From digital image series to 3D models - Arctron

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From digital image series to 3D models interface allows correlating reference points within the point cloud to independently measured control points. By deploying a so-called multi-point transformation, the point clouds are transformed into the target coordinate system, whereupon the errors are checked and can be optimized in general (Fig. 2). Fig. 2: Scaling and georeferencing in aSPECT 3D . Left: Photographs of a house façade as a SFM image series. Middle: Single reference points are recorded with a total station and processed in aSPECT 3D . Right:: Using a multi-point transformation, the scaling and georeferencing is carried out with the computation of the errors. © ArcTron 3D GmbH, 2012. Larger objects (e.g. roof landscapes, façades) sometimes consist of several point clouds merged together, because they were recorded from very different points of view – e.g. via airborne photography or from the ground. For this purpose, aSPECT 3D includes an ICPalgorithm, which can fuse overlapping point clouds together. This is especially suited for image series with very little overlap. Once this step is done, the 3D point model is finished. How precise are 3D models? 3D models have to depict the original object as precisely as possible for restoration, preservative or archaeological purposes. Size and shape need to have the smallest possible deviation. Depending on the deployed survey procedure (total station, laser scanner etc.), the accuracies can vary. The creators of the software, which generates models based on SFM-images, asked themselves if aSPECT 3D could compete with other 3D methods. In order to answer this question, ArcTron 3D carried out accuracy analyses during different projects 5 . The results (below) show, that for both small and large objects, significantly precise 3D models can be generated. 5 Publications of the author concerning the topic are currently in print or preparation. Compare also: Kersten, T. &Mechelke, K., Fort Al Zubarah in Katar – 3D-Modell aus Scanner- und Bilddaten im Vergleich. In: Luhmann/Müller (Ed.), Photogrammetrie, Laserscanning, Optische 3D-Messtechnik. Beitr. Oldenburger 3D- Tage 2012, S. 89-98.
From digital image series to 3D models Fig. 3: Project “wall documentation”. Veitsberg, Bad Neustadt a.d. Saale (in cooperation with the University of Jena, Professorship for Prehistoric and Protohistoric Archaeology). Left: Photographic documentation airborne and from the ground. Middle: Reference surveys with total station and laser scanner. Right: Deviation result after a “best fit” registration of the SFM point cloud onto the terrestrial laser scan with standard deviation of below 1 cm. © ArcTron 3D GmbH, 2012. Example 1: In cooperation with the University of Jena, ArcTron 3D surveyed a small representative area of wall fragments in Veitsberg showing early medieval Carolingian remains (Fig. 3). For the accuracy analysis, the model generated by aSPECT 3D was compared to models generated in a different way. In one test various control points placed in the area were recorded via total station. In a second test the reference area was scanned with a terrestrial 3D laser scanner which produces accuracies of about 3 mm. The aSPECT 3D model was computed from airborne pictures taken by a so-called ‘Octocopter’ drone and pictures taken from the ground. The comparison of the reference data from SFM point cloud and laser scan showed a very small standard deviation of about 1 cm. If only a total station is employed, the standard deviation still remains smaller than 3 cm. When a reference difference (e.g. a scale photographed with the object) is the only device for scaling the point cloud, excavation findings of the respective size provide accuracies of around 5 cm. Fig. 4: Project “Enigma 3D” (in cooperation with the “celtic + roman museum” Manching). Left: Angular “bread loaf idol” in photorealistic and shaded depiction. Right: The highly precise structured light scanner PT-M 1280 serves as reference measuring device. For the automated, photographic SFM-documentation, two Nikon cameras with macro-lenses and a turn table were set up in a homogenous white light room. The deviation analysis proved accuracies better than 0.2 mm. © ArcTron 3D GmbH, 2012. Example 2: Another accuracy test showed that the software is also suited for small objects. It was carried out in cooperation with the “celtic + roman museum” Manching 6 and focussed on 6 M. Schaich, „Aenigma 3D“. Zur Generierung hochaufgelöster 3D-Modelle mit Fotos einer handelsüblichen Spiegelreflexkamera – eine Fallstudie mit ArcTron’s Software aSPECT 3D am Beispiel der „Brotlaibidole“. In W. David (Ed.), Aenigma. Katalog zur Ausstellung (2012) – in preparation.
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From digital image series to 3D models - Arctron

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