How to edit cells Shapefile in R? - r

Question

I downloaded a shapefile of the world map from gadm, however, there are typos that happen with importing the shape file. For example "Aland" shows up as "Ã…land" in the shapefile. There are handful of countries I want to make changes to.
world map shapefile, the one that says "You can also download this version as six separate layers (one for each level of subdivision/aggregation), as a geopackage database or as shapefiles" :https://gadm.org/download_world.html
I imported the shapefile using:
worldmap <- readOGR("file/gadm36_0.shp")
I tried using the following code:
levels(wordlmap$NAME_0)[5] <- "Aland"
However I got this message:
Error in `levels<-`(`*tmp*`, value = c(NA, NA, NA, NA, "Aland")) :
factor level [2] is duplicated
Could you suggest how this code can be made better or an alternative.Thanks in advance

Answer 1

Since you did not provide a shapefile, I just worked with a publicly-available shapefile of Indian states. The long and short of it is to use the sf package. It loads shapefiles as (quasi) dataframes--with the longitudes and latitudes stored in the geometry variable. Then, you should be in familiar territory. Here is some code to change a state name variable:
# clear environment
rm(list=ls(all=TRUE))
# let's take admin 1 (states)
# note: already in WGS84 format
library(sf)
india_shape <- st_read("india_shape/gadm36_IND_1.shp", stringsAsFactors=FALSE)
# Let's pick something to change (state name)
> india_shape$NAME_1[1]
[1] "Andaman and Nicobar"
# Now change it
> india_shape$NAME_1[1] <- "New State Name"
> india_shape$NAME_1[1]
[1] "New State Name"

Answer 2

I can tell you a few things about how I manage those shape files downloaded from, www.gadm.org site.
First, a shape file has several other related files that do not have the .shp extension. These files must remain together in the same folder. All these files are included in the shape zip file from the gadm website.
The rgdal package provides the, readOGR() function. This function is normally in the form: readOGR(dsn = " ", layer = " " )
The, dsn is "data source name". Use quotes.
The, layer is the name of shape file without the .shp extension. Use quotes.
Proper file management is required to make things work and to maintain good file management. I already have a USA folder within my dataset folder.
I just downloaded the gadm USA shape file. So first, I will add a new folder, named USA_map, in the USA folder. And also create a new folder named, data in this new USA_map folder.
C:/python/datasets/usa/usa_map/data # usa_map/data are new
Copy the downloaded gadm36_USA_shp.zip from the "download" folder and paste into the new "USA_map" folder. Then, open the GADM zip folder and extract the entire contents of the zip folder into the new "data" folder. Then the zip file can new be deleted because all the files have been copied into the "data" folder. All's done and ready.
Now use the readOGR() function to read the shape file and assign to new variable, called usmap
usmap <- readOGR(dsn = "c:/python/datasets/USA/USA_map/data", layer = "gadm36_USA_1")
The trick is to follow the correct file management so the readOGR() function works as designed.
Next, you need to learn how to navigate through this type of data.

Answer 3

If there is more than one polygon with the same wrong name, you can make like that :
w <- length(wordlmap)
for (i in 1:w){
if (wordlmap$NAME_0[i] == "Ã…land") {
wordlmap$NAME_0[i] <- "Aland"
}}