Introduction
The Irish Grid (EPSG:29903) is a
coordinate reference system spanning the island of Ireland. Created by
the Ordnance Survey Ireland (now Tailte
Éireann) and Ordnance
Survey of Northern Ireland, locations within the Irish Grid are
defined by an X and Y coordinate.
Irish grid references are an alternative way of recording locations
on the Irish Grid. An Irish grid reference consists of a letter,
optionally followed by an easting and northing. The letter (e.g. “N”)
refers to a particular 100 km square on the Irish Grid. There are 25
such 100 km squares - the letter “I” is skipped. The easting and
northing refer to a square within the 100 km square. An Irish grid
reference with a 1-digit easting and northing (e.g. “N85”) refers to a
particular 10 km square wtihin the 100 km square “N”. A 5-digit easting
and northing (e.g. “N 12345 67890”) refers to a particular 1 m
square.
There are many R packages facilitating spatial data analysis and
visualisation. The igr package is designed to simplify using
datasets containing Irish grid references in R. It translates Irish grid
references to and from Irish Grid coordinates or sf objects that other R
packages can work with. The igr package supports precisions of Irish
grid reference in multiples of ten from 1 m to 100 km, mixes of
precision, and Irish grid references with or without whitespace between
letter, easting and northing.
When converting Irish grid references to point locations (using
igr_to_ig(), or st_igr_as_sf() with the
default polygons=FALSE), the south west corners of the
relevant Irish Grid squares are returned. When converting to polygons
(using st_igr_as_sf() with polygons=TRUE), the
polygons returned span the entire square of each grid reference.
The functions st_igr_as_sf() and
st_irishgridrefs() convert to and from sf (simple feature) objects
in any coordinate reference system.
Checking Irish grid references
The validity of Irish grid references can be checked with the
function igr_is_valid().
# Sample of valid and invalid Irish grid references
igrs <- c("A", "A16", "A123678", "BAD", "I12", "", "B125", "Z")
igr_is_valid(igrs)
#> [1] TRUE TRUE TRUE FALSE FALSE FALSE FALSE TRUE
Converting from Irish grid references
To convert a vector of Irish grid references to Irish Grid
coordinates use igr_to_ig().
igrs <- c("A", "D12", "J53", "M5090", "N876274", "S1234550000", "W")
igr_to_ig(igrs)
#> $x
#> [1] 0 310000 350000 150000 287600 212345 100000
#>
#> $y
#> [1] 400000 420000 330000 290000 227400 150000 0
The Irish grid references may have spaces between the grid letter,
easting and northing:
igrs <- c("A", "D 12", "J 5 3", "M 50 90", "N 876 274", "S 12345 50000", "W")
igr_to_ig(igrs)
#> $x
#> [1] 0 310000 350000 150000 287600 212345 100000
#>
#> $y
#> [1] 400000 420000 330000 290000 227400 150000 0
These sample Irish grid references have a mix of precisions. The
precisions in metres can be returned by supplying a column name for the
precision data:
igr_to_ig(igrs, precision = "prec")
#> $x
#> [1] 0 310000 350000 150000 287600 212345 100000
#>
#> $y
#> [1] 400000 420000 330000 290000 227400 150000 0
#>
#> $prec
#> [1] 1e+05 1e+04 1e+04 1e+03 1e+02 1e+00 1e+05
A list or data.frame of Irish grid references can be converted to an
sf object using st_igr_as_sf(). By default the geometries
returned are points located at the south west corner of each grid
reference, and the coordinate reference system is Irish Grid (EPSG:29903):
igr_df <- data.frame(igr = igrs)
points_sf <- st_igr_as_sf(igr_df, "igr")
points_sf
#> Simple feature collection with 7 features and 1 field
#> Geometry type: POINT
#> Dimension: XY
#> Bounding box: xmin: 0 ymin: 0 xmax: 350000 ymax: 420000
#> Projected CRS: TM75 / Irish Grid
#> igr geometry
#> 1 A POINT (0 4e+05)
#> 2 D 12 POINT (310000 420000)
#> 3 J 5 3 POINT (350000 330000)
#> 4 M 50 90 POINT (150000 290000)
#> 5 N 876 274 POINT (287600 227400)
#> 6 S 12345 50000 POINT (212345 150000)
#> 7 W POINT (1e+05 0)
These points can be plotted by the tmap package. A basic outline
of Ireland and the United Kingdom can be retrieved from the maps package.
if (requireNamespace("maps", quietly = TRUE) &
requireNamespace("tmap", quietly = TRUE)) {
library(maps)
library(tmap)
ie_uk_sf <- maps::map("world",
regions = c("Ireland", "UK"),
plot = FALSE,
fill = TRUE
) |>
sf::st_as_sf(ie_uk) |>
sf::st_transform(29903)
tm_shape(points_sf, ext = 1.4) +
tm_dots(size = 1, col = "cornflowerblue") +
tm_text("igr", ymod = 1) +
tm_shape(ie_uk_sf) +
tm_borders()
}
The Irish grid references can also be converted to polygon
geometries. Each polygon spans the full extent of a grid reference, so a
set of grid references of varying precision will result in a set of
polygons with different sizes.
polygons_sf <- st_igr_as_sf(igr_df, "igr", polygons = TRUE)
polygons_sf
#> Simple feature collection with 7 features and 1 field
#> Geometry type: POLYGON
#> Dimension: XY
#> Bounding box: xmin: 0 ymin: 0 xmax: 360000 ymax: 5e+05
#> Projected CRS: TM75 / Irish Grid
#> igr geometry
#> 1 A POLYGON ((1e+05 5e+05, 1e+0...
#> 2 D 12 POLYGON ((320000 430000, 32...
