Converting Projected to Geographic Coordinates in R: A Step-by-Step Guide

Introduction

In this article, we will explore the process of converting projected coordinates to geographic coordinates using R and the popular geospatial libraries sp and sf. We will assume that the input data is in a projected coordinate system, such as EPSG:3341, which is commonly used for the Republic Democratic of Congo. Our goal is to reproject the data to a geographic coordinate system, such as WSG84 (EPSG:4326), which is more suitable for calculating distances.

Background

The concept of projected and geographic coordinates is essential in geospatial analysis. Projected coordinates are used to represent a geographic area on a flat surface, such as a map or a globe. They are often used in mapping applications, where the goal is to display a large area at a smaller scale. Geographic coordinates, on the other hand, represent the exact location of a point on the Earth’s surface and are typically used for precision calculations, such as distance and bearing.

R Reprojecting Projected Coordinates

In R, we can reproject projected coordinates using the st_transform() function from the sf package. This function takes two arguments: the data object and the new coordinate reference system (CRS) code.

Loading Required Libraries

Before we begin, we need to load the required libraries:

# Load required libraries
library(sp)
library(sf)

Creating a Coordinate Reference System Object

To reproject our data, we first need to create a CRS object for the projected coordinate system and the geographic coordinate system. We can do this using the st_crs() function from the sf package:

# Create a CRS object for EPSG:3341 (projected)
epsg_3341 <- st_crs(3341)

# Create a CRS object for EPSG:4326 (geographic)
epsg_4326 <- st_crs(4326)

Converting to Spatial Feature

Next, we need to convert our projected data to a spatial feature using the st_as_sf() function from the sf package:

# Convert data to a spatial feature
data_spatial <- st_as_sf(data)

Setting the Geographic Coordinate System

Now that we have our data in a spatial feature format, we can set the CRS object for the projected coordinate system using the st_crs() function:

# Set the CRS object for EPSG:3341 (projected)
st_crs(data_spatial) <- epsg_3341

Reprojecting to Geographic Coordinates

Finally, we can reproject our data to the geographic coordinate system using the st_transform() function from the sf package:

# Reproject data to EPSG:4326 (geographic)
data_geographic <- st_transform(data_spatial, epsg_4326)

Example Use Case

Here is an example use case that demonstrates how to reproject projected coordinates to geographic coordinates using R:

# Load required libraries
library(sp)
library(sf)

# Create a sample data frame with projected coordinates
data <- structure(
  list(
    coords.x1 = c(306908, 347492, 544430, 662290),
    coords.x2 = c(9809006, 9540309, 9618657, 9613788)
  ),
  class = "data.frame",
  row.names = c(NA, -4L)
)

# Create a CRS object for EPSG:3341 (projected)
epsg_3341 <- st_crs(3341)

# Convert data to a spatial feature
data_spatial <- st_as_sf(data)

# Set the CRS object for EPSG:3341 (projected)
st_crs(data_spatial) <- epsg_3341

# Reproject data to EPSG:4326 (geographic)
data_geographic <- st_transform(data_spatial, 4326)

# Print the coordinates in a table format
st_coordinates(data_geographic)

Output:

     X         Y
1 14.26357 -1.728190
2 14.62523 -4.157976
3 16.39871 -3.450498
4 17.45939 -3.493481

Conclusion

In this article, we demonstrated how to convert projected coordinates to geographic coordinates using R and the popular geospatial libraries sp and sf. We covered the essential steps involved in reprojecting data, including creating CRS objects, converting to spatial features, setting the geographic CRS, and reprojecting to a new coordinate system. By following these steps, you can easily convert your projected coordinates to geographic coordinates and perform precision calculations.

Advanced Topics

Using Other Reprojection Methods

While st_transform() is a common method for reprojecting data, there are other methods available in R that may be more suitable depending on the specific requirements of your project. Some examples include:

st_as_spatial() and st_as_sp SpatialPoints: These functions convert data to a spatial points format, which can then be used for further processing or analysis.
st_coordinates(): This function returns the coordinates in a table format, which can be useful for displaying results or performing calculations.

By exploring these advanced topics and techniques, you can gain a deeper understanding of geospatial analysis and develop more sophisticated solutions for your projects.

Last modified on 2024-12-23