NZ 8m Digital Elevation Model (2012)

The Geographx New Zealand Digital Elevation Model version 2.1 covers the three main islands of New Zealand (North Is, South Is, Rakiura/Stewart Is) at a cell resolution of 8 metres.
It is prepared primarily from 1:50 000 topographic data from Toitū Te Whenua Land Information New Zealand (the LINZ classes used are contour, height, coastline, island, lake, lagoon, pond, reservoir, sand, mud, shingle, reef, shoal, rock (polygons and points) and rock outcrop), with 3-second SRTM data (USGS, Shuttle Radar Topography Mission (February 2000), unfilled finished version B (2006). Global Land Cover Facility, University of Maryland - www.landcover.org) used in a supporting role.

The main criterion in its production is the detailed and accurate depiction of natural landforms. It is therefore suitable primarily for cartographic visualisation. It may also be useful for terrain analysis, before the acquisition of high-resolution data for a region of interest.

Spatial accuracy is nominally the same as for the LINZ source data: 90% of well-defined points are within ±22 metres horizontally and within ±10 metres vertically.

The DEM is initially derived by constrained triangulation, with post-processing using the DEST algorithm (Favalli M and M T Pareschi, “Digital elevation model construction from structured topographic data: the DEST algorithm”, J Geophysical Research 109 (2004)), followed by signal detection and filtering to suppress artifacts while preserving natural form. Processing is performed in Manifold System 8.

Waterbody elevations
In version 2.1, all LINZ waterbody classes (lake, lagoon, pond and reservoir) are given either a validated LINZ height, or an estimated height.
Validation was performed by correlating LINZ waterbody elevations to the height of the underlying contour, the correlation being used to identify outlying values, which were validated, excluded or (in the case of obvious typographical errors) corrected.

A revised correlation, along with constrained SRTM heights and information about local convexity or concavity, was then used to estimate all waterbody heights. Parameters were adjusted to minimize RMS difference between estimated height and validated LINZ heights.
Waterbodies with no LINZ elevation (or with an invalidated elevation) were then given the
estimated height.

Coastal elevations
Coastal features have been assigned representative elevations. The LINZ coastline and island
outlines depict not mean sea level, but high water mark (generally the line of visible debris), and
are assigned a constant elevation of 1.5m. Coastal sand, mud and shingle are given elevation 1.0m, while coastal rock, reefs and shoals are assigned 0.5m.

This layer was modified 01 March 2016 using 'gdalwarp' to covert the values in ocean regions to 'no data'. Ocean regions are determined by overlaying the LINZ Topo50 Coastlines and Polygons (https://data.linz.govt.nz/layer/1153-nz-coastlines-and-islands-polygons-topo-150k/)and converting those values outside the coastlines layer. This action was performed for each individual DEM tiles in a batched process. The projection used for the entire operation is NZTM.

For Windows: FOR %i in (*.tif) do (gdalwarp -s_srs EPSG:2193 -cutline ~\topo50_coastline.shp %i ~\dem_clip\%i)

For LINUX: find . -name '*.gdb' | xargs -P 3 -t -n1 -I % gdalwarp -s_srs EPSG:2193 -cutline ~\topo50_coastline.shp % ~\dem_clip\%

The result is a DEM for only the land regions of NZ.