PROJ

The goal of PROJ is to provide generic coordinate system transformations in R with a functional requirement for the system library to be provided by the libproj package.

This is a shared goal with the reproj package, and PROJ provides the infrastructure for later versions of the underlying library.

PROJ provides basic coordinate transformations for generic numeric data in matrices or data frames. Transforming spatial data coordinates is a basic task independent of storage format.

PROJ is strictly for modern versions of the PROJ library, now via libproj package at version 7. Alternative pathways to PROJ via the proj4 package are available but only used for expert testing.

Because we are using modern PROJ there is no forward/inverse transformation, only integrated source/target idioms. This is the same approach taken by the reproj package- the source must be provided as well as the target. When a data set has an in-built CRS projection recorded, then methods can be written for that use-case with that format.

We can use ‘auth:code’ forms, PROJ.4 strings, full WKT2, or the name of a CRS as found in the PROJ database, e.g ‘WGS84’, ‘NAD27’, etc. Full details are provided in the PROJ documentation.

Things to be aware of

Note that PROJ in your system is not used, PROJ imports the standalone libproj package installed from CRAN.

• Input can be a data frame or a matrix, but internally input is assumed to be x, y, z, and time. So the output is always a 4-column matrix.
• You can’t use strings like “+init=epsg:4326” any more, it must be “EPSG:4326”.
• You should know what your target projection is, and also what your source projection is. This is your responsibility.
• PROJ assumes longitude/latitude order always by setting the PROJ library context proj_normalize_for_visualization.

Please see PROJ library documentation for details on this.

Installation

On all systems do

install.packages("PROJ")

or

remotes::install_cran("PROJ")

To install the development version from Github do

remotes::install_github("hypertidy/PROJ")

Example

Minimal code example, two lon-lat coordinates to LAEA, and back.

library(PROJ)
lon <- c(0, 147)
lat <- c(0, -42)
dst <- "+proj=laea +datum=WGS84 +lon_0=147 +lat_0=-42"
src <- "+proj=longlat +datum=WGS84"

## forward transformation
(xy <- proj_trans( cbind(lon, lat), dst, source = src))
#> $x_
#> [1] -8013029        0
#> 
#> $y_
#> [1] -8225762        0

## inverse transformation
proj_trans(cbind(xy$x_, xy$y_), src, source = dst)
#> $x_
#> [1]   0 147
#> 
#> $y_
#> [1] -3.194835e-15 -4.200000e+01


## note that NAs propagate in the usual way
lon <- c(0, NA, 147)
lat <- c(NA, 0, -42)

proj_trans(cbind(lon, lat), src, source = dst)
#> $x_
#> [1]       NA       NA 147.0018
#> 
#> $y_
#> [1]        NA        NA -42.00038

A more realistic example with coastline map data.

library(PROJ)
w <- PROJ::xymap
lon <- na.omit(w[,1])
lat <- na.omit(w[,2])
dst <- "+proj=laea +datum=WGS84 +lon_0=147 +lat_0=-42"
xy <- proj_trans(cbind(lon, lat), dst, source = "epsg:4326")
plot(xy$x_, xy$y_, pch = ".")

lonlat <- proj_trans(xy, src, source = dst)
plot(lonlat$x_, lonlat$y_, pch = ".")

Convert projection strings

We can generate PROJ or within limitations WKT2 strings, format 0, 1, 2 for WKT, proj4string, projjson respectively.

cat(wkt2 <- proj_crs_text("EPSG:4326"))
#> GEOGCRS["WGS 84",
#>     DATUM["World Geodetic System 1984",
#>         ELLIPSOID["WGS 84",6378137,298.257223563,
#>             LENGTHUNIT["metre",1]]],
#>     PRIMEM["Greenwich",0,
#>         ANGLEUNIT["degree",0.0174532925199433]],
#>     CS[ellipsoidal,2],
#>         AXIS["geodetic latitude (Lat)",north,
#>             ORDER[1],
#>             ANGLEUNIT["degree",0.0174532925199433]],
#>         AXIS["geodetic longitude (Lon)",east,
#>             ORDER[2],
#>             ANGLEUNIT["degree",0.0174532925199433]],
#>     USAGE[
#>         SCOPE["unknown"],
#>         AREA["World"],
#>         BBOX[-90,-180,90,180]],
#>     ID["EPSG",4326]]

proj_crs_text(wkt2, format = 1L)
#> [1] "+proj=longlat +datum=WGS84 +no_defs +type=crs"

