, and kindly contributed to R-bloggers)

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In our last post mapedit and leaflet.js >
1.0
we
discussed remaining tasks for the
RConsortium funded project
mapedit. mapedit 0.5.0 fixes
a couple of lingering issues, but primarily focuses on bringing the
power of Leaflet.pm as an
alternate editor.
Leaflet.draw,
the original editor in mapedit provided by leaflet.extras, is a
wonderful tool but struggles with snapping and those pesky holes that we
commonly face in geospatial tasks. Depending on the task, a user might
prefer to continue using Leaflet.draw, so we will maintain full
support for both editors. We’ll spend the rest of the post demonstrating
where Leaflet.pm excels to help illustrate when you might want to
choose editor = "leafpm".

Install/Update

At a minimum, to follow along with the rest of this post, please update
mapedit and install the new standalone package leafpm. While we are
it, we highly recommend updating your other geospatial dependencies.

install.packages(c("sf", "leaflet", "leafpm", "mapview", "mapedit"))
# lwgeom is optional but nice when working with holes in leaflet.pm
# install.packages("lwgeom")

Holes

mapedit now supports holes. Let’s look at a quick example in which we
add, edit, and delete holes.

library(sf)
library(leaflet)
library(mapview)
library(mapedit)
library(leafpm)
# make a contrived polygon with holes for testing
outer1 = matrix(c(0,0,10,0,10,10,0,10,0,0),ncol=2, byrow=TRUE)
hole1 = matrix(c(1,1,1,2,2,2,2,1,1,1),ncol=2, byrow=TRUE)
hole2 = matrix(c(5,5,5,6,6,6,6,5,5,5),ncol=2, byrow=TRUE)
outer2 = matrix(c(11,0,11,1,12,1,12,0,11,0),ncol=2, byrow=TRUE)
pts1 = list(outer1, hole1, hole2)
pts2 = list(outer2)
pl1 = st_sf(geom = st_sfc(st_polygon(pts1)))
pl2 = st_sf(geom = st_sfc(st_polygon(pts2)))
mpl = st_sf(geom = st_combine(rbind(pl1, pl2)), crs=4326)
tst = editFeatures(mpl, editor = "leafpm")
# look at our creation
mapview(tst)

screenshot of hole editing

Please note that right mouse click deletes vertexes. For a more real
world application franconia[5,] from mapview has a hole. Try to edit
it with the following code.

library(sf)
library(leaflet)
library(mapview)
library(mapedit)
library(leafpm)
editFeatures(franconia[5,], editor="leafpm")

Snapping

Leaflet.pm gives us a very pleasant snapping experience, so if you
want to snap, set editor = "leafpm" and snap away. Snapping is
particular important when drawing/digitizing features from scratch. Here
is how it looks with the example from above.

screenshot of snapping

Snapping is enabled by default.

Fixes For Lingering Issues

GeoJSON Precision

Robin Lovelace discovered that at
leaflet zoom level > 17 we lose coordinate precision. Of course,
this is not good enough, so we will prioritize a fix as discussed in
issue. Hopefully,
this leaflet.js pull
request
will make this
fix fairly straightforward.

I am happy to report that we have found a solution for the loss of
precision. Please let us know if you discover any remaining problems.

Mulitlinestring Editing

Leaflet.js and multilinestrings don’t get along as Tim
Appelhans
reported in
issue.
For complete support of sf, mapedit should work with
multilinestring, so we have promoted this to issue
62
.

We backed into a solution with MULTILINESTRING since Leaflet.pm’s
approach fits better with MULTI* features. As an example, let’s edit
one of the trails from mapview.

library(sf)
library(leaflet)
library(mapview)
library(mapedit)
library(leafpm)
editFeatures(trails[4,], editor="leafpm")

screenshot of MULTILINESTRING editing

Conclusion and Thanks

As of this post we have reached the end of the extremely generous
RConsortium funding of mapedit.
Although the funding is over, we still expect to actively maintain and
improve mapedit. One feature that we had hoped to implement as part of
the mapedit toolset was editing of feature attributes. This turned out
to be very ambitious, and unfortunately we were not able to implement a
satisfactory solution for this feature during the funding period. We
plan, however, to develop a solution. Your participation, ideas, and
feedback are as vital as ever, so please continue to engage. Thanks to
all those who have contributed so far and thanks to all open source
contributors in the R and JavaScript communities.

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