Understanding sf::st_transform() and Orthographic Projections in R Studio

sf is a popular package for working with simple features (such as polygons, lines, and points) in R. One of its most useful functions is st_transform(), which allows you to reproject shapes from one coordinate reference system (CRS) to another. In this article, we’ll explore how to use sf::st_transform() for orthographic projections and address a common issue where the function returns empty geometries.

Introduction to Orthographic Projections

Orthographic projections are a type of map projection that aims to preserve angles between features and minimize distortions. Unlike Mercator or Gall-Peters projections, which distort sizes and shapes, orthographic projections maintain a fixed aspect ratio and scale. These projections are useful for illustrating the appearance of features on a globe without distorting their shape.

sf::st_transform() Function

The sf::st_transform() function in R Studio is used to reproject simple features (polygons, lines, points) from one CRS to another. This can be necessary when working with data that has been collected or projected using different coordinate systems.

## Usage:
## st_transform(object, crs)

## Parameters:

object: A simple feature object containing the geometry you want to transform.
crs: The new CRS for which you want to reproject the geometry.

## Returns:
A new simple feature object with the same features but in the specified CRS.

Common Issues with sf::st_transform()

When using sf::st_transform(), there are some common issues that can arise, including:

• Empty Geometries: In certain cases, the resulting geometry might be empty. This could be due to a variety of factors such as invalid or incomplete data, incorrect CRS conversions, or overlapping features.
• Incorrect Projections: If the new CRS is not compatible with the original geometry, it may result in an empty geometry.

Solving Empty Geometries Using sf::st_transform()

To resolve issues with empty geometries when using sf::st_transform(), you need to identify and address any potential errors or inconsistencies that might be causing this issue. Here are some possible solutions:

• Check Data Quality: Verify the integrity of your data by checking for any missing values, invalid CRS conversions, or overlapping features.
• Verify CRS Conversions: Make sure that the new CRS is compatible with the original geometry and that all necessary calculations have been performed correctly.
• Use Stacked Projections: In some cases, using stacked projections can help resolve issues related to empty geometries. This approach involves reprojecting both the feature and its background separately before combining them.

Example: sf::st_transform() with Orthographic Projection

To demonstrate how sf::st_transform() works with orthographic projections, we’ll use the example provided in the original question. We’ll create a simple feature object containing the mainland Russia polygon and reproject it using the specified CRS.

library(sf)
library(giscoR) # for the countries dataset only

# projection string used for the polygons & ocean background
crs_string <- "+proj=ortho +lat_0=20 +lon_0=0 +x_0=0 +y_0=0 +a=6371000 +b=6371000 +units=m +no_defs"

# create a simple feature object with mainland Russia polygon
world <- gisco_countries %>% 
  st_intersection(ocean %>% st_transform(4326)) %>% # select visible area only
  st_transform(crs = crs_string) # reproject to ortho

# display the resulting geometry
ggplot(data = world, aes(geometry = st_geometry)) +
  geom_sf() +
  theme_void()

By examining this example and understanding how sf::st_transform() works with orthographic projections, you can resolve issues related to empty geometries and create accurate visualizations of your data.

Conclusion

Using sf::st_transform() for orthographic projections is a powerful tool for working with simple features in R Studio. By following the steps outlined above and addressing common issues such as empty geometries, you can ensure that your data is accurately reprojected and displayed correctly.

Last modified on 2025-02-20