Elevation model

Elevation model in relation to Terrains, Height sectors and Elevation GeoTIFFs.

An elevation model (or DEM, Digital Elevation Model), is a model representation of terrain heights; the quantitative measurement of vertical elevation change in a landscape. On a small scale, differences in terrain height can affect shade and water distribution. On a larger scale, terrain height has an effect on weather and climate patterns.

Implementation in the Tygron Platform

Source: https://commons.wikimedia.org/wiki/File:DTM_DSM.svg

The elevation model (also referred to as DEM) in the Tygron Platform is a 2-dimensional grid with values indicating the terrain height in meters relative to datum. Terrain in this context meaning the soil and the subsurface.

The 3D Visualization displays the elevation model inherently as the bottom plane upon which all other features, such as buildings, are placed. Changes in terrain height are similarly changes in the height of that plane.

In the Tygron Platform, the elevation model offers terrain heights representation as both a DTM (Digital Terrain Model) and as a DSM (Digital Surface Model). A DTM represents the bare ground surface without any objects like plants and buildings. In contrast; a DSM represents the earth's surface including all objects (e.g. houses, trees, etc.) on it. This means features such as buildings are explicitly not a part of the DTM, but are a part of the DSM. In locations with water, the elevation model follows the bottom of the water body for both the DTM and the DSM.

The 3D Visualization displays the elevation model inherently as the bottom plane upon which all other features, such as buildings, are placed. Changes in terrain height are similarly changes in the height of that plane.

The elevation model consists of a single data set, which is generated when a Project is first created. Any changes to the elevation model are applied directly to that data set, including manually drawn changes as well as imported changes.

Elevation model generation

The elevation model is generated when a Project is first created. The advanced options allow you to select the used data sources and resolution for the elevation model.

The advanced options allow you to select the used data sources and resolution for the elevation model.

In the Netherlands, the following data sources are used to construct the elevation model:

Name	Resolution	Description	Source
ESRI DTM	3m by 3m	Dataset supplied by Rijkswaterstaat to ESRI, based on AHN2.	https://www.arcgis.com/home/item.html?id=58a541efc59545e6b7137f961d7de883
AHN DSM	0.5m by 0.5m, 1m by 1m, or 2.5m by 2.5m, depending on selected options.	Raw data from Actueel Hoogte Bestand	AHN2: https://www.arcgis.com/home/item.html?id=55df27e20c0d42e3a8bd48bc74c5c8cc AHN3: https://www.arcgis.com/home/item.html?id=b87c68ea246c4b17807cdad486279a47 AHN4: https://www.arcgis.com/home/item.html?id=3aa24b7eeed74fdf8b71b68b05d13fab
AHN DTM	0.5m by 0.5m, 1m by 1m, or 2.5m by 2.5m, depending on selected options.	Interpolated (surface level) data based on Actueel Hoogte Bestand	AHN2: https://www.arcgis.com/home/item.html?id=fc00e93ca66546399d5b7827f61af1db AHN3: https://www.arcgis.com/home/item.html?id=9d62ff8889ba41b2b712a68c7493bd16 AHN4: https://www.arcgis.com/home/item.html?id=1fea0e68b7124dda88373d8538822435
Features		Presence of features of the built environment	https://www.pdok.nl/introductie/-/article/basisregistratie-grootschalige-topografie-bgt- (BGT) and https://www.pdok.nl/introductie/-/article/basisregistratie-topografie-brt-historie (Top10NL)

By default the AHN3 is used for the DTM and DSM. AHN2, AHN3, or AHN4 can be selected.
If AHN4 is used in a location where coverage is incomplete, no fallback data will be available which means data will be missing in that part of the project.
By default the BGT is used for the features in the 3D Visualization. If the BGT is deactivated in the advanced options panel, TOP10NL is used.

DTM (Terrain height)

The DTM is generated as follows:

The ESRI DTM is resampled to the same resolution as the AHN DTM and AHN DSM.
The AHN DTM serves as the base data for the resulting elevation model DTM.
In locations where the AHN DTM has NO_DATA values (such as waterbodies), the ESRI DTM is used.
Optionally, if the option for applying thresholds is selected, the AHN DSM heights are used instead of the so far determined DTM heights, if the difference between the two does not exceed a certain threshold. The following features can have the DSM heights applied instead:
- Bare land
- Roads
- Crop fields/agriculture

DSM (Building height)

For buildings, the average height of the AHN DSM is determined for the polygon defining the building's footprint.That average height of the AHN DSM is then determined to be the height of the building.

Using image recognition techniques, the footprint polygon of the buildings may be split into multiple Building sections, each with their own polygon. The AHN DSM's average height is then determined per individual section, so that the variability in the rooftops of the building can be taken into account.

The resulting building heights are not made part of the DTM. Instead, they are stored as properties of the buildings in the Project.

Water

Water bodies, found in terrain surface types, are lowered a few meters relative to the elevation of the surrounding surface. The following parameters are used:

The default water depth of the determined Terrain Type of the waterbody is used.
The angle of repose of the Underground Terrain Type present at or near the sides of the waterbodies.

From the DTM a point is selected at the waterbody's edge, or as close as possible. The Angle of repose value of the underground Terrain Type present in that point is determined. A downward slope is created at the determined angle of repose. The downward slope continues until either it meets the slope generated from the other end, or it reaches the determined default water depth relative to the DTM. This means that thin waterways will not be as deep as their default water depth allows them to be, and that large water bodies are likely to have flat water bottoms.

The resulting terrain height changes are made part of the DTM.

The angle of repose and water depth m attributes determine the slope and depth of the waterway. When the waterway is wide enough, the water bottom will be flat.

Because of the angle of repose, the slopes of a narrow waterway can end in a point, despite the fact that the waterway is not yet as deep as determined in the water depth m attribute.

To improve the water depth with your own data, see:

Elevation model generation (outside the Netherlands)

For the elevation model in projects abroad, the ESRI DTM is used. The resolution differs per location, based on the used data sources for creating the ESRI DTM layer.

Name	Resolution	Description	Source
ESRI DTM	Differs per location, see the metadata for more information	Ground height dataset of ESRI, based on multiple sources.	https://www.arcgis.com/home/item.html?id=58a541efc59545e6b7137f961d7de883

For the DSM height, the heights of buildings are used. If available, the OSM building heights are used, otherwise the default heights. The default heights differ per function, based on the default floors and floor height.

Data format

The height is stored in Height sectors, which are square 500m x 500m tiles floating point values which store the height data in meters, with a precision of 3 decimals. The amount of points per height sector depend on the DEM resolution configured in advanced options of the New Project Wizard, before project creation, and cannot be increased or decreased later on.

Heightmap Overlays

More insight into the elevation model can be provided by adding the Digital Terrain Model Overlay or the Digital Surface Model Overlay, which are grid overlays showing the (average) height per grid cell. The amount of averaging depends on the configured grid cell size.

Elevation model

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