#> 3 J 5 3 POLYGON ((360000 340000, 36...
#> 4 M 50 90 POLYGON ((151000 291000, 15...
#> 5 N 876 274 POLYGON ((287700 227500, 28...
#> 6 S 12345 50000 POLYGON ((212346 150001, 21...
#> 7 W POLYGON ((2e+05 1e+05, 2e+0...
Plotting these polygons:
if (exists("ie_uk_sf")) {
tm_shape(ie_uk_sf, bbox = points_sf, ext = 1.4) +
tm_borders() +
tm_shape(polygons_sf, ext = 1.2) +
tm_polygons(size = 1, col = "cornflowerblue", alpha = 0.5) +
tm_text("igr", ymod = -1)
}
Ignoring invalid Irish grid references
To be consistent with the behaviour of sf::st_as_sf(),
an error is returned if any of the Irish grid references supplied to
st_igr_as_sf() are invalid. To avoid this scenario, any
invalid Irish grid references can be filtered out using
igr_is_valid():
some_invalid_df <- data.frame(igr = c(igrs, "BAD", "I", "", "123", "N1"))
some_invalid_df
#> igr
#> 1 A
#> 2 D 12
#> 3 J 5 3
#> 4 M 50 90
#> 5 N 876 274
#> 6 S 12345 50000
#> 7 W
#> 8 BAD
#> 9 I
#> 10
#> 11 123
#> 12 N1
valid_sf <- st_igr_as_sf(some_invalid_df[igr_is_valid(some_invalid_df$igr), , drop = FALSE])
valid_sf
#> Simple feature collection with 7 features and 1 field
#> Geometry type: POINT
#> Dimension: XY
#> Bounding box: xmin: 0 ymin: 0 xmax: 350000 ymax: 420000
#> Projected CRS: TM75 / Irish Grid
#> igr geometry
#> 1 A POINT (0 4e+05)
#> 2 D 12 POINT (310000 420000)
#> 3 J 5 3 POINT (350000 330000)
#> 4 M 50 90 POINT (150000 290000)
#> 5 N 876 274 POINT (287600 227400)
#> 6 S 12345 50000 POINT (212345 150000)
#> 7 W POINT (1e+05 0)
In tidy r this can be written as:
valid_df <- some_invalid_df |>
dplyr::filter(igr_is_valid(igr)) |>
st_igr_as_sf()
Converting to Irish grid references
Irish Grid coordinates can be converted to Irish grid references
using ig_to_igr(). The first two columns in the matrix
supplied must be the Irish Grid X coordinate and Y coordinate
respectively. Any other columns are ignored.
p <- matrix(c(0, 490000, 400000, 0, 453000, 4000), ncol = 2, byrow = TRUE)
colnames(p) <- c("x", "y")
p
#> x y
#> [1,] 0 490000
#> [2,] 400000 0
#> [3,] 453000 4000
ig_to_igr(p)
#> [1] "A000900" "Z000000" "Z530040"
Spaces or other strings can be inserted between the letter, easting
and northing:
ig_to_igr(p, sep = " ")
#> [1] "A 000 900" "Z 000 000" "Z 530 040"
The generated Irish grid references can be limited to a particular
precision by specifying the number of digits for both easting and
northing. Between 0 (for 100 km precision) and 5 (for 1 m precision)
digits are supported.
ig_to_igr(p, digits = 1) # 10 km precision
#> [1] "A09" "Z00" "Z50"
ig_to_igr(p, digits = 5) # 1 m precision
#> [1] "A0000090000" "Z0000000000" "Z5300004000"
Precision can also be specified number of metres. Multiples of 10
from 1 m to 100,000 m (100 km) are supported. Any value supplied for
precision overrides any values for digits.
ig_to_igr(p, precision = 10000) # 10 km precision
#> [1] "A09" "Z00" "Z50"
ig_to_igr(p, precision = 1) # 1 m precision
#> [1] "A0000090000" "Z0000000000" "Z5300004000"
A data.frame of Irish Grid coordinates can be converted:
p_df <- data.frame(p)
p_df
#> x y
#> 1 0 490000
#> 2 400000 0
#> 3 453000 4000
ig_to_igr(p_df)
#> [1] "A000900" "Z000000" "Z530040"
An sf object containing geometries of type POINT can be converted to
Irish grid references using st_irishgridrefs():
p_sf <- sf::st_as_sf(p_df,
crs = 29903,
coords = c("x", "y")
)
p_sf
#> Simple feature collection with 3 features and 0 fields
#> Geometry type: POINT
#> Dimension: XY
#> Bounding box: xmin: 0 ymin: 0 xmax: 453000 ymax: 490000
#> Projected CRS: TM75 / Irish Grid
#> geometry
#> 1 POINT (0 490000)
#> 2 POINT (4e+05 0)
#> 3 POINT (453000 4000)
st_irishgridrefs(p_sf)
#> [1] "A000900" "Z000000" "Z530040"
To append Irish grid references (with spaces) to the original sf
object using base r:
p_sf$igr <- st_irishgridrefs(p_sf, sep = " ")
p_sf
#> Simple feature collection with 3 features and 1 field
#> Geometry type: POINT
#> Dimension: XY
#> Bounding box: xmin: 0 ymin: 0 xmax: 453000 ymax: 490000
#> Projected CRS: TM75 / Irish Grid
#> geometry igr
#> 1 POINT (0 490000) A 000 900
#> 2 POINT (4e+05 0) Z 000 000
#> 3 POINT (453000 4000) Z 530 040
In tidy r this can be written as:
p_sf <- p_sf |>
dplyr::mutate(igr = st_irishgridrefs(sep = " "))