This document explains the View Geometry Extension to the SpatioTemporal Asset Catalog (STAC) specification.
View Geometry adds metadata related to angles of sensors and other radiance angles that affect the view of resulting data. It will often be combined with other extensions that describe the actual data, such as the eo, sat or sar extensions.
The fields in the table below can be used in these parts of STAC documents:
-90-90). Negative values indicate the sun is below the horizon, e.g. sun elevation of -10° means the data was captured during nautical twilight. view:moon_azimuth number Moon azimuth angle. From the scene center point on the ground, this is the angle between truth north and the moon. Measured clockwise in degrees (0-360). view:moon_elevation number Moon elevation angle. The angle from the tangent of the scene center point to the moon. Measured from the horizon in degrees (0-90).
The angles off_nadir and incidence_angle are angles measured on a 2d plane formed by the sensor location, the sub-satellite point on the earth, and the center of the target area as shown in the diagram below. Grazing angle is shown as it is frequently used but it is not included in this extension because it is simply the complement of the incidence angle. When the off-nadir angle is low (low incidence angle) then the two angles are approximately equal. However, at high off-nadir angles with high altitude sensors the curvature of the earth has an impact and the two angles are no longer equivalent.
The azimuth (sun_azimuth, moon_azimuth), and the elevation (sun_elevation, moon_elevation) angles are measured as per the diagram below. Azimuth angles are measured as degrees from North, and elevation angles are measured up from the target plane to the body (satellite, sun, or moon).
One of the best practices is to use Asset Roles to provide clients with more information about the assets in an item. The following list includes a shared vocabulary for some common EO assets. This list should not be considered definitive, and implementors are welcome to use other asset roles. If consensus and tooling consolidates around these role names then they will be specified in the future as more standard than just 'best practices'. The roles listed below all tend to be additional files that contain specific values for every single pixel. It is recommended to use them all with the role of 'metadata'.
Role Name Description incidence-angle Points to a file with per-pixel incidence angles. azimuth Points to a file with per-pixel azimuth angles. sun-azimuth Points to a file with per-pixel sun azimuth angles. sun-elevation Points to a file with per-pixel sun elevation angles. moon-azimuth Points to a file with per-pixel moon azimuth angles. moon-elevation Points to a file with per-pixel moon elevation angles. terrain-shadow Points to a file that indicates whether a pixel is not directly illuminated due to terrain shadowing. terrain-occlusion Points to a file that indicates whether a pixel is not visible to the sensor due to terrain occlusion during off-nadir viewing. terrain-illumination Points to a file with coefficients used for terrain illumination correction are provided for each pixel.All contributions are subject to the STAC Specification Code of Conduct. For contributions, please follow the STAC specification contributing guide Instructions for running tests are copied here for convenience.
The same checks that run as checks on PR's are part of the repository and can be run locally to verify that changes are valid. To run tests locally, you'll need npm, which is a standard part of any node.js installation.
First you'll need to install everything with npm once. Just navigate to the root of this repository and on your command line run:
Then to check markdown formatting and test the examples against the JSON schema, you can run:
This will spit out the same texts that you see online, and you can then go and fix your markdown or examples.
If the tests reveal formatting problems with the examples, you can fix them with:
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