Convert shapefile to LonLat (R)

Question

Just a small disclaimer: I don't have any background in cartography, hence I might have missed the answer to my question in search just because I didn't even know the keywords to look for.

So, I am using R to plot statistical data on maps. It was all fine and understandable when all of the materials came in same units of measurments (lon/lat in form of decimal degrees or degrees/minutes/seconds which have obvious and easy conversion).

So what I have encountered - doing routine fortify(shapeMap) to plot data with ggplot2 yields dataframe with values:

    long    lat order   hole    piece   group   id
1   563656.6    373826.0    1   FALSE   1   0.1 0
2   563864.6    373664.3    2   FALSE   1   0.1 0
3   564272.9    373895.4    3   FALSE   1   0.1 0
4   564491.6    373805.5    4   FALSE   1   0.1 0
5   565151.1    372709.0    5   FALSE   1   0.1 0
6   565328.3    372347.0    6   FALSE   1   0.1 0

...

As far as I understand that this is UTM coordinate system. And hence the question - is there an easy way to convert these coordinates to decimal degrees? I have dug through quite a few materials and online convertors but I can't really seem to get the expected result.

Just for reference - shapefile coordinates above should be somewhere in Latvia. And Latvia is split among 3 UTM grid cells: 34V; 35V and 35U (if that is relevant at all).

Update: I have found out that coordinates are in LKS92 system. Hence now there is a clear objective. Convert LKS92 to WGS84. Guess now it is up to figuring out spTransform() function from rgdal package. Unfortunately it is not an obvious function with "from" and "to" arguments. Or at least at first glance.

Update2: Reference for those that will encounter the same problem:

1. You have to be sure what coordinate system you want to conver from/into. In my case I found out it was LKS92.

2. Find specific parameters for the coordinate system on spatialreference.org. In my case parameters look like "+proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=-6000000 +ellps=GRS80 +units=m +no_defs". Not that these are parameters for coordinate system FROM which I want to convert. If this is not WGS84 then you will want another parameter string for target coordinate system.

3. R package proj4 has function project(). An example of LKS92 -> WGS84 conversion:

xy <- cbind(c(508148.442), c(319423.294)) project(xy, "+proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=-6000000 +ellps=GRS80 +units=m +no_defs", inverse = T) [,1] [,2] [1,] 24.1342 57.01823

Note the inverse=T which means convert to WGS84. Making inverse = F suggests that input coordinates are in WGS84 format.

Another small update:

To convert whole shapefile you would still need rgdal package:

proj4string(shapeMap) <- CRS("+proj=tmerc +lat_0=0 +lon_0=24 +k=0.9996 +x_0=500000 +y_0=-6000000 +ellps=GRS80 +units=m +no_defs")
shapeMap <- spTransform(shapeMap, CRS("+proj=longlat"))

First line defines the CRS of shapefile, second line tells what CRS to transform into.

Answer 1

Use proj4 package for R. There is a function project():

project(xy, proj, inverse = FALSE, ellps.default="sphere")

You need to use it with option inverse = TRUE in order to project from other projection to lat/long. And notice that documentation for this package might be ambiguous: seems that you should pass long/lat coordinates not lat/long.