Converting Rasters from GeoTIFF (.tif) to ASCII (.asc) Format

Author details: Dr Ryan Newis

Editor details: Xiang Zhao

Contact details: support@ecocommons.org.au

Copyright statement: This script is the product of the EcoCommons platform. Please refer to the EcoCommons website for more details: https://www.ecocommons.org.au/

Date: Feb 2026

Script and Data Information

This notebook provides a simple, reproducible example of converting a raster dataset stored as a GeoTIFF (.tif) into an ASCII grid (.asc) format using the terra package in R. It is intended as a lightweight template that can be adapted to other raster-based spatial datasets and workflows.

Introduction

GeoTIFF is a widely used raster format in modern spatial analysis workflows due to its efficiency, flexibility, and broad software support. However, ASCII grid files remain important for interoperability with legacy systems, environmental models, and software tools that require text-based raster inputs.

Although format conversion is often treated as a straightforward technical step, preserving spatial metadata during this process is essential. Changes to the coordinate reference system (CRS), spatial extent, or grid resolution can introduce subtle spatial misalignment that may not be immediately visible but can affect downstream analyses and modelling results.

This notebook demonstrates a structured and transparent approach to converting a GeoTIFF raster to an ASCII grid while explicitly verifying that key spatial properties including CRS, extent, and resolution are maintained. By incorporating validation steps into the workflow, the notebook promotes reproducibility and safeguards spatial data integrity.

Key Concepts

K.1 Raster data formats

Raster datasets represent spatial information as a grid of cells, where each cell contains a value representing a measured or modelled variable. Different raster formats vary in efficiency, storage, and compatibility with analysis tools.

K.2 Raster data handling in R with terra

The terra package provides a modern framework for reading, manipulating, and writing raster data in R. It supports a wide range of raster formats, including GeoTIFF and ASCII grids, and is designed for performance, clarity, and reproducibility.

K.3 Converting between raster formats

Converting raster data between formats (e.g. GeoTIFF -> ASCII) is a common preprocessing step that supports compatibility with modelling tools, data sharing requirements, and legacy workflows while preserving spatial structure and metadata. A robust conversion workflow must ensure that key spatial metadata including coordinate reference system (CRS), spatial extent, and resolution are preserved during the process to avoid unintended spatial misalignment or projection errors.

Notebook Objectives

1. Demonstrate how to read a GeoTIFF (.tif) raster file into R as a SpatRaster using the terra package.
2. Perform a basic visual check (plot) to confirm that the raster has been loaded correctly.
3. Save the raster to an ASCII grid (.asc) format while preserving the original file naming.
4. Verify that key spatial metadata (CRS, extent, and resolution) are preserved during the conversion process.

Workflow Overview

This notebook follows a simple, linear workflow to convert a GeoTIFF raster to an ASCII grid format:

• Step 1: Set up the R environment and load the required package (terra).
• Step 2: Define and validate the file path to the input GeoTIFF (.tif) raster.
• Step 3: Read the GeoTIFF into R as a SpatRaster object and confirm that a valid CRS is defined.
• Step 4: Perform a quick visual check by plotting the raster to confirm it was read correctly.
• Step 5: Write the raster to disk as an ASCII grid (.asc) file, preserving the original file name and providing a transparent output location.
• Step 6: Validate that CRS, spatial extent, and resolution are preserved after conversion.

In the near future, this material may form part of comprehensive support materials available to EcoCommons users. If you have any corrections or suggestions to improve the efficiency, please contact the EcoCommons team.

Step 1. Install and load required packages

Install and load R packages and libraries that are not present in the work environment. For this notebook, only the terra package is required for the GeoTIFF (.tif) to ASCII (.asc) conversion and googledrive to download the example GeoTIFF file. .

