Datenpaket: Terrain Aspect of Bavaria

The map of terrain aspect is based on the Digital Terrain Model of Bavaria with a spatial resolution of 5 m (DGM5) and characterizes the exposure of the actual terrain surface. Aspect was calculated according to Horn (1981), considering the eight immediately adjacent raster cells. The local orientation of the terrain is determined from Elevation differences within a 3 × 3 cell neighborhood. Aspect is expressed in degrees (°) and corresponds to a Compass direction, with 0° or 360° representing North, 90° East, 180° South, and 270° West. Intermediate values represent the corresponding intermediate directions. The DGM5 was used in its original terrain geometry. Consequently, both natural and anthropogenic terrain features, such as embankments, dams, or cuttings, may influence the derived aspect values. The dataset provides a comprehensive description of topographic exposure and can support the characterization of radiation and site conditions as well as agricultural, forestry, and ecological analyses.

This dataset provides a Bavaria-wide representation of terrain aspect derived from the Digital Terrain Model of Bavaria (DGM5) at 5 m spatial resolution.
Terrain aspect describes the orientation of the terrain surface relative to the cardinal directions. It was calculated according to Horn (1981), using elevation differences within a 3 × 3 raster-cell neighbourhood comprising the eight cells immediately adjacent to each central raster cell.
Aspect is expressed as an azimuth angle in degrees (°), with 0° or 360° representing north, 90° east, 180° south, and 270° west. Intermediate values represent the corresponding intermediate orientations.
The DGM5 used as input is a preprocessed digital terrain model. No additional hydrological conditioning was applied prior to the calculation of terrain aspect. In particular, the terrain was not modified specifically to enforce hydrological connectivity or drainage patterns for the purpose of this analysis.
Consequently, the resulting aspect values characterize the local orientation of the terrain surface represented in the DGM5 rather than a hydrologically conditioned terrain surface. Terrain structures represented in the DGM5, including anthropogenic features such as embankments, dams, cuttings, or other artificial modifications of the terrain, may therefore influence the calculated aspect values.
The dataset provides a spatially comprehensive characterization of topographic orientation across Bavaria and can support analyses of radiation and site conditions as well as agricultural, forestry, ecological, and other environmental applications.

INPUT DATA
Digital Terrain Model of Bavaria (DGM5)
• Spatial resolution: 5 m
• Data provider: Bavarian Surveying Administration / Bayerische Vermessungsverwaltung
• Product: Digitales Geländemodell 5 m (DGM5)
• License: CC BY 4.0
• Source: Open Data Bayern

DATASET SPECIFICATIONS
• Spatial extent: Bavaria, Germany
• Spatial resolution: 5 m
• Unit: degrees (°)
• Variable: terrain aspect
• Data type: raster
• File format: GeoTIFF / Cloud Optimized GeoTIFF (COG)
• Coordinate reference system: ETRS89 / UTM zone 32N
• EPSG: 25832

PROCESSING
Input: Digital Terrain Model of Bavaria (DGM5), 5 m spatial resolution.
Method: Terrain aspect was calculated according to Horn (1981).

The local orientation of the terrain surface is derived from elevation differences within a 3 × 3 raster-cell neighborhood. The eight cells immediately adjacent to each central raster cell are considered in the calculation. The resulting aspect describes the direction of maximum downward slope and is expressed as an azimuth angle in degrees.

The DGM5 used as input is a preprocessed digital terrain model. No additional hydrological conditioning of the terrain was applied prior to aspect calculation. The derived aspect values therefore represent the local orientation of the terrain geometry contained in the DGM5 rather than that of a hydrologically conditioned terrain representation. Anthropogenic terrain structures represented in the DGM5 may consequently influence the calculated aspect values.

PARAMETER SETTINGS
• Neighborhood: 3 × 3 raster cells
• Neighboring cells considered: 8
• Raster cell size: 5 m
• Output unit: degrees (°)
• Aspect convention: azimuth

The aspect calculation is based on local elevation differences within the immediate 3 × 3 cell neighborhood. At a raster resolution of 5 m, the calculated values therefore describe terrain orientation at the local scale represented by the DGM5.

VALUE INTERPRETATION
Aspect values represent azimuth directions: 0° / 360° - North, 45° - Northeast, 90° - East, 135° - Southeast, 180° - South, 225° - Southwest, 270° - West, 315° - Northwest.

Intermediate values represent the corresponding intermediate orientations.
For flat or nearly flat terrain, aspect is geometrically undefined or may be unstable because no meaningful downslope direction exists. Depending on the implementation of the processing algorithm and numerical characteristics of the input data, such cells may contain numerical aspect values. Aspect values in areas with very low terrain gradients should therefore be interpreted with caution.

INTERPRETATION AND LIMITATIONS
The dataset represents the local geometric orientation of the terrain surface represented in the DGM5 and should be interpreted accordingly.
In particular:
• The aspect calculation is derived from the terrain geometry represented in the DGM5.
• The input DGM5 is a preprocessed digital terrain model.
• No additional hydrological conditioning was performed specifically for the aspect calculation.
• The dataset therefore does not represent aspect derived from a hydrologically conditioned terrain model.
• Anthropogenic terrain structures represented in the DGM5, including embankments, dams, cuttings, and similar features, may influence local aspect values.
• Aspect is calculated locally from the immediate 3 × 3 raster-cell neighborhood and therefore reflects terrain orientation at the spatial scale represented by the 5 m DGM5.
• Aspect is geometrically undefined for completely flat terrain and may be unstable in areas with very low terrain gradients.
• Aspect is a circular variable: values close to 0° and 360° represent similar orientations despite their numerical difference. This should be considered in statistical analyses.
• The dataset describes terrain geometry and does not by itself provide information on hydrological connectivity, surface runoff, or active geomorphological processes.

RECOMMENDED USE
The dataset can be used as a terrain-related spatial variable in regional and Bavaria-wide analyses, including:
• characterization of terrain exposure and topographic orientation;
• solar-radiation and microclimatic analyses;
• agricultural and forestry applications;
• ecological and biodiversity modelling;
• habitat and species-distribution modelling;
• analysis of topographically influenced site conditions; and
• landscape and environmental modelling.
For statistical analyses, aspect should be treated as a circular variable. Depending on the application, transformation into northness and eastness components or other circular representations may be appropriate.
For applications requiring explicit representation of surface runoff, drainage networks, flow accumulation, or hydrological connectivity, hydrologically conditioned elevation data and dedicated hydrological terrain derivatives should be used instead.

REFERENCE
Horn, B. K. P. (1981). Hill shading and the reflectance map. Proceedings of the IEEE, 69(1), 14–47.

DATA SOURCE
Digitales Geländemodell 5 m (DGM5), Bavarian Surveying Administration / Bayerische Vermessungsverwaltung, Open Data Bayern, licensed under CC BY 4.0.