r - Converting point to lat lon?

I have received a shapefile with measurement points to be analyzed, but no matter how I try, I can't find how to convert to lat lon. I suppose it would be good to know the projection but when I look into the file (with R and summary()) it says "Is projected: NA" and "proj4string : [NA]".

One measurement-point I know is close to the border Peru-Chile has x,y = 354521, 7997417.8.

How can I find out how to convert this point (and many others) to lat lon?

I tried to follow the instructions of similar questions here, but to no avail.

Answer

It looks like you are using R, so try the proj4 package. Using @mkennedy's guess of UTM zone 19S, you can query a proj4string, and use the package:

library(proj4)

proj4string <- "+proj=utm +zone=19 +south +ellps=WGS84 +datum=WGS84 +units=m +no_defs "

# Source data
xy <- data.frame(x=354521, y=7997417.8)

# Transformed data
pj <- project(xy, proj4string, inverse=TRUE)
latlon <- data.frame(lat=pj$y, lon=pj$x)
print(latlon)

Produces:

        lat       lon
1 -18.10714 -70.37495

Also, if you need more precision in the output, set options("digits"=12), and you will see more digits used to display the coordinates:

            lat            lon
1 -18.107144101 -70.3749500491