## 4 Results
### User Study
At the beginning of each session, the 4D browser was introduced via a live tour presenting different features and data. Subsequently, the participants were encouraged to use the application themselves to solve a defined task and provide feedback to the session leaders. At the end, they were asked to complete an online survey on specific design questions. The demo session approach was reworked into a more extensive test and was carried out with art history students from the University of Munich in July 2023 and with digital humanities scholars in October 2023. Both groups are among the application target users. The two sessions again included a live tour of the 4D browser introducing different features. After a brief discussion to clarify questions concerning the data and the interface, the participants completed an online questionnaire with specific tasks to solve using the 4D browser. The questionnaire allowed every participant to work at their own pace, individually or in groups. The tasks were designed to test the functionalities of the application using the little available test data und to see if users approach solutions as intended during development and are satisfied with the results. The tasks were very simple scenarios connected to art history research and analysis:  
* Identifying buildings in a photograph. * Gathering information for a building (footprint, roof shape) and finding images from all sides of the building. * Analyzing which perspective of a building was most frequently photographed. * Analyzing differences between a digitized painting and the city skyline. * Comparing the ways two photographers staged a certain building. * Reconstructing the biography of a building. * Identifying a certain statue in a photograph. The questionnaire provided all necessary information concerning the task. Additionally, it was possible to seek assistance by displaying a step-by-step approach to the solution, even offering different ways to solve the task. The participants were able to provide feedback on the features and navigation of the application as well as the assigned tasks. After each test, the feedback was reviewed and the application was adapted accordingly. Subsequent tests revealed if the changes were perceived positively. Limitations  
A general issue in historical research is that available information and data are always fragmented in terms of time and space. Validated knowledge is bound to single points in time, which may then be interpolated. Historical photographs capture a certain moment in the past, but the exact date when they were taken is often unknown. Similarly, the dates when a building was constructed or demolished may be uncertain. This affects the reliability of our visualized data, particularly if images are projected as textures of 3D building models. Due to the different temporal restraints and uncertain knowledge, a building may have the wrong facade texture at specific dates. Inaccuracies in spatialization and in transformations of the photograph, or not using central perspective cameras, lead to mismatches between the projected texture and the 3D surface. If objects have been captured in the foreground of a photograph, they appear bigger when projected onto the building, hiding parts of the facade. Currently, where data are missing, building footprints extrapolated to 3D geometry and maps are queried from OSM to fill in the gaps. However, these contemporary data only show the current building situation. When exploring the past, buildings may be shown that were not existent at that time. Additionally, since the footprints are extrapolated, they may be hard to identify. This applies in particular to more complex buildings like churches. Furthermore, not all OSM data are properly tagged with a height value or the number of levels for an estimation of the building heights. Despite using data compression, the amount of data loaded can still be very extensive, which is a crucial issue especially for the mobile application, where the bandwidth may be limited.