Sun elevation correction

Introduction

One scene peculiarity is how the scene is illuminated. Consider an area at an appreciable latitude, such as the Netherlands. The illumination of the area will be quite different in winter than in summer (overall brightness, shadows, etc.), because of differences in Sun elevation. 

Illumination differences will cause problems if we want to analyse sequences of images of a particular area that were taken on different dates (or images of the same date taken at different time), or if we would like to make mosaics of such images or if we process an image with a very large swath width. We can apply a simple Sun elevation correction knowing the incident angle of the radiance with the terrain.

How to

The trick is to normalize the images as if they were taken with the Sun at its zenith. We can achieve this normalization by dividing every pixel value of an image by the sine of the Sun elevation angle at the time of data acquisition. The Sun elevation angle is usually given in the meta-data file, which is supplied with an image. Obviously this is an approximate correction as it does not take into account the effect of elevation and height differences in the scene, nor atmospheric effects.

Prior knowledge

Outgoing relations

Learning paths