I have the following coordinates in DMS format. I need to convert them to decimal degrees.

# Libraries
> library(sp)
> library(magrittr)

# Latitide & Longitude as strings
> lat <- '21d11m24.32s'
> lng <- '104d38m26.88s'

I tried:

> lat_d <- char2dms(lat, chd='d', chm='m', chs='s') %>% as.numeric()
> lng_d <- char2dms(lng, chd='d', chm='m', chs='s') %>% as.numeric()

> print(c(lat_d, lng_d))
[1]  21.18333 104.63333

Although close, this result is different from the output I get from this website. According to this site, the correct output should be:

Latitude: 21.190089

Longitude: 104.6408

It seems that sp::char2dms and as.numeric are rounding the coordinates. I noticed this issue when converting a large batch of DMS coordinates using this method because the number of unique values decreases drastically after the conversion.

CodePudding user response：

You are right! To tell you the truth, I didn't notice this problem. To get around this, here is a solution with the use of the package measurements:

REPREX:

install.packages("measurements") 
library(measurements)

lat <- conv_unit('21 11 24.32', from = "deg_min_sec", to = "dec_deg")
long <- conv_unit('104 38 26.88' , from = "deg_min_sec", to = "dec_deg")

print(c(lat, long))
#> [1] "21.1900888888889" "104.6408"

^{Created on 2021-10-07 by the reprex package (v2.0.1)}

Edit from OP

This can also be solved by adding 'N' or 'S' to latitude and 'E' or 'W' to longitude.

# Add character to lat & long strings
> lat_d <- char2dms(paste0(lat,'N'), chd='d', chm='m', chs='s') %>% as.numeric()
> lng_d <- char2dms(paste0(lng,'W'), chd='d', chm='m', chs='s') %>% as.numeric()
> print(c(lat_d, lng_d))
[1]   21.19009 -104.64080