reproj: Reproject coordinates.

In reproj: Coordinate System Transformations for Generic Map Data

Reproject coordinates.

Description

Reproject coordinates from a matrix or data frame by explicitly specifying the 'source' and 'target' projections.

Usage

## S3 method for class 'sc'
reproj(x, target = NULL, ..., source = NULL)

## S3 method for class 'mesh3d'
reproj(x, target, ..., source = NULL)

## S3 method for class 'quadmesh'
reproj(x, target, ..., source = NULL)

## S3 method for class 'triangmesh'
reproj(x, target, ..., source = NULL)

reproj(x, target, ..., source = NULL, four = FALSE)

## S3 method for class 'matrix'
reproj(x, target, ..., source = NULL, four = FALSE)

## S3 method for class 'data.frame'
reproj(x, target, ..., source = NULL, four = FALSE)

reproj_xy(x, target, ..., source = NULL)

reproj_xyz(x, target, ..., source = NULL)

Arguments

x	coordinates
target	target specification (PROJ.4 string or epsg code)
...	arguments passed to proj4::ptransform()
source	source specification (PROJ.4 string or epsg code)
four	if TRUE, and PROJ version 6 is available return four columns xyzt (not just three xyz)

Details

We currently use the proj4 package.

The reproj() and related functions drive proj4::ptransform() and sort out the requirements for it so that we can simply give coordinates in data frame or matrix form, with a source projection and a target projection.

If using PROJ, reproj can pass in a wider variety of source and target strings, not just "proj4string" and we are completely subject to the new rules and behaviours of the PROJ library. We always assume "visualization order", i.e. longitude then latitude, easting then northing (as X, Y).

The basic function reproj() takes input in generic form (matrix or data frame) and returns a 3-column matrix, by transforming from map projection specified by the source argument to that specified by the target argument. Only column order is respected, column names are ignored.

This model of working also allows adding methods for specific data formats that already carry a suitable source projection string. Currently we support types from the silicate and quadmesh and rgl packages, and only the target string need be specified.

This model has obvious flexibility, for packages to import the generic and call it with the correct source (from the data format) and the target from user, or process controlled mechanism.

The source argument must be named, and if it is not present a light check is made that the source data could be "longitude/latitude" and transformation to target is applied (this can be controlled by setting options).

The function reproj() always returns a 3-column matrix unless four = TRUE, and PROJ::ok_proj6() is TRUE and then a 4-column matrix is returned.

Functions reproj_xy() and reproj_xyz() are helpers for reproj() and always return 2- or 3-column matrix respectively.

Note that any integer input for source or target will be formatted to a character string like "EPSG:<integer_code>" as a simple convenience. Note that there are other authorities besides EPSG, so the pattern "AUTH:code" is a general one and you should really be explicit.

Until recently the proj4 package was the only one available for generic data that will transform between arbitrary coordinate systems specified by source and target coordinate systems and with control over 'xy' versus 'xyz' input and output. This package adds some further features by wrapping the need to convert longitude/latitude data to or from radians.

Other R packages for transforming coordinates are geared toward data that's in a particular format. It's true that only GDAL provides the full gamut of available geographic map projections, but this leaves a huge variety of workflows and applications that don't need that level of functionality.

Value

numeric matrix of the transformed coordinates, either 2, 3, or 4 columns depending on the shape of the input, or the argument 'four' in reproj(). Use reproj_xy() or reproj_xyz() for those specific 2- and 3-column cases.

Dependencies

• The PROJ package is a stub atm and is not used.

The proj4 package works perfectly well with the PROJ-lib at versions 4, 5, 6, or 7 and if this is preferred reproj can be set to ignore the PROJ R package (see reproj-package).

Global options

Assuming longitude/latitude input

The behaviour is controlled by user-settable options which on start up are reproj.assume.longlat = TRUE and reproj.default.longlat = "OGC:CRS84".

If the option reproj.assume.longlat is set to FALSE then the source argument must be named explicitly, i.e. reproj(xy, t_srs, source = s_srs), this is to help catch mistakes being made. The target is the second argument in reproj though it is the third argument in proj4::ptransform. This function also converts to radians on input or output as required.

If the option reproj.assume.longlat is set to TRUE and the input data appear to be sensible longitude/latitude values, then the value of reproj.default.longlat is used as the assumed source projection.

Controlling use or PROJ or proj4

See reproj-package for another option set reproj.mock.noproj6 for package testing for expert use.

Warning

There are a number of limitations to the PROJ library please use at your own risk. The sf package provides a better supported facility. The libproj package will be used if it makes it to CRAN.

Examples

reproj(cbind(147, -42), target = "+proj=laea +datum=WGS84",
                         source = getOption("reproj.default.longlat"))

reproj documentation built on Oct. 29, 2022, 1:09 a.m.