From points to pixels — creating digital elevation models from OpenTopography point clouds

LINZ
LINZ
Jun 4 · 7 min read
Digital Elevation Model of The Remarkables, Otago

DEM and DSM Nomenclature

Land Information New Zealand defines:

The two ‘DEM’ rasterization methods available on OpenTopography

Creating our DEMs and DSMs — Clarification

This hillshaded image of ground elevation (DEM) near Lake Ruataniwha highlights the Ohau A Hydro Station and the impact of the Ohau River on the shape of the land. Can you spot the fault traces in this image?
  1. Selecting the local binning method and making sure the maximum elevation value option has been checked.

From Points to Pixels

The two main methods of generating surfaces (rasters) from points in OpenTopography are:

  • Local Binning Method
Choosing Ground — Return Classification means that all Elevation Models derived from the data represent ground points only (DEM). Conversely selecting Unclassified + Ground for this dataset, means that the Elevation Models derived represent the highest (DSM) providing that max (z) elevation value has been selected

Delauney Triangulation TIN Method

The TIN (triangular irregular network) method uses Delauney Triangulationto create a DEM surface. This surface formed by triangular planes of different shapes and sizes, defined by nodes and edges which is then rasterized. What makes Delauney Triangulation different from the other TIN methods, is that each triangle satisfies the Delaunay Triangle Criterion. This criterion states that no vertex lies within the interior of any of the circumcircles of the triangles in the network. This ensures the largest possible sized triangular area between points, therefore creating the ‘cleanest’ possible surface.

Delaunay Criterion 2D Example — Creating a planar surface from XYZ lidar points
This point cloud tile showing ground points only. The data surveyed over dense vegetation has much less points versus areas that are classified as urban. This is showing an area north of Brooklyn, Tasman.

Local Binning/Gridding Method — Our options

Circles with a specific radius are used to calculate elevation values.
Irishman Creek Fault near Lake Tekapo, Canterbury unveiled by a DEM.
Digital Surface Model (DSM) of Lincoln Township, Canterbury. On the top left hand corner you can see the old river channels flowing through the town, as well as the Lincoln University Campus (the largest building).

Tips using the Binning Method

If the pixel size is three to four times larger than the average point distance, you can safely use binning. If the cell size is smaller than that, you can try binning with void filling turned off.

Bins versus Triangles — Pros and Cons

A literal representation of the differences between these methods.
Variations between TIN (with no gaps) and Local Binning Method (with gaps)

Licensors and Attribution

The LiDAR data used in this story is licensed by Auckland Council, Environment Canterbury Regional Council, Nelson City Council, Otago Regional Council and Tasman District Council, for re-use under the Creative Commons Attribution 4.0 International license.


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On Location

Updates from LINZ on location information and open source software