Creating a Transect Line

The goal is to set up a transect line with evenly spaced points defined by latitude and longitude. We will use this transect to download ocean remote-sensing variables at those locations.

First we will set up our two points.

library(data.table)
df <- data.frame(
  Latitude=c(9.9413972, 9.2),
  Longitude=c(76.2, 74)
  )
data.table::setDT(df)

Here is where they are on a map.

library(rworldmap)
newmap <- rworldmap::getMap(resolution = "low")
plot(newmap, xlim = c(75,76), ylim = c(8,13), asp = 1)
points(df$Longitude, df$Latitude, col = "red", cex = .6)

Next we will interpolate points every 10km on this line.

dist.between = 10000 #in meters

library(geotools)
library(geosphere)    # for distHaversine
get.dist <- function(lon, lat) geosphere::distHaversine(tail(cbind(lon,lat),-1), head(cbind(lon,lat),-1))
# The := function is in data.table
df[,dist:=c(0,cumsum(get.dist(Longitude,Latitude)))]

interp.dist <- function(var,dist) approx(dist,var,xout=seq(min(dist),max(dist),by=dist.between))$y
result <- setDT(df)[,lapply(.SD,interp.dist,dist=dist), 
                    .SDcols=c("Latitude","Longitude","dist")]

Now we can plot these points.

# plot the result
newmap <- rworldmap::getMap(resolution = "low")
plot(newmap, xlim = c(74,76), ylim = c(8,13))
points(df$Longitude, df$Latitude, col = "red", cex = .6)
points(Latitude~Longitude,result, col="blue")
axis(side=1)
axis(side=2)

We can plot with ggplot() also.

library(ggplot2)
india <- rworldmap::getMap(resolution = "low")

(with_india <- ggplot() +
    geom_path(data = india, 
        aes(x = long, y = lat, group = group),
        fill = NA, colour = "black") + 
    geom_point(data = result, 
        aes(x = Longitude, y = Latitude)) +
  xlim(74,80) +
  ylim(7,13) +
    coord_quickmap() +  # Prevents stretching when resizing
    theme_classic() +  # Remove ugly grey background
    xlab("Longitude") +
    ylab("Latitude"))

That’s really ugly so let’s improve it.

Download bathymetry data from NOAA.

library(marmap)#if it is not in your package list install it simply by marmap on the dialog box
xlims <- c(73,80)
ylims <- c(6,15)
bathydata<- getNOAA.bathy(xlims[1], xlims[2], ylims[1], ylims[2])#Be sure the locations are same

Plot the data with our transect line. You have to tweak the plot region otherwise you will have white space around your plot. It has to do with setting asp=1 (default) to not distort your plot. You can change to asp=NA once your plot size is close to asp=1.

# code adapted from 
# https://pepprbook.com/2018/02/17/mapping-in-r/
#Load the bathymetry data from NOAA using marmap
library(rworldmap)
library(rworldxtra)
library(maps)
#Make some colour combinations.Here I took blues and greys

blues <- c("lightsteelblue4", "lightsteelblue3",
 "lightsteelblue2", "lightsteelblue1")
greys <- c(grey(0.6), grey(0.93), grey(0.99))
#Now plot the data 'bathydata' with the colours we created
#par(mar=c(5, 10, 5, 10) + 0.1)
plot(bathydata, image = TRUE, land = TRUE, lwd = 0.03,
 bpal = list(c(0, max(bathydata), greys),
 c(min(bathydata), 0, blues)), asp=1)
#make thickened contour lines
plot(bathydata, n = 4, lwd = 0.4, add = TRUE)
#Add the map overlay
map("world","India",col = "grey",fill = TRUE,add=TRUE,xlim=xlims,ylim=ylims, lwd=2)
#Add your sampling points(pch=25 makes it inverted triangles)
points(result$Longitude,result$Latitude,pch=16,col="black",cex=.5)
box(lty="solid", lwd=2)

We can also make the plot with ggplot().

autoplot(bathydata, geom=c("r", "c")) + 
  scale_fill_etopo() +
  ylim(7,13) +
  xlim(73,79)+
  geom_point(aes(x=Longitude, y=Latitude), data=result, alpha=1)