Evaluation of Digital Elevation Models created from different satellite images

Dr. K.S.Siva Subramanian, Amitabh Singh, Manda Sudhakar
RMSI Private Limited, A - 7 Sector 16, NOIDA 201 301, India
Tel +91 120 251 1102, 251 2101, Fax +91 120 251 1109, 251 0963


Abstract
The generation of Digital Elevation Models (DEMs) from satellite images scores over conventional methods of DEM generation using topographic maps and aerial photographs. This global availability of satellite images allows for quicker data processing for an equivalent area. This paper discusses the evaluation of DEM, generated using stereo pairs of different satellites. The various satellite datasets taken for study include Radarsat standard beam mode data, IRS 1-C Pan, ASTER VNIR bands, and Spot Pan data. PCI Geometica OrthoEngine v8.2 software has been used for DEM generation, and Erdas Imagine v8.4 has been used for the DEM evaluation. Each of the source image pairs used for DEM creation have their own merits and demerits. The Radar images are generally processed in two different ways viz. interferometric method and stereoscopic method. An analysis of the literature survey shows that the interferometric method yields better results in a relatively low relief area but not so in moderate relief areas. Conversely the stereoscopic method gives better results for moderate relief areas falling in flat topographic terrains.

In addition the beam mode selection too plays a key role in the output DEM quality. A larger beam mode separation for the stereo pair (say S2-S5) results into a good parallax. The automated pixel matching method adopted by the software, however fails when the parallax is too high. A low beam mode separation (say S2-S3), on the other hand results in errant values in low relief areas. The problem is further compounded by the presence of speckles.

Alternatively hand stereo pairs obtained by satellites, operating in optical mode yield more consistent results over different kinds of terrains. The only method of processing these images is the stereoscopic method of measuring parallax by pixel matching.

Studies reveal that of the three different pairs processed, the ASTER images yield the best results. This is possible because the temporal difference between the stereo pair is only a few seconds, whereas pairs acquired through other satellites require a larger time cycle. Results also show that raster match is good in case of ASTER images due to short temporal differences. Further the unique fixed b/h ratio yields more consistent and better results over various terrains.