Quality

Sun elevation angle

How high the sun was at capture time. Affects shadows, illumination consistency, and time-series comparability.

SUN ELEVATION · SHADOW GEOMETRY45°shadow · 1.00× hh
Shadow length
1.00× building height
Illumination
Good analytical light
Fig. 1 Sun elevation angle controls shadow length and illumination uniformity. Low sun emphasises terrain and micro-topography but casts long shadows that occlude urban canyons and forest canopies. High sun gives flat, uniform illumination preferred for analytics. Shadow length on flat ground scales with cot(elevation).

Low sun means long shadows, which is bad for seeing into urban canyons or forest canopies but great for terrain visualisation and picking out subtle topography. High sun means flatter, more uniform illumination, which is what you want for most analytics work.

Time-series consistency

Sun elevation matters enormously for multi-date stacks where you want consistent illumination across the time series. Forest monitoring, for example, needs every image taken at similar sun angles so that shadow patterns don't contaminate the change signal. This is one reason surface reflectance correction is essential — it normalises for sun angle differences.