Create a buffer

This post is about how to create a spatial buffer of 1 km around a point.

Miriam Lerma true
2024-04-04

Intro

In this blog, a buffer subsetting environmental data would be created and the average values within the buffer calculated.

1. Download data

Load the package rerddap

library("rerddap")
SST_info <- info('erdMWsstd1day_LonPM180')

To subset the data select the coordinates of a smaller area.

lonmin<--111
lonmax<--109
latmin<-23
latmax<-27

To download, provide the parameters such as the server, the coordinates and the time frame. It takes some time to download.

SST_values<-griddap(SST_info, 
                    latitude= c(latmin, latmax), longitude = c(lonmin, lonmax), 
                    time = c('2023-03-01T00:00:00Z','2023-03-30T00:00:00Z'), 
                    fields = 'sst')

To access the data as data frame, extract the data.

SST_df<-SST_values$data

Using functions from the package tidyverse, remove all NAs.

library(tidyverse)
SST_dfclean<-SST_df %>%
  filter(sst!='NaN')

Map

Check data downloaded using ggplot.

library(ggplot2)

Download data from land from the package rworldmap.

library(rworldmap)

The function getMap() loads the map in your environment.

world_map <- getMap()

To plot using ggplot a data frame is recommended. Use the function fortify to be able to get the data in a data frame format.

world_points <- fortify(world_map)
world_points$region <- world_points$id
world_df <-world_points[,c("long","lat","group", "region")]
ggplot() +
 geom_raster(data=SST_dfclean,aes(x=longitude, y=latitude, fill = sst))+ scale_fill_viridis_c(option = "H")+
 geom_polygon(data = world_df, aes(x = long, y = lat, group = group), colour = '#403d39', fill = "#e5e5e5") +
 coord_sf(xlim = c(-109.7,-109.1),ylim = c(24.6,25.2))

2. Create buffer

Load package sp.

library(sp)

Give coordinates from the central point.

This_point<-data.frame(Longitude=-109.37728617302638,Latitude=24.92572600240793)

Use this custom function to create a buffer. Note that it uses epsg 4236, adjust if necessary.

create_buffer<-function(central_point=central_point, buffer_km=buffer_km){
  central_spatial<- sp::SpatialPoints(cbind(central_point$Longitude,central_point$Latitude)) 
  sp::proj4string(central_spatial)= sp::CRS("+init=epsg:4326") 
  central_spatial <- sp::spTransform(central_spatial, sp::CRS("+init=epsg:4326"))
  buffer_dist<-buffer_km/100
  central_buffer<-rgeos::gBuffer(central_spatial, width=1*buffer_dist)
  return(central_buffer)
  }

The central point and the kilometers are the arguments needed.

This_buffer<-create_buffer(central_point=This_point,buffer_km=20)

Use the projection to convert the buffer into a SpatialPolygons.

polygon_proj <- proj4string(This_buffer)

Confirm that the buffer is not a SpatialPolygons.

class(This_buffer)
[1] "SpatialPolygons"
attr(,"package")
[1] "sp"

Map

Use ggplot to create the map.
Use the functions geom_raster to plot the SST data.
The function geom_polygon to plot the base maps data.
The function geom_polygon to plot the buffer.
Adjust to your corresponding area on the coord_sf.

ggplot() +
 geom_raster(data=SST_dfclean,aes(x=longitude, y=latitude, fill = sst))+ scale_fill_viridis_c(option = "H")+
 geom_polygon(data = world_df, aes(x = long, y = lat, group = group), colour = '#403d39', fill = "#e5e5e5") +
 coord_sf(xlim = c(-109.7,-109.1),ylim = c(24.6,25.2))+
 geom_polygon(data=This_buffer,aes(x = long, y = lat, group = group),colour='black', fill='transparent', linetype='dashed')+
 geom_point(data=This_point, aes(x=Longitude, y=Latitude), colour = "black", size = 4)

3. Extract information

Convert the SST data into a SpatialPointsDataFrame.

SST_sp <- SST_dfclean
sp::coordinates(SST_sp) <- ~longitude + latitude
sp::proj4string(SST_sp) = sp::CRS("+init=epsg:4326")

Use the function over from the package sp to extract the SST data using the buffer.

SST_buffer<- over(SST_sp,This_buffer)

The SST values that fall inside the buffer are now added as a column named sst_inside in the data frame.

SST_dfclean$sst_inside <- as.numeric(SST_buffer)

The values that fall inside the buffer are shown as 1.
To keep only the data inside the buffer select only those rows with values inside the buffer using the function filter.

SST_inside<-SST_dfclean %>%
  filter(sst_inside==1)%>%
  drop_na(sst)

Subset

To plot the SST data, select the information from the new data frame SST_inside and the function geom_raster to plot the information.

ggplot() +
  geom_raster(data=SST_inside,aes(x=longitude, y=latitude, fill = sst))+ scale_fill_viridis_c(option = "H")+
  geom_polygon(data = world_df, aes(x = long, y = lat, group = group), colour = '#403d39', fill = "#e5e5e5") +
  coord_sf(xlim = c(-109.7,-109.1),ylim = c(24.6,25.2))+
  geom_polygon(data=This_buffer,aes(x = long, y = lat, group = group),colour='black', fill='transparent', linetype='dashed')+
  geom_point(data=This_point, aes(x=Longitude, y=Latitude), colour = "black", size = 4)+
  theme(panel.background = element_blank(),
        panel.border = element_rect(colour = "black", fill='transparent'))

Use the values inside the buffer to calculate the average SST

mean(SST_inside$sst)
[1] 20.61711
sd(SST_inside$sst)
[1] 1.66074

End of document