Introduction

Changes in the representation of coordinate reference systems (CRS), and of operations on coordinates, that have been occurring over decades must now be implemented in the way spatial objects are handled in R packages. Up to the 1990s, most spatial data simply used the coordinates given by the local mapping authority; for example, the Meuse bank data set used a planar representation in metres, which turned out to be EPSG:28992, “Amersfoort / RD New”. A major resource for finding out why CRS were specified as they were are Clifford J. Mugnier’s columns in Photogrammetric Engineering & Remote Sensing, references to which are available in rgdal; the Netherlands were covered in the February 2003 column:

td <- tempfile()
dir.create(td)
Sys.setenv("PROJ_USER_WRITABLE_DIRECTORY"=td)
library(rgdal)
## Loading required package: sp
## rgdal: version: 1.5-16, (SVN revision 1050)
## Geospatial Data Abstraction Library extensions to R successfully loaded
## Loaded GDAL runtime: GDAL 3.1.2, released 2020/07/07
## Path to GDAL shared files: /usr/local/share/gdal
## GDAL binary built with GEOS: TRUE 
## Loaded PROJ runtime: Rel. 7.1.0, August 1st, 2020, [PJ_VERSION: 710]
## Path to PROJ shared files: /tmp/Rtmpaz2aWe/file736921313f71:/usr/local/share/proj:/usr/local/share/proj
## PROJ CDN enabled: FALSE
## Linking to sp version:1.4-2
## To mute warnings of possible GDAL/OSR exportToProj4() degradation,
## use options("rgdal_show_exportToProj4_warnings"="none") before loading rgdal.
data("GridsDatums")
GridsDatums[grep("Netherlands", GridsDatums$country),]
##                               country      month year ISO
## 50 Aruba and the Netherlands Antilles     (July) 2002 ABW
## 57     The Kingdom of the Netherlands (February) 2003 NLD

While most national mapping agencies defined their own standard geographical and projected CRS, supranational bodies, such as military alliances and colonial administrations often imposed some regularity to facilitate operations across national boundaries. This also led to the creation of the European Petroleum Survey Group (EPSG), because maritime jurisdiction was not orderly, and mattered when countries sharing the same coastal shelf tried to assign conflicting exploration concessions. Experts from oil companies accumulated vast experience, which fed through to the International Standards Organization (ISO, especially TC 211) and the Open Geospatial Consortium (OGC).

Defining the CRS became necessary when integrating other data with a different CRS, and for displaying on a web map background. Many legacy file formats, such as the ESRI Shapefile format, did not mandate the inclusion of the CRS of positional data. Most open source software then used PROJ.4 strings as a flexible representation, but as internationally accepted standards have been reached, in particular ISO 19111, and improved over time by iteration, it is really necessary to change to a modern text representation, known as WKT2 (2019). Now it looks as though almost all corporations and mapping agencies accommodate this representation, and it has been adopted by sp through rgdal, sf and other packages.

demo(meuse, ask=FALSE, package="sp", echo=FALSE)
library(mapview)
x <- mapview(meuse, zcol="zinc")
mapshot(x, file="meuse.png")
knitr::include_graphics(system.file("misc/meuse.png", package="rgdal"))