Cloud and Cloud-Shadow Detection in SPOT5 HRG Imagery with Automated Morphological Feature Extraction

Adrian Fisher

doi:10.3390/rs6010776

Abstract

Detecting clouds in satellite imagery is becoming more important with increasing data availability, however many earth observation sensors are not designed for this task. In Satellite pour l’Observation de la Terre 5 (SPOT5) High Resolution Geometrical (HRG) imagery, the reflectance properties of clouds are very similar to common features on the earth’s surface, in the four available bands (green, red, near-infrared and shortwave-infrared). The method presented here, called SPOTCASM (SPOT cloud and shadow masking), deals with this problem by using a series of novel image processing steps, and is the first cloud masking method to be developed specifically for SPOT5 HRG imagery. It firstly detects marker pixels using image specific threshold values, and secondly grows segments from these markers using the watershed-from-markers transform. The threshold values are defined as lines in a 2-dimensional histogram of the image surface reflectance values, calculated from two bands. Sun and satellite angles, and the similarity between the area of cloud and shadow objects are used to test their validity. SPOTCASM was tested on an archive of 313 cloudy images from across New South Wales (NSW), Australia, with 95% of images having an overall accuracy greater than 85%. Commission errors due to false clouds (such as highly reflective ground), and false shadows (such as a dark water body) can be high, as can omission errors due to thin cloud that is very similar to the underlying ground surface. These errors can be quickly reduced through manual editing, which is the current method being employed in the operational environment in which SPOTCASM is implemented. The method is being used to mask clouds and shadows from an expanding archive of imagery across NSW, facilitating environmental change detection.

Highlights

Clouds and their shadows are often a significant problem when conducting remote sensing of the earth’s surface as they can locally obscure surface features and alter reflectance
SPOTCASM appears to detect the majority of thick clouds and their shadows within most of the 313 New South Wales (NSW) images that have been validated
SPOTCASM is being routinely used in the OEH Satellite pour l’Observation de la Terre 5 (SPOT5) High Resolution Geometrical (HRG) image processing chain, producing automated cloud and cloud-shadow masks that have an overall accuracy of 85%

Summary

Introduction

Clouds and their shadows are often a significant problem when conducting remote sensing of the earth’s surface as they can locally obscure surface features and alter reflectance. Their masking, and exclusion from analysis, is an important pre-processing step in many applications. Automating the masking is especially important in multi-temporal studies over large areas where hundreds or thousands of images require processing. The research into cloud masking presented here was conducted to facilitate large area vegetation monitoring by the Office of Environment and Heritage (OEH) in New South Wales (NSW), Australia. At the time of this research, the archive contained 313 SPOT5 HRG images contaminated by cloud acquired between 2004 and 2012. All images are assessed for cloud contamination by a visual quality control process, and their locations are shown on Figure 1

Methods

Results

Discussion

Conclusion