# Install and load required packages (if not already installed)
required_packages <- c(
  "terra",
  "googledrive"
)

for (pkg in required_packages) {
  
  # Install if missing
  if (!pkg %in% installed.packages()[, "Package"]) {
    install.packages(pkg)
    cat("Package installed successfully:", pkg, "\n")
  }
  
  # Load quietly
  suppressPackageStartupMessages(
    library(pkg, character.only = TRUE)
  )
  
  cat("Package loaded successfully:", pkg, "\n")
}

Package loaded successfully: terra 
Package loaded successfully: googledrive 

Step 2. Define the path to the input GeoTIFF file

We have provided an example GeoTIFF (.tif) for this notebook via OPTION 2 – Google Drive. If you would like to use your own GeoTIFF, choose OPTION 1, commenting out this line:

tif_file <- NULL

and uncomment this line with your updated path:

tif_file <- "/path_to_your_file"

The example GeoTIFF is an continuous annual mean temperature layer, using Australia as the boundary. The original data was obtained from https://www.worldclim.org/data/worldclim21.html

Note: The input GeoTIFF raster must have a valid Coordinate Reference System (CRS) defined. If the GeoTIFF file does not contain CRS information, the conversion will stop.

# OPTION 1: User-defined local GeoTIFF
tif_file <- NULL
#tif_file <- "/path_to_your_file"


# OPTION 2: Download example GeoTIFF if no local file
if (is.null(tif_file)) {

  drive_deauth()

  workspace <- getwd()
  datafolder_path <- file.path(workspace, "data")

  if (!dir.exists(datafolder_path)) {
    dir.create(datafolder_path, recursive = TRUE)
  }

  tif_file_id <- "11k1icyISQOGVNJmLR4Cy-Zhwn-X-a_RJ"

  # Get file metadata (to retrieve original name)
  file_info <- drive_get(as_id(tif_file_id))

  # Build full local path using original filename
  tif_file <- file.path(datafolder_path, file_info$name)

  # Download to that exact file path
  drive_download(
    as_id(tif_file_id),
    path = tif_file,
    overwrite = TRUE
  )
}

File downloaded:

• 'australia_annual_mean_temp.tif' <id: 11k1icyISQOGVNJmLR4Cy-Zhwn-X-a_RJ>

Saved locally as:

• '/Users/xiangzhaoqcif/dev/notebook-blog/notebooks/raster_format/data/australia_annual_mean_temp.tif'

# Final safety check
if (!file.exists(tif_file)) {
  stop("Input GeoTIFF file not found.")
}

# Extract base file name (used downstream)
base_name <- tools::file_path_sans_ext(basename(tif_file))

Step 3. Read GeoTIFF raster into R

The rast() function reads the GeoTIFF file into R as a SpatRaster object.

#Read GeoTIFF raster into R
input_raster <- rast(tif_file)

cat("CRS of input raster:\n")

CRS of input raster:

print(crs(input_raster))

[1] "GEOGCRS[\"WGS 84\",\n    ENSEMBLE[\"World Geodetic System 1984 ensemble\",\n        MEMBER[\"World Geodetic System 1984 (Transit)\"],\n        MEMBER[\"World Geodetic System 1984 (G730)\"],\n        MEMBER[\"World Geodetic System 1984 (G873)\"],\n        MEMBER[\"World Geodetic System 1984 (G1150)\"],\n        MEMBER[\"World Geodetic System 1984 (G1674)\"],\n        MEMBER[\"World Geodetic System 1984 (G1762)\"],\n        MEMBER[\"World Geodetic System 1984 (G2139)\"],\n        MEMBER[\"World Geodetic System 1984 (G2296)\"],\n        ELLIPSOID[\"WGS 84\",6378137,298.257223563,\n            LENGTHUNIT[\"metre\",1]],\n        ENSEMBLEACCURACY[2.0]],\n    PRIMEM[\"Greenwich\",0,\n        ANGLEUNIT[\"degree\",0.0174532925199433]],\n    CS[ellipsoidal,2],\n        AXIS[\"geodetic latitude (Lat)\",north,\n            ORDER[1],\n            ANGLEUNIT[\"degree\",0.0174532925199433]],\n        AXIS[\"geodetic longitude (Lon)\",east,\n            ORDER[2],\n            ANGLEUNIT[\"degree\",0.0174532925199433]],\n    USAGE[\n        SCOPE[\"Horizontal component of 3D system.\"],\n        AREA[\"World.\"],\n        BBOX[-90,-180,90,180]],\n    ID[\"EPSG\",4326]]"

# Stop if CRS is missing
if (is.na(crs(input_raster)) || crs(input_raster) == "") {
  stop(
    "❌ Input GeoTIFF raster has NO CRS defined.\n",
    "Please ensure the file has a valid CRS before proceeding."
  )
}

Step 4. Plot raster

Plot the raster to confirm that the GeoTIFF was read correctly into R. At this stage, the raster exists only as an in-memory object and has not yet been written to disk.

plot(input_raster, main = base_name)

Step 5. Save ASCII output

By default, the output is written to the same directory as the input GeoTIFF file.

