Despite the growing number of satellite optical and radar sensors and the continuous effort in optimising the technical specifications (e.g. spatial and spectral resolution), the underlying physical and technical principles for analysing the relevant remote sensing data remain practically unchanged and can be broken down into a number of elementary processing steps, such as radiometric and geometric calibration, orthorectification, cloud screening, atmospheric correction, and validation. The harmonisation of these steps and of the required ancillary data is therefore a fundamental prerequisite for improving interoperability of the various missions.

This harmonisation requirement is nevertheless a long-term goal, while the current situation, especially for what concerns Third Party Missions, shows an extremely heterogeneous landscape, with a large variety of different theoretical approaches and procedures for addressing the same problem. This is one of the most important issues that is currently hindering the interoperability of the various sensors, since the observed inconsistencies are difficult to understand and, therefore, to resolve.

The main objective of the EDAP Multi-Mission Studies is to address all multi-mission common requirements in terms of satellite data processing and Cal/Val protocols, so as to improve the consistency of similar measurement types and ease of interoperability.

The following study is considered part of the Multi-Mission domain of EDAP. Further studies are expected to be added throughout the project.

This study is related to the use of Digital Elevation Models (DEMs) for different missions and their impact on data quality, specifically over critical mountainous regions.

Image data acquired by satellites are affected by systematic sensor and platform-induced geometry errors, which introduce terrain distortions when the sensor is not pointing directly at the nadir location of the sensor. In order to accurately remove the image distortions, a DEM is used to perform image orthorectification.

A DEM is a dedicated catalogue that represents the relief of a surface between points of known elevation. Orthorectification is the process of removing internal and external distortions to assign more accurate coordinates to the final image. The goal of orthorectification is to create a final product whereby every pixel in the image is depicted as if it were viewed at nadir (i.e. directly overhead), removing the effects of hills, valleys, etc., on the data. However, the orthorectification process is less accurate over areas with varied relief (such as mountains), or over cities (where the bare earth representation given by DEM is very challenging to be assessed due to the presence of human artefact, such as buildings).

The first step in this study is to assess the quality of the global DEMs over these difficult areas, using defined areas of comparison and evaluating the quality using standard statistical parameters such as Root Mean Square Error (RMSE) or Linear Error at 90% and 95% confidence. 

Global DEMs Documentation

• Technical Note on Quality Assessment for Global DEM

Copernicus DEM Documentation

• Technical Note on Quality Assessment for Copernicus DEM
• Technical Note on Planimetric Misregistration Assessment

VHR DEMs Documentation

• Technical Note on VHR DEMs
• Technical Note on Sentinel-1 ARD from VHR DEM