convert to local time zone using latitude and longitude?

Question

I have a data set with the following information: latitude, longitude, EST time. For example, for one observation

lat = 13
long = -2
time1 = as.POSIXlt(\"201         


        

Answer 1

lat = 13
long = -2
time1 <- as.POSIXct("2014-02-12 17:00:00", tz = "EST")
# https://developers.google.com/maps/documentation/timezone/
apiurl <- sprintf("https://maps.googleapis.com/maps/api/timezone/%s?location=%s,%s&timestamp=%d&sensor=%s", 
                  "xml", 
                  lat, 
                  long, 
                  as.numeric(time1), 
                  "false")
library(XML)
tz <- xmlParse(readLines(apiurl))[["string(//time_zone_id)"]]
as.POSIXct(format(time1, tz=tz))
# [1] "2014-02-12 22:00:00 CET"

or, as suggested by @SymbolixAU, use their googleway package:

res <- googleway::google_timezone(c(lat, long), time1, key = NULL)
as.POSIXct(format(time1, tz=res$timeZoneId))
# [1] "2014-02-12 22:00:00 CET"

Answer 2

An alternate approach is to intersect the target point coordinates with a shapefile of global time zones and subsequently convert the EST timestamps to the local time at each spatial point. For example, boundaries of the world's time zones are available from Timezone Boundary Builder (direct download).

library(sf)

## example data
dat = data.frame(lon = -2
                 , lat = 13
                 , time1 = as.POSIXlt("2014-02-12 17:00:00", tz = "EST"))

## convert to 'sf' object
sdf = st_as_sf(dat, coords = c("lon", "lat"), crs = 4326)

## import timezones (after extraction) and intersect with spatial points
tzs = st_read("timezones.geojson/combined.json", quiet = TRUE)
sdf = st_join(sdf, tzs)

## convert timestamps to local time
sdf$timeL = as.POSIXlt(sdf$time1, tz = as.character(sdf$tzid))
sdf$timeL
# [1] "2014-02-12 22:00:00 GMT"

Note that the corresponding time zone Africa/Ouagadougou is GMT.