Transforming an sf feature without M-values into one with M-values

Question

Basic question

Working with the sf library in R, how can I add M-values to a geometry that doesn't already have them?

Or put differently, given an sf feature without M-values, how can I create a new object with the same coordinates as the original, but which also contains M-values (for example, just a bunch of zeroes)?

Example & code

Suppose I have an object called ml which contains a MultiLineString, as shown below:

library(sf)

wkt = "MULTILINESTRING((0 0, 0 1), (1 1, 1 2, 1 3))"
ml = st_as_sfc(wkt, 
               crs='epsg:4326')

Notice that the ml object was created from a WKT string that specified the geometry as a MultiLineString, not a MultiLineString M.

So how can I generate a new geometry that has the same coordinates as ml but also has M-values enabled? The M-values could be all zero - what matters aren't the individual M-values themselves. Instead, what matters is that, at the end of this process, I have a MultiLineString M geometry.

What I've tried

I tried using the st_cast function, but that doesn't seem to work:

st_cast(ml, "MULTILINESTRING M")
# Error in which_sfc_col(to) : st_cast for MULTILINESTRING M not supported

Manually editing the original WKT string is also a possibility here, but that won't really solve my problem since what I've described here is just a little small-scale version of the real bigger problem I'm trying to solve which involves a large feature with many, many coordinates. I want to avoid manually editing that thing as much as I can.

Answer 1

WKT-based work-around

As pointed out by Jindra Lacko, the st_as_sfc function can be used to generate a MultiLineString Z, but not a MultiLineString M. But this gave me the idea of doing a simple WKT-based work-around: use st_as_sfc to add the Z dimension, transform back to WKT, then search-and-replace "MULTILINESTRING Z" with "MULTILINESTRING M" and then create a geometry again! Here is the solution in code:

# Loading tidyverse to have access to the piping %>% operator
library(tidyverse) 

ml_m = wkt %>% 
  st_as_sfc() %>%
  st_zm(drop = F, what = "Z")  %>%
  st_as_text() %>%
  str_replace('MULTILINESTRING Z', 'MULTILINESTRING M') %>%
  st_as_sfc()

And Bob's you're uncle! You've got the ml_m object that's a proper MULTILINESTRING M object.

Thanks again to Jindra Lacko for the idea!!!