vrgti

vrgti

Convert GDAL VRT mosaics to the GDAL Tile Index (GTI) format.

Installation 

# install.packages("pak")
pak::pak("hypertidy/vrgti")

Usage 

library(vrgti)

# Convert a VRT mosaic to a GTI-compatible GeoPackage
vrt_to_gti(
  dsn = "/vsicurl/https://raw.githubusercontent.com/mdsumner/rema-ovr/main/REMA-2m_dem_ovr.vrt",
  output = "rema_tiles.gpkg"
)

# Open the result as a raster dataset
library(gdalraster)
ds <- new(GDALRaster, "GTI:rema_tiles.gpkg")
ds$dim()
ds$res()
ds$close()

Let’s try that example output directly:

dsn <- "/vsicurl/https://github.com/mdsumner/vrgti/releases/download/latest/rema_v2_tiles.gti.gpkg"
library(gdalraster)
#> GDAL 3.13.0dev-7924dc998e (released 2026-01-27), GEOS 3.12.1, PROJ 9.7.0
ds <- new(GDALRaster, dsn)
ds$getDriverLongName()
#> [1] "GDAL Raster Tile Index"
ds$dim()
#> [1] 2725100 2921100       1
ds$res()
#> [1] 2 2
ds$bbox() ## xmin,ymin,xmax,ymax
#> [1] -2700100 -2500100  2750100  3342100
ds$close()

What it does

  1. Parses the VRT XML (via xml2) to extract tile source paths and pixel-space layout
  2. Validates that the VRT is a simple mosaic — only SimpleSource elements, consistent across bands, no resampling
  3. Computes geographic tile footprints from DstRect pixel coordinates + GeoTransform
  4. Writes a GTI-compatible GeoPackage via gdalraster’s GDALVector, with polygon footprints, a location field, and layer metadata (RESX, RESY, extent, band count, data type, nodata, SRS)

Why GTI over VRT?

GTI is an improved mosaic format over VRT for large tile collections:

  • Spatial indexing — GeoPackage/FlatGeoBuf backends provide efficient spatial queries, avoiding the need to scan all source entries
  • Scale — handles hundreds of thousands of tiles where VRT’s XML-in-memory approach struggles
  • Flexibility — tiles can have different SRS (on-the-fly reprojection), z-order control, alpha band handling
  • Compact — smaller index files than equivalent VRT XML

Non-simple VRTs

The function will stop with a diagnostic message if the VRT contains:

  • AveragedSource, KernelFilteredSource, or other non-simple source types (note: ComplexSource is allowed — it’s the standard nodata-aware mosaic source)
  • VRTWarpedDataset or VRTDerivedRasterBand subclasses
  • Sources where SrcRect and DstRect dimensions differ (within-VRT resampling)
  • Bands with inconsistent source file/layout across bands

Dependencies

  • xml2 for VRT XML parsing
  • gdalraster (>= 2.0.0) for GeoPackage writing via GDALVector and layer metadata

Code of Conduct

Please note that the vrgti project is released with a Contributor Code of Conduct. By contributing to this project, you agree to abide by its terms.