Understanding the tract_choropleth Function in Choroplethr v3.6.0 for R
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In this article, we will delve into the world of choropleth mapping using the tigris package in R, specifically focusing on the tract_choropleth function in Choroplethr v3.6.0. We’ll explore common pitfalls and potential solutions to issues that may arise during data manipulation and visualization.
Background
Choroplethr is an R package designed for creating choropleth maps, which are a type of map where areas (such as countries, states, or census tracts) are colored based on some attribute. Choroplethr v3.6.0 is the latest version available on CRAN (Comprehensive R Archive Network). The tigris package, used by Choroplethr, provides an interface to download shapefiles from various sources, including the United States Census Bureau.
The Problem
We’ll examine a Stack Overflow post where the user reports difficulty mapping areas outside of New York using the tract_choropleth function. Despite being able to import underlying tract data for all areas, the function throws an error when trying to map non-NY tracts.
Error Analysis
The error message returned is quite informative:
Error in ogrInfo(dsn = dsn, layer = layer, encoding = encoding,
use_iconv = use_iconv, : Cannot open layer In addition: Warning
message: In unzip(file_loc, exdir = cache_dir, overwrite = TRUE) :<br/>
error 1 in extracting from zip file
The warning message suggests a problem with the download of shapefile data from the Census Bureau website. This can happen if the downloaded files are corrupted.
Solution
To resolve this issue, we’ll explore two potential solutions:
Solution 1: Enable Tigris Refresh
One solution to this problem is to enable tigris refresh, which will direct the package to re-download shapefile data from the Census Bureau website. This can be achieved by running:
library(tigris)
options(tigris_refresh = TRUE)
By setting tigris_refresh to TRUE, we ensure that the package will automatically download the latest version of the shapefiles whenever it’s needed.
Solution 2: Disable Tigris Refresh
However, this may not be desirable in all cases. To control future re-downloads, you can set options(tigris_refresh = FALSE):
library(tigris)
options(tigris_refresh = FALSE)
This will disable the automatic refresh feature for tigris.
Additional Considerations
When working with choropleth maps and shapefiles, there are several additional considerations to keep in mind:
Cache Control
Tigris uses caching to improve performance. However, this can sometimes lead to issues if the cache is outdated or corrupted. By enabling tigris refresh, we ensure that our package will use the latest version of the shapefiles.
Shapefile Formats
The tigris package supports various shapefile formats, including ESRI (Shp) and GeoPackage (.gp). If you’re working with a specific format, make sure to select it in your options(tigris_format) setting:
library(tigris)
options(tigris_format = "ESRI")
Spatial Reference Systems
It’s essential to match the spatial reference system (SRS) of your shapefiles and map projections. Choroplethr uses the ESRI SRS (EPSG 4326), so make sure to use this as well:
library(tigris)
options(tigris_srs = "EPSG:4326")
Best Practices
To avoid common pitfalls when working with choropleth maps and shapefiles:
Always Check the Source Data
Verify that your shapefile data is correct and free from errors. This can be done by inspecting the file or using tools like QGIS to analyze its contents.
Validate Spatial References
Make sure you’re working with the correct SRS for your shapefiles and map projections. This will help ensure accurate color transitions between adjacent areas.
Use Caching Wisely
While caching can improve performance, be cautious of stale data. Regularly update your cache by re-downloading shapefile data or setting tigris_refresh to TRUE.
By understanding the inner workings of the tract_choropleth function and following these best practices, you’ll be well-equipped to create high-quality choropleth maps using Choroplethr v3.6.0 in R.
Conclusion
In conclusion, this article has explored common pitfalls and potential solutions for issues that may arise during data manipulation and visualization with the tract_choropleth function in Choroplethr v3.6.0. By following the advice outlined above, you’ll be able to create beautiful and informative choropleth maps with ease.
Additional Resources
For more information on using tigris and Choroplethr, please refer to the official documentation:
You can also explore additional resources on spatial data analysis, shapefiles, and choropleth mapping.
By following the guidelines outlined in this article, you’ll be well on your way to creating stunning choropleth maps that effectively communicate complex information.
Last modified on 2025-02-15