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Converting Latitude-Longitude Coordinates to Polar Stereographic

Notebook Filename: reproject_epsg4326_to_3412.Rmd
Authors: RG. Cutter, D. Kinzey, J. Sevadjian

When working in the polar regions, we often need to integrate data that are in different projections. This example demonstrates how to use R to convert latitude-longitude coordinates to polar stereographic coordinates in order to integrate in-situ Argo float data with satellite sea ice concentration data.

This R code demonstrates the following techniques:

  • Accessing an in-situ dataset with lat-lon coordinates from the PolarWatch ERDDAP server
  • Converting data with latitude-longitude coordinates (WGS84, EPSG:4326) to projected coordinates (EPSG:3412).
  • Creating a maps of in-situ and projected satellite data together

Install required packages and load libraries

The required packages are maptools, rgdal, sp, oce, ocedata, mapdata, ggplot2, RColorBrewer, ggspatial, ncdf4, lubridate, and scales. The mapdata package is used for a coastline/basemap in ggplot.

# Function to check if pkgs are installed, install missing pkgs, and load
pkgTest <- function(x)
{
  if (!require(x,character.only = TRUE))
  {
    install.packages(x,dep=TRUE,repos='http://cran.us.r-project.org')
    if(!require(x,character.only = TRUE)) stop(x, " :Package not found")
  }
}

list.of.packages <- c("maptools","rgdal","sp", "oce","ocedata", "mapdata", "ggplot2", "RColorBrewer", "ggspatial", "ncdf4", "lubridate","scales")

# create list of installed packages
pkges = installed.packages()[,"Package"]
for (pk in list.of.packages) {
  pkgTest(pk)
}

Download in-situ data with lat/lon coordinates

We will use the SOCCOM BGC Argo float in-situ dataset on the PolarWatch ERDDAP server. This dataset has latitude-longitude coordinates (WGS84).

argo_url <- 'https://polarwatch.noaa.gov/erddap/tabledap/SOCCOM_BGC_Argo_Snapshot_Archive.csv0?latitude%2Clongitude&time%3E=2020-08-23T00%3A00%3A00Z&time%3C=2020-08-30T02%3A34%3A00Z&latitude%3C=-50'

download.file(argo_url, destfile='lat_lon_coordinates_input.csv')

Read in the Argo float coordinates from the .csv file

  • Read in the float coordinates .csv file from our working directory
  • Create a dataframe of lat-lon coordinate points
  • The coordinates are the locations of individual Argo float profiles
infn = "lat_lon_coordinates_input.csv"

indata = read.csv( infn, header=FALSE)

Create Spatial Dataframe

Convert the Argo float coordinate points dataframe to a spatial object (spatial dataframe)

latitude <- indata$V1
longitude <- indata$V2

# Longitude for this dataset goes from 0-360 but the transform library uses -180/180
# Convert from 360 to -180/180
lon180 <- ((longitude + 180) %% 360) - 180
indata$V2 <- lon180

dfcoords = cbind(lon180,latitude)      # coords in lon, lat order
sppoints = SpatialPoints(coords = indata)
spdf     = SpatialPointsDataFrame(coords = dfcoords, indata)

# Verify initial coordinates of spatial dataframe
coordsinit <- spdf@coords

# Define coordinate reference systems
crslonglat       = CRSargs(CRS("+init=epsg:4326")) # order is longitude latitude in R CRS
crsseaicepolster3412 = CRSargs(CRS("+init=epsg:3412"))

Map the Argo Float Locations with ggplot

To get a sense for the Argo float locations in latitude-longitude, we will create a ggplot map using the lat-lon float coordinates.

  • This map uses latitude-longitude for the axes
xlim <- c(-180,180)
ylim <- c(-80,-45)
coast <- map_data("worldHires", ylim = ylim, xlim = xlim)
myplot<-ggplot(data = indata, aes(x = V2, y = V1)) +
  geom_polygon(data = coast, aes(x=long, y = lat, group = group), fill = "grey80") +
  theme_bw(base_size = 15) + ylab("Latitude") + xlab("Longitude") +
  coord_fixed(2.7,xlim = xlim, ylim = ylim) +
  geom_point(data=indata, aes(x=V2, y=V1), size=1, shape=21, color="blue") +
  ggtitle("Argo Float locations, Lat-Lon Coordinates with ggplot") +
  theme(plot.title = element_text(hjust = 0.5))
  
myplot

Map the Argo Float Locations on a Polar Stereographic Map

To get a better sense of the float locations, let’s plot the same float location data again, this time with a polar stereographic view.

  • This map is made using the OCE package

  • This plotting package allows us to specify the projection of the map while still using lat-lon coordinates for the float locations

# define map extents
ylim <- c(-79,-50)
xlim <- c(-170,70)
data("coastlineWorldMedium") # included in ocedata

# set plot margins (bottom, left, top, right)
par(mar=c(2, 6, 2, 6))

# make a base
mapPlot(coastlineWorldMedium, 
        projection=crsseaicepolster3412,
        col="lightgray", 
        longitudelim=xlim, 
        latitudelim=ylim,
        main="Argo Float Locations on a Polar Stereographic OCE Map"
        )

## add float location points
mapPoints(lon180, latitude, col = "blue", pch = 20)

Transform the Argo float coordinates from latitude-longitude to polar stereographic

We ultimately want to analyze the Argo float data in conjunction with a sea ice dataset that is in polar stereographic coordinates. So, next we will demonstrate converting the coordinates of the Argo float data into the projected coordinates of a satellite sea ice data set.

