warp_scale: Compute warp resampling scale

In terra: Spatial Data Analysis

Compute warp resampling scale

Description

Compute the ratio between source and destination pixel sizes for use with project. These values correspond to the GDAL warp options XSCALE and YSCALE. By default, GDAL computes them independently for each processing chunk, which can produce visible block-boundary artifacts when the ratio varies across the raster (particularly in projections with significant deformation). Setting fixed values via project(..., xscale=, yscale=) eliminates these artifacts.

The function samples a grid of points in the source raster, projects them to the destination CRS, and returns summary statistics of the local resampling ratios.

Usage

warp_scale(x, y, n=21)

Arguments

x	SpatRaster. The source raster
y	SpatRaster or character. A template raster or a CRS description for the destination
n	integer. Number of sample points along each axis (the total number of sample points is n*n)

Value

A list with two named numeric vectors (xscale and yscale), each containing the quantiles (0%, 25%, 50%, 75%, 100%) of the local resampling ratio across the sampled points. The median ("50%") is typically the best choice for a global fixed scale. Values equal one for no resampling, below one for downsampling, and above one for upsampling.

See Also

project

Examples

## Not run: 
a <- rast(ncols=360, nrows=180, xmin=-180, xmax=180, ymin=-90, ymax=90,
          crs="+proj=longlat +datum=WGS84")
values(a) <- 1:ncell(a)

sc <- warp_scale(a, "+proj=robin")
sc
# Use the median values
b <- project(a, "+proj=robin", xscale=sc$xscale["50%"], yscale=sc$yscale["50%"])

## End(Not run)

terra documentation built on Aug. 22, 2026, 5:07 p.m.