Speed comparisons

library(reproj)
library(rgdal)
library(lwgeom)
library(sf)
#> Linking to GEOS 3.8.0, GDAL 3.0.4, PROJ 7.0.0
lon <- w[,1]
lat <- w[,2]
lon <- rep(lon, 25)
lat <- rep(lat, 25)
ll <- cbind(lon, lat)
z <- rep(0, length(lon))
llproj <- "+proj=longlat +datum=WGS84"

xyz <- cbind(lon, lat, z)
xyzt <- cbind(lon, lat, z, 0)

rbenchmark::benchmark(
          PROJ = proj_trans(ll, target = dst, source = llproj),
          reproj = reproj::reproj(ll, target = dst, source = llproj),
          rgdal = project(ll, dst),
          sf_project = sf_project(llproj, dst, ll),
        # lwgeom = st_transform_proj(sfx, dst),
        # sf = st_transform(sfx, dst),
        replications = 100) %>%
  dplyr::arrange(elapsed) %>% dplyr::select(test, elapsed, replications)
#>         test elapsed replications
#> 1 sf_project   8.820          100
#> 2       PROJ   9.033          100
#> 3     reproj   9.929          100
#> 4      rgdal  10.390          100

A geocentric example, suitable for plotting in rgl.

xyzt <- proj_trans(cbind(w[,1], w[,2]), z_ = rep(0, dim(w)[1L]), target = "+proj=geocent +datum=WGS84", source = "EPSG:4326")
plot(as.data.frame(xyzt[1:3]), pch = ".", asp = 1)

Geocentric transformations aren’t used in R much, but some examples are found in the quadmesh and anglr packages.

Why PROJ?

PROJ was created before libproj existed, and now leverages it.

• Why not proj4? It’s not maintained in a way that works for me.
• Why not sf? It brings a lot of baggage, and can’t do geocentric transformations.
• Why not rgdal? Still baggage, no transformations possible without special data formats, no geocentric.
• Why not lwgeom? That package is format-specific, and does not work with generic data coordinates so is unsuitable for many straightforward and efficient data-handling schemes.
• Why not mapproj? This is unusable for real-world projections in my experience, it seems to be written for some basic graphics cases.
• Why not reproj? reproj will be improved by importing PROJ. This is an extension for reproj, to bridge it from PROJ version 4 and 5, to version 6 and 7 and beyond.

The reproj package wraps the very efficient proj4::ptransform() function for general coordinate system transformations. Several package now use reproj for its consistency (no format or plumbing issues) and efficiency (directly transforming bulk coordinates). The proj4 package is not updated regularly, and depends on a local installation of the system library on some sytems. . So reproj requires a new wrapper around PROJ (PROJ.4) itself.

Since the 1990s PROJ.4 has been the name of the common standard library for general coordinate system transformations (for geospatial). That 1994 release has been modernized and now has versions PROJ 5 and PROJ 6. There’s a bit of traction in the name PROJ.4, so it has stuck

I’ll use “PROJ (PROJ.4)” to distinguish the system library from this package.

There are a few links to the PROJ (PROJ.4) library in R.

• rgdal Provides low level project(matrix, inv = TRUE/FALSE), and engine behind sp::spTransform().
• proj4 Provides low level project() and ptransform.
• sf Provides low level sf::sf_project() transformation of matrices. Provides high level st_transform() which works via the GDAL library and its own internal version of PROJ (PROJ.4). The high level function converts coordinates in list heirarchies of matrices into WKB for the transformations.
• lwgeom Provides high level st_transform_proj() also converts coordinates in list hierarchies of matrices into WKB, but internally uses the PROJ (PROJ.4) library directly.

(The mapproj package uses all its own internal code).

Packages sf, rgdal and proj4 provide raw access to coordinate transformations for R vectors. sf::sf_project() is the winner, but is embedded in a package that does many other things. Rgdal only has project forward and project inverse and always works in degrees proj4 has the more general ptransform() but requires manual conversion of degree values into radians. PROJ (PROJ.4) internally works only with radians.

The rgdal function project() won’t transform with a third Z coordinate. The sf functions do work with geocentric coords.

The packages rgdal, sf, lwgeom are now compatible with PROJ 5 (and 6) and don’t need any further attention in this regard. They work fine within their chosen context.

---

Please note that the PROJ project is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.