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Merging Shape Files

When we work with spatial anaylsis, it is quite often we need to deal with data in different format and at different scales. For example, I have nc data with global pm2.5 estimation with \(0.01\times 0.01\) resolution. But I want to see the pm2.5 estimation in municipal level. I need to integrate my nc file into my municipality shp file so that I can group by the data into municipal level and calculate the mean. Then, I can make a map of it.

In this page, I will use Brazil’s pm2.5 estimation and its shp file in municipal level.

Keep in Mind

  • It doesn’t have to be nc file to map into the shp file, any format that can read in and convert to a sf object works. But the data has to have geometry coordinates(longitude and latitude).

Implementations

R

Unusually for LOST, the example data files cannot be accessed from the code directly. Please visit this page and download both files to your working directory before running this code.

It is also strongly recommended that you find a high-powered computer or cloud service before attempting to run this code, as it requires a lot of memory.

# If necesary
# install.packages(c('ncdf4','sp','raster','dplyr','sf','ggplot2','reprex','ggsn'))
# Load packages
library(ncdf4)
library(sp)
library(raster)
library(dplyr)
library(sf)
library(ggplot2)
library(reprex)

### Step 1: Read in nc file as a dataframe*
pm2010 = nc_open("GlobalGWRwUni_PM25_GL_201001_201012-RH35_Median_NoDust_NoSalt.nc")
nc.brick = brick("GlobalGWRwUni_PM25_GL_201001_201012-RH35_Median_NoDust_NoSalt.nc")
# Check the dimensions
dim(nc.brick)

# Turn into a data frame for use
nc.df = as.data.frame(nc.brick[[1]], xy = T)

head(nc.df)

### Step 2: Filter out a specific country.
# Global data is very big. I am going to focus only on Brazil.
nc.brazil = nc.df %>% filter(x >= -73.59 & x <= 34.47 & y >= -33.45 & y <= 5.16)
rm(nc.df)
head(nc.brazil)

### Step 3: Change the dataframe to a sf object using the st_as_sf function
pm25_sf = st_as_sf(nc.brazil, coords = c("x", "y"), crs = 4326, agr = "constant")
rm(nc.brazil)
head(pm25_sf)

### Step 4: Read in the Brazil shp file. we plan to merge to
Brazil_map_2010 = st_read("geo2_br2010.shp")
head(Brazil_map_2010)

### Step 5: Intersect pm25 sf object with the shp file.*
# Now let's use a sample from pm25 data and intersect it with the shp file. Since the sf object is huge, I recommend running the analysis on a cloud server
pm25_sample = sample_n(pm25_sf, 1000, replace = FALSE)

# Now look for the intersection between the pollution data and the Brazil map to merge them
pm25_municipal_2010 = st_intersection(pm25_sample, Brazil_map_2010)
head(pm25_municipal_2010)

### Step 6: Make a map using ggplot
pm25_municipal_2010 = pm25_municipal_2010 %>%
  select(1,6)
pm25_municipal_2010 = st_drop_geometry(pm25_municipal_2010)
Brazil_pm25_2010 = left_join(Brazil_map_2010, pm25_municipal_2010)
ggplot(Brazil_pm25_2010) +
  # geom_sf creates the map we need
  geom_sf(aes(fill = -layer), alpha=0.8, lwd = 0, col="white") +
  # and we fill with the pollution concentration data
  scale_fill_viridis_c(option = "viridis", name = "PM25") +
  ggtitle("PM25 in municipals of Brazil")+
  ggsn::blank()