Sf: st_perimeter

[etiennebr]

I think it would be useful to have an st_perimeter. Is it equivalent to have st_cast("MULTILINESTRING") %>% st_length?

[edzer]

(how) does it deal with holes in polygons?

[etiennebr]

library(sf)
#> Linking to GEOS 3.5.0, GDAL 2.1.1, proj.4 4.9.3

m <- rbind(c(0,0), c(1,0), c(1, 1), c(0,1), c(0,0))
n <- (m - 0.5) / 2 + 0.5
s <- matrix(c(2, 0), 5, 2, byrow = TRUE)

single <- list(m)  %>% st_polygon()
multi <- list(list(m), list(m + s)) %>% st_multipolygon()
half <- list(n)  %>% st_polygon()

hole <- st_difference(single, half)
single %>% st_cast("MULTILINESTRING") %>% st_length()
#> [1] 4
half %>% st_cast("MULTILINESTRING") %>% st_length()
#> [1] 2
hole %>% st_cast("MULTILINESTRING") %>% st_length()
#> [1] 6

multi %>% st_cast("MULTILINESTRING") %>% st_length()
#> [1] 8

[edzer]

seems like its the sum of all lengths - I don't think that justifies a separate function.

[etiennebr]

Ok, then I don't want to insist too much, but what about enabling st_length for polygons?, so it can be applied to all geometries (points are 0).

[etiennebr]

I believe this is a good solution.

[edzer]

This has been revisited; anything not a LINE (not having dimension 1) returns length 0.

[etiennebr]

This would indeed be conform to OGC specifications (and postgis). So you'd add a st_perimeter() ?

[edzer]

I don't see st_perimeter anywhere in the SFA standard doc. Is it part of PostGIS, and if yes, is it in liblwgeom?

[etiennebr]

You're right, I couldn't find it in the specification either. I took it from postgis st_length

• This method implements the OpenGIS Simple Features Implementation Specification for SQL 1.1. s2.1.5.1
• This method implements the SQL/MM specification. SQL-MM 3: 7.1.2, 9.3.4

It's available from lwgeom_length()

[edzer]

SQL MM (ISO/IEC 13249-3) defines st_perimeter, and postGIS has st_perimeter, and liblwgeom has it implemented. So, this can be added to lwgeom.