Cell Operations

In vaster: Tools for Raster Grid Logic

knitr::opts_chunk$set(
  collapse = TRUE,
  comment = "#>"
)

library(vaster)

Overview

Cell operations are at the heart of raster grid logic. This vignette covers:

• Converting between cell indices and x,y coordinates
• Converting between cell indices and row/column positions
• Working with extents from cells
• Finding cells within an extent

All functions use the standard vaster convention: dimension is c(ncol, nrow) and extent is c(xmin, xmax, ymin, ymax).

Cell Indexing

Cells are numbered starting from 1 at the top-left corner, proceeding row-wise (left-to-right, then top-to-bottom):

 1  2  3  4
 5  6  7  8
 9 10 11 12

## define a simple 4x3 grid
dimension <- c(4, 3)   # 4 columns, 3 rows
extent <- c(0, 4, 0, 3) # xmin=0, xmax=4, ymin=0, ymax=3

n_cell(dimension)
n_col(dimension)
n_row(dimension)

Cells ↔ Coordinates

From cells to coordinates

Use xy_from_cell() to get the centre coordinates of cells:

## single cell
xy_from_cell(dimension, extent, 1)

## multiple cells
xy_from_cell(dimension, extent, c(1, 4, 9, 12))

## all cells
xy_from_cell(dimension, extent, seq_len(n_cell(dimension)))

For just x or y coordinates:

x_from_cell(dimension, extent, 1:4)
y_from_cell(dimension, extent, c(1, 5, 9))

From coordinates to cells

Use cell_from_xy() to find which cell contains given coordinates:

## coordinates at cell centres
xy <- cbind(c(0.5, 1.5, 2.5), c(2.5, 1.5, 0.5))
cell_from_xy(dimension, extent, xy)

## coordinates outside the grid return NA
xy_outside <- cbind(c(-1, 5), c(1.5, 1.5))
cell_from_xy(dimension, extent, xy_outside)

## coordinates on cell boundaries go to the cell below/right
## except at the grid edge where they go to the last cell
xy_edge <- cbind(c(1.0, 4.0), c(2.0, 0.0))
cell_from_xy(dimension, extent, xy_edge)

Cells ↔ Row/Column

From cells to row/column

## get row and column for cells
rowcol_from_cell(dimension, extent, 1:12)

For just row or column:

row_from_cell(dimension, 1:12)
col_from_cell(dimension, 1:12)

Note that row_from_cell() and col_from_cell() only need dimension since row/column positions don't depend on extent.

From row/column to cells

Get cells for specific row/column combinations:

## single cell at row 2, column 3
cell_from_row_col(dimension, row = 2, col = 3)

## multiple cells (vectorized, with recycling)
cell_from_row_col(dimension, row = 1:3, col = 1:3)  # diagonal cells

Get all cells in specific rows or columns:

## all cells in row 2
cell_from_row(dimension, 2)

## all cells in column 3
cell_from_col(dimension, 3)

Get the cross-product of rows and columns (all combinations):

## all cells in rows 1-2 AND columns 2-3
cell_from_rowcol_combine(dimension, row = 1:2, col = 2:3)

Rows/Columns ↔ Coordinates

Convert directly between row/column indices and coordinates:

## x coordinate of each column (centre)
x_from_col(dimension, extent, 1:4)

## y coordinate of each row (centre)
y_from_row(dimension, extent, 1:3)

## which column contains this x coordinate
col_from_x(dimension, extent, c(0.5, 2.5, 3.9))

## which row contains this y coordinate
row_from_y(dimension, extent, c(2.5, 1.5, 0.1))

Cells ↔ Extents

Extent from cells

Get the bounding box that covers a set of cells:

## extent of a single cell
extent_from_cell(dimension, extent, 1)

## extent covering multiple cells
extent_from_cell(dimension, extent, c(1, 4))  # top row
extent_from_cell(dimension, extent, c(1, 12)) # full grid

Cells from extent

Find all cells within a given extent:

## cells within a sub-extent
sub_extent <- c(1, 3, 1, 2)  # xmin=1, xmax=3, ymin=1, ymax=2
cell_from_extent(dimension, extent, sub_extent)

The extent is automatically aligned to cell boundaries before finding cells.

Handling Invalid Inputs

Functions return NA for invalid inputs:

## invalid cell numbers
xy_from_cell(dimension, extent, c(0, 13, -1))

## coordinates outside grid
cell_from_xy(dimension, extent, cbind(c(-1, 10), c(1, 1)))

## invalid row/column
cell_from_row_col(dimension, row = 0, col = 1)
cell_from_row_col(dimension, row = 4, col = 1)  # only 3 rows exist

Practical Example: Sampling Grid Points

Create a stratified sample by selecting one random point from each cell:

set.seed(42)
dimension <- c(5, 4)
extent <- c(0, 10, 0, 8)

## get cell centres
centres <- xy_from_cell(dimension, extent, seq_len(n_cell(dimension)))

## add random jitter within each cell
res <- c(x_res(dimension, extent), y_res(dimension, extent))
jitter_x <- runif(n_cell(dimension), -res[1]/2, res[1]/2)
jitter_y <- runif(n_cell(dimension), -res[2]/2, res[2]/2)

sample_points <- cbind(
  x = centres[, 1] + jitter_x,
  y = centres[, 2] + jitter_y
)

head(sample_points)

## verify all points fall in expected cells
all(cell_from_xy(dimension, extent, sample_points) == seq_len(n_cell(dimension)))

See Also

• vignette("grid-fundamentals"): Basic grid concepts
• vignette("extent-alignment"): Aligning extents to grids
• vignette("gdal-interop"): GDAL geotransform operations

vaster documentation built on March 10, 2026, 5:08 p.m.