Drawing Accurate Ground Plans Using Optical Triangulation Data Kevin Cain INSIGHT kevin@insightdigital.org Philippe Martinez cole normale supØrieure / INSIGHT pmartine@ens.fr Abstract Here we consider optical triangulation scanning as a means of creating permanent architectural archives in the form of accurate ground plans and other orthographic views. We present plan drawings created with laser scan data and use these documents to make comparisons with existing documents. Finally, we present a new technique for decreasing the laser scanning field time required to create plans and other views. 1. Preface This brief paper introduces an effort to create accurate ground plans for a Fatimid-era monument in Cairo. The Egyptian Antiquities Project of the American Research Center in Egypt (EAP, ARCE), having undertaken conservation of the Zawiya and Sabil of Sultan Farag ibn Barquq (Al-Dehisha) in Cairo, Egypt, requested digital documentation as a basis for their physical restoration. This monument, shown below, is listed as Supreme Council of Antiquities Islamic and Coptic Antiquities Monument No. 203. Fig. 1Digital photograph of north faade, Farag Ibn Barquq 2. A Simple Review of Optical Triangulation Scanning Practice Laser scanning can be used to create a representation of an object in space, but the construction of this image comes by a very different method than is used in conventional photography. When a conventional photo is taken, the image is captured through a lens. The specifications of the lens determine the observed perspective in the final image. Also, the lens introduces distortions that make it difficult to extract accurate drawings or measurements. The orthographic drawings commonly used by architects and engineers are drawn without perspective in order that the relationships between any given points on the drawing can be measured at a constant scale. During optical triangulation scanning, a sensor measures the distance from the scanner to a specific point on a given object. By making these measurements, the relationship between the gathered points can be expressed in the constructed 3D space of the computer. By taking a many measurements, a cloud of points emerges that accurately describes the subject being scanned. Finally, when rendered from the point of view of a synthetic camera in the 3D world space of the computer, digital drawings are generated. 3. Our Work at Farag ibn Barquq The scanning and registration process at Barquq involved multiple viewpoints, which were later correlated. Fig. 2Digital photograph of the monuments west faade Four main techniques were used to document selected parts of the mosque: photography, acetate drawings, acoustic measurement, and laser scanning. Conventional ink drawings on acetate were used to record the epigraphy set in limestone near the mosques entry. Freehand drawings were made of all areas in the 1063-6919/03 $17.00 © 2003 IEEE