Landslide susceptibility (global, NASA, slope-gated 90 m)

Source: NASA Global Landslide Susceptibility (Stanley & Kirschbaum) × GIRI / NGI maximum terrain slope Year: 2017 Category: Physical risks · Mitigating services · Geological Coverage: Global Format: Raster grid (92.8 m) Used in risk analysis: No (contextual layer; not used in materiality scoring) — the flag and the priced loss read Landslide susceptibility (global, NASA)

What it shows

The same six-level NASA landslide susceptibility scale as the global NASA layer (0 None, 1 Very low, 2 Low, 3 Moderate, 4 High, 5 Very high), carried onto a 90 m grid and set to 0 wherever the terrain is too flat for a slide to begin.

The point of the layer is the slope gate. A susceptibility class built from terrain, geology and land cover spreads over a whole neighbourhood; the ground that can actually fail is the steep part of it. At 2 km, a valley floor and the slope above it share one pixel and one class. At 90 m with the gate applied, the flat floor reads 0 and the slope keeps its class.

How it is built

The NASA susceptibility raster is carried onto a 92.766 m web-mercator grid (EPSG:3857, Float64), and every cell whose maximum terrain slope is below 6 degrees is set to class 0. The slope comes from the GIRI maximum terrain slope surface (UNEP-GRID / CIMA / NGI).

Two consequences follow from that construction, and both matter when reading a value.

Class 0 means "none or too flat". It merges two different statements: NASA assigning no susceptibility, and the slope gate zeroing a cell NASA did classify. The legend label says so, and the 2 km layer's class 0, which reads simply "None", is not the same quantity.

It is initiation only. The gate answers where a slide can start. Runout is not modelled, so a flat cell immediately below a steep, high-class slope reads 0 while remaining exposed to material arriving from above. That is the main way this layer can be read too favourably.

How to read it

Higher classes mean a greater inherent likelihood that the ground fails at that location. The classes are an ordinal hazard scale from NASA, not a probability and not a physical quantity in units.

This is a screening layer, not a siting tool. The slope gate sharpens the picture rather than certifying it: below 6 degrees it removes the class, and above 6 degrees it leaves NASA's own classification untouched, at a resolution the source classification was never built at. Reading a single 90 m cell as a statement about a specific parcel asks the underlying map for more than it carries.

How it differs from the 2 km layer

Both layers carry the same NASA classification, and the catalogue keeps both.

The 2 km layer is the operating one: it raises the "Soil and sediment retention" flag at Moderate and above, and it is the raster the Direct Physical Loss engine samples for ground movement. This 90 m layer is published for reading, so that a flag raised at 2 km can be inspected at a resolution where slope is visible. Nothing is computed from it today, and moving the flag onto it would change which sites are flagged, which is a decision for the layer team rather than a by-product of the finer grid.

Class thresholds

This is a categorical susceptibility layer. It publishes six ordinal classes and no proximity rule, so it crosses no threshold in the platform.

How these thresholds were set. The six-class scheme and the rules that assign a cell to a class belong to NASA's Global Landslide Susceptibility Map methodology (Stanley & Kirschbaum) and are not reproduced in Darwin's configuration. The in-house elements are the 90 m grid and the 6 degree slope gate, a Darwin cut-off on the GIRI maximum-slope surface rather than a published landslide-initiation standard.

Reference: NASA Global Landslide Susceptibility Map (Stanley & Kirschbaum; Stanley et al., 2017).

Source

NASA Global Landslide Susceptibility Map (Stanley & Kirschbaum, 2017). Maximum terrain slope from GIRI (UNEP-GRID / CIMA / NGI), CC BY 3.0 IGO.

Comparison with the WWF Risk Filter Suite

This layer maps to WWF Biodiversity Risk Filter indicator S3_1 Landslides and draws on the same NASA source family (Stanley et al., 2017). It departs from WWF in two ways: the classes are carried at 90 m rather than at the coarse grid WWF publishes, and the slope gate removes ground the WWF indicator would still score.

Legend

Symbolised field: Landslide susceptibility

Generated from darwin/layers/layer-landslide-slope-resolved.toml (develop).