Converting NAD83 state plane coordinates to WS84 standard lon/lat in degrees

Question

I tried to use this instruction to convert a set of xy coordinates in NAD83 State Plane Coordinates to regular Lan/Lat coordinates in degrees. I could reproduce for the example given in this post but it fails to give right answer to my set! Following is what I have tried and what I obtained [wrong answer].

library(rgdal)
nad83_coords <- data.frame(x=c(577430), y=c(2323270)) # My coordinates in NAD83
coordinates(nad83_coords) <- c('x', 'y')
proj4string(nad83_coords)=CRS("+init=esri:102272") # West Illinois
coordinates_deg <- spTransform(nad83_coords,CRS("+init=epsg:3436")) # West Illinois

> x             y
1 1894451.52045 7622263.00755

The correct solution must be:

lat= 38.2525     deg
lon=-90.07364722 deg

I have a code to do this job in MATLAB, but I am really interested to know if "rgdal" or any other library in R can do this. This way, my world would be more beautiful! Thank you for reading.

Answer 1

In order to achieve Latitude and Longitude coordinates you need to go from you projected coordinate system (NAD 83) to a geographic coordinate system (WGS 84). In the case of your data you are using a projection in feet, so your West Illinois projection is correct. However, your mistake in the orignal post and highlighted below was that you spTransform from NAD83 to NAD83 giving you erroneous data. More information on the difference between projected and geographic coordinate systems can be found here . Instead you need to use a WGS84 projection in your transformation, as follows: ** as noted by Jim, the conversion from feet to meters is now incorporated.

library(rgdal)
   nad83_coords <- data.frame(x=c(577430), y=c(2323270)) # My coordinates in NAD83
nad83_coords <- nad83_coords *.3048 ## Feet to meters
    coordinates(nad83_coords) <- c('x', 'y')
    proj4string(nad83_coords)=CRS("+init=esri:102272") # West Illinois
    ## Erroneous code, NAD83 to NAD83
    ## coordinates_deg <- spTransform(nad83_coords,CRS("+init=epsg:3436"))
    ## Corrected with WGS84 to yield Lat/Long
    coordinates_deg <- spTransform(nad83_coords,CRS("+init=epsg:4326"))
    coordinates_deg
    SpatialPoints:
                x        y
    [1,] -96.57822 42.86484
    Coordinate Reference System (CRS) arguments: +init=epsg:4326 +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 

In the event you did not want to convert we can use the following projection EPSG: 3531

library(rgdal)
   nad83_coords <- data.frame(x=c(577430), y=c(2323270)) # My coordinates in NAD83
    coordinates(nad83_coords) <- c('x', 'y')
    proj4string(nad83_coords)=CRS("+init=EPSG:3531") # West Illinois
    ## Erroneous code, NAD83 to NAD83
    ## coordinates_deg <- spTransform(nad83_coords,CRS("+init=epsg:3436"))
    ## Corrected with WGS84 to yield Lat/Long
    coordinates_deg <- spTransform(nad83_coords,CRS("+init=epsg:4326"))
    coordinates_deg
    SpatialPoints:
                x        y
    [1,] -96.57821 42.86485
    Coordinate Reference System (CRS) arguments: +init=epsg:4326 +proj=longlat +datum=WGS84 +no_defs +ellps=WGS84 +towgs84=0,0,0 

I see what you're saying about where the coordinates should be, looking at the spatial reference site, your coordinates are within the window of that projection, but they aren't coming out as expected. Still digging.