Note: Writing an ASCII grid may create additional companion files alongside the .asc file, such as:

• .prj — stores the coordinate reference system (CRS) definition
• .aux.xml — stores auxiliary metadata

These files are automatically generated by GDAL/terra and should be kept together with the .asc file (i.e. same folder). Unlike GeoTIFF, the ASCII grid format does not always retain EPSG identifiers explicitly, even when the CRS definition remains spatially equivalent.

# Save .asc output file to the same folder as the original GeoTIFF input raster 
out_dir <- dirname(tif_file)

# Build the output file name (using previously defined base_name)
out_file  <- file.path(out_dir, paste0(base_name, "_converted.asc"))

# Write the raster to disk as an ASCII grid file
writeRaster(
  input_raster,
  out_file,
  filetype = "AAIGrid",
  NAflag = -9999,
  overwrite = TRUE
)

cat("ASCII grid written to:\n", out_file, "\n")

ASCII grid written to:
 /Users/xiangzhaoqcif/dev/notebook-blog/notebooks/raster_format/data/australia_annual_mean_temp_converted.asc 

Step 6. Verify spatial metadata after writing ASCII

Reload the written ASCII grid and verify that key spatial metadata including CRS, spatial extent, and resolution have been preserved during conversion.

# Reload written ASCII file
written_raster <- rast(out_file)

# CRS check

# Extract CRS info for input
input_crs_info <- terra::crs(input_raster, describe = TRUE)
input_crs_name <- input_crs_info$name
input_epsg <- input_crs_info$code

# Extract CRS info for output
output_crs_info <- terra::crs(written_raster, describe = TRUE)
output_crs_name <- output_crs_info$name
output_epsg <- output_crs_info$code

cat("\nInput CRS name:", input_crs_name, "\n")

Input CRS name: WGS 84 

cat("Input EPSG code:", input_epsg, "\n")

Input EPSG code: 4326 

cat("\nOutput CRS name:", output_crs_name, "\n")

Output CRS name: WGS 84 

cat("Output EPSG code:", output_epsg, "\n")

Output EPSG code: NA 

# Validate CRS
if (!same.crs(input_raster, written_raster)) {
  stop("❌ CRS mismatch detected after writing ASCII.")
}

# Extent check
cat("\nInput extent:\n")

Input extent:

print(ext(input_raster))

SpatExtent : 113.166666666667, 153.625, -43.625, -10.7083333333333 (xmin, xmax, ymin, ymax)

cat("\nOutput extent:\n")

Output extent:

print(ext(written_raster))

SpatExtent : 113.166666666667, 153.625000000324, -43.625, -10.70833333307 (xmin, xmax, ymin, ymax)

if (!isTRUE(all.equal(ext(input_raster), ext(written_raster)))) {
  stop("❌ Extent mismatch detected after writing ASCII.")
}

# Resolution check
cat("\nInput resolution:\n")

Input resolution:

print(res(input_raster))

[1] 0.04166667 0.04166667

cat("\nOutput resolution:\n")

Output resolution:

print(res(written_raster))

[1] 0.04166667 0.04166667

if (!isTRUE(all.equal(res(input_raster), res(written_raster)))) {
  stop("❌ Resolution mismatch detected after writing ASCII.")
}

cat("\n✅ All spatial metadata successfully preserved during conversion.\n")

✅ All spatial metadata successfully preserved during conversion.

Summary

This notebook demonstrated a simple, reproducible workflow for converting a GeoTIFF (.tif) raster into an ASCII grid (.asc) format using the terra package in R, while explicitly verifying that key spatial metadata were preserved during conversion.

A companion notebook (link below) demonstrates the reverse workflow, converting an ASCII grid (.asc) to GeoTIFF (.tif) format.

Converting Rasters from ASCII (.asc) to GeoTIFF (.tif) Format