DIGITAL RECONSTRUCTION OF THE CHURCH OF SAN ILDEFONSO AT ZAMORA (SPAIN) USING ORTHOWARE

Abstract. This paper describes the surveying process performed on the Church of San Ildefonso in Zamora (Spain) using Orthoware^©, an innovative photogrammetric software tool specially designed for the digital reconstruction of cultural heritage. This software is the result of intense R&D at Metria Digital after several years producing heritage true orthoimages, plans and full color 3D models in a commercial environment. Orthoware allows true measurements and produces high quality true orthoimages and full color 3D models starting out from conventional digital photographs. This is a progressive tool designed for non-specialist users, providing intuitive and quick methods to visually diagnose the quality of the results.

The Church of San Ildefonso in Zamora was built over a previously existing temple whose Romanesque construction dates back to the 11th century, although it is nowadays hidden among extensions and remodelings carried out up until 18th century. With a length of more than 30 meters, the original floor plan of the Church consists on three naves and three apses, although only one nave is visible now, covered by groin vault, and one semicircular apse partially hidden by the current building. The south front maintains the greater part of its Romanesque origins, in spite of being hidden and higher than the present ground level. At the feet of the temple a tower rises, whose first stage is Romanesque but which has been altered by numerous Baroque elements.

The objective of the photogrammetric reconstruction was the integral survey of the monument, including its four façades, interior faces and roofs for the production of some true-orthoimages, cross-sections, longitudinal-sections and ground plans at scale 1/50 with an accuracy of 10 mm for the drawings and a pixel size below 10 mm for the synthetic imagery.

The usual photogrammetric workflow for producing true-orthoimages and digital 3D models in cultural heritage depends to a great extent on finding and matching homologous features amongst various images. This is a time-consuming, repetitive and blind recognition process requiring some spatial intuition and experience from the user. Orthoware makes use of automatic image analysis and computer vision techniques to improve this processing. Its innovative graphic user interface allows full interactivity and corrections or refinements to be performed at any time providing intuitive diagnostics from partial results. This flow involves very intensive calculations achieved through a software development specific for GPU (Graphic Processing Unit), using OpenGL (Open Graphics Library) and DirectX. Therefore, a combination of manual and automatic techniques makes up the ideal tool for a production environment. The workflow presented herein allow users to reduce orthoimage production time from months to just a few weeks.