  • Define each of the coordinate reference systems
  • The satellite sea ice dataset is the NOAA Sea Ice Concentration CDR which is in the NSIDC Polar Stereographic South projection. The code for this projection is EPSG:3412.
  • The Latitude-Longitude projection of the Argo float dataset is known as WGS84 with a code of EPSG:4326.
  • Transform the lat-lon coordinates to polar stereographic with spTransform
  • Check new coordinates with coords and bbox
# Set initial CRS of spatial dataframe
proj4string(spdf) = CRS(crslonglat)
lat_lon_bbox       = spdf@bbox
print(lat_lon_bbox)
##               min     max
## lon180   -179.641 166.398
## latitude  -73.909 -51.006

# Check the CRS 
crs_set = proj4string(spdf)

# Converts from existing lat-lon crs to polar stereo (3412)
spdfProjected = spTransform(spdf, CRS(crsseaicepolster3412))  
crs_projected = proj4string(spdfProjected)

coordsproj = spdfProjected@coords
bbox       = spdfProjected@bbox
print( bbox )
##               min     max
## lon180   -4385774 3967157
## latitude -3924592 3401168

Get polar projected sea ice satellite data from ERDDAP

Next we will download the sea ice satellite data from the PolarWatch ERDDAP. This dataset is in NSIDC Polar Stereographic South (EPSG:3412) projection. Here we use a preconfigured ERDDAP data request url to access the NOAA Sea Ice Concentration CDR dataset from ERDDAP.

For more information about accessing this dataset from ERDDAP, including step-by-step examples of subsetting the dataset and moving between projected and lat-lon coordinates, see the “Accessing Projected Datasets” R tutorial in our course materials. https://github.com/CoastWatch-WestCoast/r_code/blob/master/accessing_projected_datasets.md

sea_ice_url <- 'https://polarwatch.noaa.gov/erddap/griddap/nsidcG02202v4sh1day.nc?cdr_seaice_conc%5B(2021-05-31T00:00:00Z):1:(2021-05-31T00:00:00Z)%5D%5B(4337500.0):1:(-3937500.0)%5D%5B(-3937500.0):1:(3937500.0)%5D'

# Download the sea ice data NetCDF file  (a single timestamp)
sea_ice_data_nc <- download.file(sea_ice_url, destfile="sea_ice_data.nc", mode='wb')
sea_ice_dataFid <- nc_open('sea_ice_data.nc')

ygrid <- ncvar_get(sea_ice_dataFid, varid="ygrid")
xgrid <- ncvar_get(sea_ice_dataFid, varid="xgrid")
seaiceCDR <- ncvar_get(sea_ice_dataFid, varid='cdr_seaice_conc')

nc_close(sea_ice_dataFid)

Plot the projected in-situ data with the NSIDC Sea Ice CDR

Lastly, we will verify the placement of the transformed float locations by viewing them a polar projected map along side the satellite sea ice dataset.

  • Plot the projected Argo float point data on top of the NOAA sea ice concentration CDR data using ggplot
  • This map has projected coordinates as the axes
  • Visually check the locations of the transformed coordinates
icemap2 <- expand.grid(xgrid=xgrid, ygrid=ygrid)

icemap2$Seaice <- array(seaiceCDR[,], dim(xgrid)*dim(ygrid))
icemap2$Seaice[icemap2$Seaice > 2] <- NA 

df_projected <- as.data.frame(spdfProjected)

main=" Polar Projected In-situ Argo Float locations\n and the NOAA Sea Ice Concentration CDR"  
myplot <- ggplot(data = icemap2, aes(x = xgrid, y = ygrid, fill=Seaice) ) + 
       geom_tile() + 
       coord_fixed(ratio = 1) + 
       scale_y_continuous(labels = comma) + 
       scale_x_continuous(labels = comma) +
       scale_fill_gradientn(colours=rev(brewer.pal(n = 1, name = "Blues")),na.value="black") +
       geom_point(data=df_projected, aes(x=lon180, y=latitude),inherit.aes = FALSE, size=2,shape=21,color="black") +
       labs(title=main)+
    theme(plot.title = element_text(hjust = 0.5))

myplot

History:
Mar 2022: Updated satellite dataset to latest version of CDR. Simplified by removing subsetting projected data portion.
Nov 2020: Converted to Rmd notebook, added ERRDAP lat-lon coordinate retrieval, J. Sevadjian.
Feb 2020: Created code snippet, simplified from: AMLR_GIS_in_R by RG. Cutter and D. Kinzey. Nov 2017: AMLR_GIS_in_R.R, G Cutter.