Today, world-wide, there isn't a consistent Digital Elevation Model (DEM).
Among the several advantages of the Synthetic Aperture Radar (SAR) Interferometry technology with respect to competing ones, the active nature of SAR systems provide weather independency, allowing to map large areas of the terrain in limited timeframes, not depending on the weather and solar illumination conditions. Although complex, the processing of interferometric SAR data is, at large extent, automated, providing accurate and cost-effective products in reduced time.
Already in the late 90's, sarmap has produced the first country-wide DEM using this technology. The Swiss DEM has a spatial resolution of 25 metres and a height accuracy ranging from 7 (moderate topography) to 15 metres (steep topography).
The combination of interferometric SAR acquired simultaneously at high frequency (X-Band), non penetrating through the tree leaves, together with lower frequency (P-Band) data allows to obtain at the same time and directly information on the surface (DSM) and terrain (DTM) height of a vegetated area.
The generation of the DSM and DTM is performed by exploiting interferometric data acquired by the OrbiSAR-1 SAR system. This system, mounted on a small airplane, is capable of acquiring three X-Band and one P-Band images of a same are simultaneously, allowing to generate DSMs with single-pass acquisitions. If DTMs shall also be generated, a repeat-pass acquisition scheme has to be implemented. The system is capable of acquiring X-Band with a selectable horizontal resolution of up to 0.5 by 0.5 m and P-Band data up to 2.5 by 2.5 m. An example of a high resolution DSM is shown in the figure below.
Spaceborne/airborne stereo-optical and SAR interferometry, spaceborne stereo-SAR, and Airborne Laser Scanning techniques are well established techniques since several years for the generation of DEMs. Each of them suffers from limitations that in many cases do not allow to obtain accurate results without the integration of other information: nevertheless, many of the problematic areas for one of these technologies can be very often covered with the other one to a high level of success. Hence, the smart combination of DEMs obtained from these technologies allows to obtain fused DEMs that may provide higher accuracy and less blunders in almost every land cover and topography conditions, exploiting complementarities of the technologies and not just considering them as mere alternatives. As example, the DEM fusion based on interferometric ALOS PALSAR-1 and SPOT-5 HRS stereo-optical data is shown in the figure below.
F. Holecz, J. Moreira, P. Pasquali, S. Voigt, E. Meier, and D. Nuesch, Height model generation, automatic geocoding and mosaicing using airborne AeS-1 InSAR data, IGARSS Symposium, Singapore, 1997.
C. Poidomani, D. Costantini, and P. Pasquali, National-scale DEM generation using ERS-Tandem data in alpine regions, ERS-ENVISAT Symposium, Gothenburg, 2000.
M. Crosetto and P. Pasquali, DSM generation and deformation measurement from SAR data, Chapter 12 in Advances in Photogrammetry, Remote Sensing and Spatial Information Science, ISPRS Congress Book, edited by P. Aplin, 2008.
P. Pasquali, A. Cantone, and M. Barbieri, Generation and validation of accurate DEMs over large areas using ERS-ASAR Cross-interferometry, European Space Agency Living Planet Symposium, Bergen, 2010.