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ReSAC participation in JRC’s study on the Orthorectification Performance of Formosat-2 Data for the Control with Remote Sensing Campaign of the CAP

At the end of 2006 and during 2007, Remote Sensing Application Center – ReSAC Bulgaria investigated the possibilities for orthorectification of FORMOSAT-2 scenes with two well established software packages in the geospatial industry – ERDAS IMAGINE and PCI Geomatica.

ReSAC worked closely with MARS-PAC and CID actions in the Agrifish unit for IPSC, DG JRC, responsible for the technical support of the “Control with Remote Sensing” (CwRS) – a European Commission Service programme for monitoring the agricultural subsidy claims with a strong deterrent effect on potential frauds. Because of the EU accelerative enlargement the number of sites to be controlled increases dramatically. Hence, JRC intended to include several more satellite sensors in order to increase the acquisition capacity and to ensure timely delivery of the necessary imagery to the MS administrations and their contractors.

JRC decided to investigate more thoroughly the usability of FORMOSAT-2, from geometric accuracy point of view, for the CwRS campaign.

From the very beginning ReSAC has played an important role in this research working in close collaboration with JRC, along with other companies such as SPOT Image (France) and Spacemetric AB (Sweden). The work followed the JRC best practices related to the conduction of the overall test set and the accuracy evaluation (Common Technical Specifications for the 2006 Campaign of Remote Sensing Control of Area-Based Subsidies, ITT no. 2005/S 233-229332, Ispra, 2005).

ReSAC has been responsible for the investigation of the best methodologies for FORMOSAT-2 imageries orthorectification in the environment of ERDAS Imagine and PCI Geomatica software suites. In general the research was carried in two phases – in the first the overall models’ functionalities and the positional accuracy of the ortho-products were studied, while in the second, the effect of the incidence angle over the final quality of the orthoimageries was in focus. The results from the first phase of the project were presented at the Toulouse Conference in November 2006 (Orthorectified Formosat-2 data performance in the CwRS campaign 2006 and future applications. JRC, ReSAC et al., 2007) and are published in the special publications edition of the conference reports.

In both phases of this thorough research over the orthorectification of FORMOSAT-2 scenes, ReSAC participated also with its geodetically precise database of DGPS point measurements. They were used primarily in the quality control carried over the orthorectification outputs. As a source for independent check points (and also ground control points – GCPs), satellite orthophotos from Ikonos and QuickBird images processed by ReSAC were also used. They were needed in order to assess the influence of the accuracy of the GCPs on the final orthophoto accuracy.

For the second phase of the JRC’s study, two sites were acquired two times each, with different incidence angles. One of the test areas was Sozopol, Bulgaria and the second was Mausanne, France. As already stated, for each test site two PAN images were available, but each was taken with different off-nadir angles – similar across track and various along track angles.

Location of the test areas: (a) Two test areas in Bulgaria – Sofia area on the left and Sozopol area on the right; (b) Mausanne area in France

The technical part of the second phase of the study is completed, but the results are currently being systematised and summarised. The results of the study will be presented on the Annual MarsPAC Conference 2007 “Geomatics in support of the CAP” which will be held in Madrid, Spain on 12th-14th Nov 2007.

Another related study that ReSAC undertook was exploring the potential influence of different DEMs to the quality of the final ortho-products.

Because the quality of an input DEM might affect the positional accuracy of the orthorectified imageries, ReSAC carried an individual research over the Sofia city scene, used in the first phase of the FORMOSAT-2 study, with various DEMs. One of these elevation models was provided from the Shuttle Radar Topography Mission (SRTM) and more specifically the SIR-C sensor, where the sampling points were distributed on a grid with an approximate spacing of 70×90m at the latitude of Bulgaria. Another DEM used was Reference3D – a product of Spot Image with a sampling distance of 30m for the area of Sofia. The last two DEMs in this study were derived from analogue topographic maps for civil use of different scale, 1/50,000 and 1/25,000, that were brought in digital format. The spatial resolution of the DEMs produced from the topographic maps in scale 1/50,000 and 1/25,000 is 30m and 15m respectively (TOPO50 and TOPO25).

The results of this study will be presented in the poster session on the Annual MarsPAC Conference 2007 “Geomatics in support of the CAP”, as well as a report will be given on the International Symposium “Modern Technologies, Education and Professional Practice in Geodesy and Related Fields” organised by the Union of Surveyors and Land Managers in Bulgaria on 9-10 Nov 2007 in Sofia, Bulgaria. The full report will be available in the symposium proceedings.

Future work envisaged by ReSAC, using FORMOSAT-2 data, include testing the influence of different DEMs on the orthorectification of images with large incidence angle.

For more information:
Remote Sensing Application Center – ReSAC