Converting WSG84 to UTM, rgdal alternatives

I have numerous existing scripts in which I used project() from the rgdal package to convert WSG84 Latitude/Longitude coordinates into UTM. rgdal has intermittently been giving me headaches by randomly stopping to work after updates, which I was always able to fix by copying an older version into my package folder instead of installing newer versions from CRAN. This also has stopped working, now, and since rgdal is going to be retired during 2023, I'm trying to convert my code using a different package for the projection. This is where I'm stuck as I don't quite understand how to adapt it for other packages like terra or sf.

somelocations<-data.frame(id=c('X', "Y", "Z"),
                   Long=c(-156.6274,-156.6457,-156.6676),
                   Lat=c(20.8081,20.8292,20.8512))

require(rgdal)

rgdal::project(as.matrix(somelocations[,c('Long', 'Lat')]), "+proj=utm +zone=4N ellps=WGS84 +units=m"))

Error in rgdal::project(as.matrix(DASARs[, c("Long", "Lat")]), "+proj=utm +zone=4N ellps=WGS84 +units=m") : 
  target crs creation failed: Invalid value for an argument
In addition: Warning message:
PROJ support is provided by the sf and terra packages among others 

Here is how you can do that with "terra".

Your example data

somelocations <- data.frame(id=c('X', "Y", "Z"),
                   Long=c(-156.6274,-156.6457,-156.6676),
                   Lat=c(20.8081,20.8292,20.8512))

The "raw" approach, which is most similar to the one with rgdal:

library(terra)
project(as.matrix(somelocations[,c('Long', 'Lat')]), 
		"+proj=longlat", "+proj=utm +zone=4 +units=m")
#         [,1]    [,2]
#[1,] 746948.4 2302727
#[2,] 745008.7 2305036
#[3,] 742693.4 2307439

Or via a SpatVector (which could be useful for other reasons such as data analysis and mapping).

v <- vect(somelocations, geom=c("Long", "Lat"), crs="+proj=longlat")
p <- project(v, "+proj=utm +zone=4 +units=m")
crds(p)
#         [,1]    [,2]
#[1,] 746948.4 2302727
#[2,] 745008.7 2305036
#[3,] 742693.4 2307439

The reason why you get an error is that your PROJ4 string "+proj=utm +zone=4N") is not valid. You could use "+proj=utm +zone=4 +north"), but adding "+north" is redundant, just like adding "unit=m" as these are the defaults. I would also leave out +ellps=WGS84 or replace it with (the default) +datum=WGS84. (see the documentation)

The {rgdal} package is by now superseded, and the package maintainer (who has since retired) is actively pushing the package users to consider other alternatives. Hence the big fat warning & pointer to {sf} and {terra}.

For converting Long Lat data to UTM zone 4N coordinates I suggest the following code.

The key parts are:

• converting from a regular data frame to {sf} data frame; this will give you the baseline for projection (where are you projecting from? are your original coordinates in decimal degrees, or meters? survey feet even?)
• apply sf::st_transform() to your sf object, converting from a known start to desired format

I have found it the easiest to specify Coordinate Reference Systems using EPSG codes, such as 4326 and 32604, rather than proj4strings as used in your code.

library(sf)
library(dplyr)

somelocations<-data.frame(id=c('X', "Y", "Z"),
                          Long=c(-156.6274,-156.6457,-156.6676),
                          Lat=c(20.8081,20.8292,20.8512))

# this is the baseline - locations in WGS84
locations_sf <- somelocations %>% 
  st_as_sf(coords = c("Long", "Lat"), crs = 4326)

# a visual check - do the locations look correct?
mapview::mapview(locations_sf)

# this is the action!
locations_projected <- locations_sf %>% 
  st_transform(crs = "EPSG:32604") # see https://epsg.io/32604 for more info...

# do the coordinates look as expected?
locations_projected
# Simple feature collection with 3 features and 1 field
# Geometry type: POINT
# Dimension:     XY
# Bounding box:  xmin: 742693.4 ymin: 2302727 xmax: 746948.4 ymax: 2307439
# Projected CRS: WGS 84 / UTM zone 4N
#   id                 geometry
# 1  X POINT (746948.4 2302727)
# 2  Y POINT (745008.7 2305036)
# 3  Z POINT (742693.4 2307439)