What is prepr ?
An R package to repair broken GIS polygons using the prepair C++ library.
prepr in action with simple polygons
This is a basic example which shows you how to solve a common problem:
A ‘bowtie’ polygon:
library(prepr) library(sf) #> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 6.3.1 p1 <- st_as_sfc("POLYGON((0 0, 0 10, 10 0, 10 10, 0 0))") st_is_valid(p1, reason = TRUE) #> [1] "Self-intersection[5 5]" p11 <- st_prepair(p1) st_is_valid(p11) #> [1] TRUE st_as_text(p11) #> [1] "MULTIPOLYGON (((0 10, 0 0, 5 5, 0 10)), ((5 5, 10 0, 10 10, 5 5)))" par(mfrow = c(1, 2)) plot(p1, main = "RAW", col = "#D7722C") plot(p11, main = "Repaired", col = "#D7722C")
Square with wrong orientation:
p2 <- st_as_sfc("POLYGON((0 0, 0 10, 10 10, 10 0, 0 0))") st_is_valid(p2, reason = TRUE) #> [1] "Valid Geometry" plot(p2, main = "RAW", col = "#D7722C")
Inner ring with one edge sharing part of an edge of the outer ring:
p3 <- st_as_sfc("POLYGON((0 0, 10 0, 10 10, 0 10, 0 0),(5 2, 5 7, 10 7, 10 2, 5 2))") st_is_valid(p3, reason = TRUE) #> [1] "Self-intersection[10 2]" p33 <- st_prepair(p3) st_is_valid(p33) #> [1] TRUE st_as_text(p33) #> [1] "MULTIPOLYGON (((10 2, 10 7, 10 10, 0 10, 0 0, 10 0, 10 2)))" par(mfrow = c(1, 2)) plot(p3, main = "RAW", col = "#D7722C") plot(p33, main = "Repaired", col = "#D7722C")
Dangling edge:
p4 <- st_as_sfc("POLYGON((0 0, 10 0, 15 5, 10 0, 10 10, 0 10, 0 0))") st_is_valid(p4, reason = TRUE) #> [1] "Self-intersection[10 0]" p44 <- st_prepair(p4) st_is_valid(p44) #> [1] TRUE st_as_text(p44) #> [1] "MULTIPOLYGON (((0 0, 10 0, 10 10, 0 10, 0 0)))" par(mfrow = c(1, 2)) plot(p4, main = "RAW", col = "#D7722C") plot(p44, main = "Repaired", col = "#D7722C")
Two adjacent inner rings:
p6 <- st_as_sfc("POLYGON((0 0, 10 0, 10 10, 0 10, 0 0), (1 1, 1 8, 3 8, 3 1, 1 1), (3 1, 3 8, 5 8, 5 1, 3 1))") st_is_valid(p6, reason = TRUE) #> [1] "Self-intersection[3 1]" p66 <- st_prepair(p6) st_is_valid(p66) #> [1] TRUE st_as_text(p66) #> [1] "MULTIPOLYGON (((10 0, 10 10, 0 10, 0 0, 10 0), (1 1, 1 8, 3 8, 5 8, 5 1, 3 1, 1 1)))" par(mfrow = c(1, 2)) plot(p6, main = "RAW", col = "#D7722C") plot(p66, main = "Repaired", col = "#D7722C")
Polygon with an inner ring inside another inner ring:
p7 <- st_as_sfc("POLYGON((0 0, 10 0, 10 10, 0 10, 0 0), (2 8, 5 8, 5 2, 2 2, 2 8), (3 3, 4 3, 3 4, 3 3))") st_is_valid(p7, reason = TRUE) #> [1] "Holes are nested[3 3]" p77 <- st_prepair(p7) st_is_valid(p77) #> [1] TRUE st_as_text(p77) #> [1] "MULTIPOLYGON (((10 0, 10 10, 0 10, 0 0, 10 0), (5 2, 2 2, 2 8, 5 8, 5 2)), ((3 4, 3 3, 4 3, 3 4)))" par(mfrow = c(1, 2)) plot(p7, main = "RAW", col = "#D7722C") plot(p77, main = "Repaired", col = "#D7722C")
Details and how to cite
Details of how we automatically repair broken polygons, and what results you can expect, are available in this scientific article by the original authors of prepair:
Ledoux, H., Arroyo Ohori, K., and Meijers, M. (2014). A triangulation-based approach to automatically repair GIS polygons. Computers & Geosciences 66:121–131. [DOI] [PDF]
If you use the R package prepr for a scientific project, please cite their original work.