02_GDE_Model.md 4.53 KB
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# The Global Dynamic Exposure (GDE) Model

The Global Dynamic Exposure (GDE) model is a high-resolution exposure model that is generated by
means of combining:

1. Aggregated exposure models.
2. Data on individual buildings from [OpenStreetMap](https://www.openstreetmap.org).
3. Remote sensing-derived built-up areas from the Global Human Settlement Layer (GHSL; Corbane
et al., 2018).

Fig. 2.1 gives an overview of how these elements are processed and combined to create the GDE
model.

<img src="images/gde_model_overview.png" width=75%>

Fig. 2.1 Conceptual overview of OpenBuildingMap (OBM) and the Global Dynamic Exposure (GDE) model.

Aggregated exposure models (such as that of the European Seismic Risk Model 2020, ESRM20,
Crowley et al., 2020) are distributed onto a grid of zoom-level 18 quadtiles proportionally to
the built-up area expected in each tile. This task is carried out by the
[gde-importer](https://git.gfz-potsdam.de/dynamicexposure/globaldynamicexposure/gde-importer/-/tree/master/),
which also processes data on building classes, their attributes (e.g. people per building,
replacement cost per building) and other relevant exposure parameters. Currently, `gde-importer`
can only import the format of the exposure model of the European Seismic Risk Model 2020
(ESRM20) (Crowley et al., 2020), but further formats can be implemented in the future. The
expected built-up areas in each tile are an output of
[obmgapanalysis](https://git.gfz-potsdam.de/dynamicexposure/openbuildingmap/obmgapanalysis),
which processes the [Global Human Settlement Layer (GHSL) built-up area layer with multi-temporal
analysis and global coverage](https://data.jrc.ec.europa.eu/dataset/jrc-ghsl-10007)
(Corbane et al., 2018) for this purpose.

Independently from this,
[rabotnik-obm](https://git.gfz-potsdam.de/dynamicexposure/openbuildingmap/rabotnik-obm)
retrieves buildings from [OpenStreetMap](https://www.openstreetmap.org) together with some
additional attributes, and assigns them occupancy types as per the
[GEM Building Taxonomy v3.0](https://github.com/gem/gem_taxonomy) (Silva et al., 2022), the
resulting product being the `OpenBuildingMap`, or OBM.

OBM buildings are assigned building classes as per the aggregated exposure model, as a function
of their geographic location and their occupancy type. Only occupancy types that belong to
occupancy cases covered by the imported aggregated exposure models can be assigned building
classes. This is done by the
[gde-core](https://git.gfz-potsdam.de/dynamicexposure/globaldynamicexposure/gde-core), as
explained
[here](04_Processing_Logic.md#retrieve-obm-buildings-and-assign-building-classes-and-probabilities-to-them).

As `OpenStreetMap` is not yet complete everywhere, that is, as not all buildings that exist in
reality are represented in `OpenStreetMap`, the final number of buildings in each tile of the
GDE model is calculated by combining numbers of buildings from the aggregated exposure model and
OpenBuildingMap, taking into account the completeness of the tile. The completeness of the tile
is determined by
[obmgapanalysis](https://git.gfz-potsdam.de/dynamicexposure/openbuildingmap/obmgapanalysis) in
an automatic fashion, by calculating the ratio between the area of building footprints from
`OpenStreetMap` and the expected built-up area as per GHSL, and using a threshold to decide
whether the tile is or not complete. The threshold was selected by analysing the built-up ratios
obtained for the region of Attica (Greece), for which a manual assessment of completeness was
carried out. If a tile is incomplete,
[gde-core](https://git.gfz-potsdam.de/dynamicexposure/globaldynamicexposure/gde-core) compares
the number of buildings in the tile as per the distribution of the aggregated exposure model
onto the grid against the number of OBM buildings. If the former is larger than the latter, a
number of so-called "remainder" buildings is calculated as the difference between the two and
assigned to the tile. These are buildings that are expected to exist in the tile but are not yet
represented in `OpenStreetMap`. If the number of OBM buildings is larger than that of the
so-called "aggregated" buildings, no remainder buildings are assigned to the tile. If the tile
is complete, no remainder buildings are assigned to the tile either. Further details can be
found [here](04_Processing_Logic.md#calculate-remainder-buildings-in-data-unit-tiles).

The resulting GDE model is thus a combination of individual OBM buildings (re-named as GDE
buildings once building classes have been assigned to them) and remainder buildings defined in
zoom-level 18 quadtiles.