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New Zealand Elevation TIFF Specifications

LiDAR Capture Base Specification

Toitū Te Whenua Land Information New Zealand publishes a National Aerial LiDAR Base Specification which defines data quality requirements and some TIFF specifications that are also covered by the following data storage specification.

These include:

  • GeoTIFFs with georeferencing information contained within each raster file. Our data processing workflows will also handle TFW World File sidecar file containing georeferencing information but this is not encouraged.
  • Data provided in the NZTM2000 coordinate system and the NZVD2016 vertical datum.
  • Void areas coded using "NODATA" value equal to -9999, identified in the appropriate location within the raster file header.

Cloud-Optimised GeoTIFFs and the specific compressions used for storage are not part of the base specification, and are outputs of LINZ data processing workflows.

Elevation Data Storage Specification

Applies to Digital Elevation Models, Digital Surface Models and Hillshades published in s3://nz-elevation after being processed by Toitū Te Whenua Land Information New Zealand.

For data processing scripts, see linz/topo-imagery.
For workflow configuration, see linz/topo-workflows.

File Format

Property Value
Container GeoTIFF (.tiff)
Profile Cloud-Optimised GeoTIFF (COG)
GDAL Driver COG (-of COG)

A summary of Cloud-Optimised GeoTIFF characteristics is provided in the GDAL docs.

Coordinate Reference Systems

Type Name EPSG Code
Projection New Zealand Transverse Mercator 2000 (NZTM2000) 2193
Vertical Datum New Zealand Vertical Datum 2016 (NZVD2016) 7839

Each TIFF contains the spatial reference information for NZTM2000. NZVD2016 is not recorded in the GeoTIFF metadata.

Spatial Resolution

1m for all LiDAR-sourced DEMs, DSMs and Hillshades.
s3://nz-elevation also contains a contour-derived DEM that is 8m spatial resolution.

Band Configuration

Data Type Bands Bit Depth Colour Interpretation Nodata Interleave
DEMs/DSMs 1 (elevation) float32 Gray -9999 Band
Hillshade 1 (greyscale) uint8 Gray 0 Band

No alpha band is used for elevation data.

COG Driver Options

The following GDAL COG driver creation options are applied to all TIFFs:

GDAL Creation Option Value Notes
-co BLOCKSIZE 512 512×512 pixel tiles (default)
-co NUM_THREADS ALL_CPUS Use all available CPUs for compression, overview generation, etc
-co SPARSE_OK TRUE Omit empty tiles (null byte) rather than writing blank data
-co BIGTIFF NO Classic TIFF only (GDAL will raise an error if output exceeds 4 GB and we target approx 1 GB TIFF sizes via automated tiling)
-co OVERVIEWS IGNORE_EXISTING Always regenerate overviews (never build from pre-existing lossy overviews)
-co STATISTICS YES Compute and embed raster statistics (ensures that GIS software can immediately accurately display the data)

Compression

DEMs/DSMs - LERC

GDAL Creation Option Value Notes
-co COMPRESS LERC LERC compression provides control over the maximum error per-pixel
-co MAX_Z_ERROR 0.001 Maximum full-resolution error of 1mm
-co MAX_Z_ERROR_OVERVIEW 0.1 Maximum overview error of 10cm
-mo AREA_OR_POINT Area Forced to Area for consistency (input DEMs and DSMs vary)
-a_nodata / --nodata -9999 Explicit nodata value set on all bands

LERC is a near-lossless compression allowing a configurable per-pixel Z error (can also be completely lossless, but this removes one of the main advantages of LERC). The 1mm full-resolution error is well within the ~20cm vertical accuracy of the source LiDAR data. Overview error is relaxed to 10cm as overviews are used for visualisation only.

Hillshades - ZSTD

GDAL Creation Option Value Notes
-co COMPRESS ZSTD ZSTD compression is a high-performance, fast, lossless compression algorithm
-co LEVEL 17 Highest compression level that produces a significantly smaller file size (at the cost of slower compression, which is acceptable for our use-cases)
-co PREDICTOR 2 Horizontal differencing predictor
-co OVERVIEW_COMPRESS ZSTD ZSTD compression for overviews as well
-co OVERVIEW_RESAMPLING LANCZOS Lanczos resampling for overview generation

File Naming Convention

Output files are named using the LINZ tile index grid name, e.g. BK27_10000_0101.tiff.

Individual 1m resolution LiDAR survey outputs are tiled to the NZ 1:10k Tile Index.
The 8m contour-derived DEM and combined national DEM, DSM and Hillshade datasets are tiled to the NZ 1:50k Tile Index.

GDAL Contributions

Toitū Te Whenua Land Information New Zealand has paid for the following contributions to GDAL:

  • -of COG driver, now the de facto raster data file format for geospatial data in cloud storage
  • -co MAX_Z_ERROR_OVERVIEW option, saving us up to ~15% file size for elevation data with overviews

Along with ongoing maintenance sponsorship.

If you benefit from any of these tools, please also consider sponsoring GDAL.