From c0580cd1d4f316afb49cdb481df53e0dbc013b80 Mon Sep 17 00:00:00 2001 From: Daniel Evans Date: Fri, 8 May 2026 10:05:58 +0100 Subject: [PATCH] Tighten up some definitions Explicitly define how nadir and sub-satellite point are defined, for anyone unfamiliar with the terms Consistently use the term "scene" rather than "target". To me, "target" could be ambiguous - is it the location that was imaged, or the location that was *desired* to be imaged? Those may be different if the satellite has a boresight misalignment or otherwise failed to point at. (Admittedly, for defining ONA/azimuth/... at a whole scene, that target vs. scene difference is pretty negligible) I'd also be interested in a more concrete definition of scene center, but I am unsure as to whether the definition is: * The location imaged by the central pixel (for a frame imager, anyway) * A centroid derived from pixels containing data * A centroid derived from a bounding box or bounding polygon * other! --- README.md | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/README.md b/README.md index 9ea2276..ef8abde 100644 --- a/README.md +++ b/README.md @@ -39,8 +39,11 @@ The fields in the table below can be used in these parts of STAC documents: | 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`). | +`Nadir` is defined as a vector running from the satellite to the center of the earth. The `sub-satellite point` +is the point where the nadir vector intersects the earth's surface. + 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 +the sub-satellite point on the earth, and the scene center point 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 @@ -50,7 +53,7 @@ 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). +the scene plane to the body (satellite, sun, or moon). Viewing Angles: Sun and Moon Elevation and Azimuth