# From Analogue to Digital Photogrammetry: Documentation of Padise Abbey in Two Different Time Stages
## 1 Introduction  
Photogrammetry is a method that is used in the fields of cultural heritage and archeology to get a documentation of cultural heritage objects [1; 2; 3; 4; 5]. Due to the photogrammetric surveys carried out in different time stages, we can study the objects and explore two different conditions of the object.  
In the past, metric analogue cameras were used for photogrammetric surveys and single-image photogrammetry and stereophotogrammetry were prevailing methods [6]. As a storing medium of photogrammetric images, glass plate negatives or photographic film were used. In the past, the metric cameras and other equipment were very expensive, and the processing required well-trained workers [7]. In 1990s, photogrammetry experienced a revolution with the arrival of digital cameras. Nowadays, close range images captured by digital cameras are often processed by effective and fast structure from motion (SfM) method. The structure from motion method has a background in computer vision science, which has different approach. The methods of computer vision were originally aimed at object recognition, navigation of autonomous vehicles, or object modelling. The internal parameters of source images for computer vision were not usually known in contrast to analogue photogrammetry, where the cameras had high optic quality with calibrated lenses and where the main topic was accuracy of a measurement [8].  
Digitizing and re-processing of analogue images of photogrammetric surveys carried out during analogue era give an opportunity to get new and potentially valuable results. For example, one of the results of the re-processing of digitized analogue images could be point cloud and orthophoto of a cultural heritage object. These results can help to study how the object has changed. Re-processing of the analogue images could have high meaning in some particular cases. For example, re-processing of images was used by [9] to make a 3D restitution of destroyed and buried historical structures of the Beaufort Castle in Lebanon. Processing of digitized analogue images of the Kronentor of the Dresden Zwinger captured by non-metric cameras with mostly unknown camera geometry was presented by [10]. Archival analogue photogrammetric images of the Great Mosque in Aleppo, Syria were used and re-processed by [11]. The mosque has been damaged during the ongoing war. The analogue images were digitized and re-processed by structure from motion method and 3D results were generated. It is possible to study the former state of the mosque on those 3D results and it can help during the future renovation. By [12], the change of Erbil Al-Qala citadel was presented on comparison of historical and contemporary data.  
Between 1972 and 1999, the group around [PERSON] from the National Design Institute for Cultural Heritage carried out several photogrammetric surveys of important cultural heritage sites in Estonia and abroad. Photogrammetric images of the surveys contain important and high-quality records about heritage site conditions and provide valuable information for heritage researchers, owners, architects, engineers, conservators, and renovators. In 1991, one of the photogrammetric surveys was a survey of the south wall of Estonian Padise Abbey. At that time, walls of the abbey were in poor condition. Nowadays, the south wall has been renovated, so it is convenient to present the re-processing of analogue images using SfM method in this case. Twelve of the analogue images (six stereo-pairs) captured by UMK metric camera were digitized and re-processed. In 2017, new images were captured by digital camera and the images were processed by SfM method. Due to resulting point clouds and orthophotos by re-processing and by new survey, it is possible to compare the former and current state of the walls.