## 2 Materials and Methods  
### Work Scheme  
The ultimate goal of this research was to develop a low-cost workflow, according to Figure 4, that allows us to generate digital twins of reality and publish these resources on different platforms, similar to what was studied in Mergin' Mode: Mixed Reality and Geoinformatics for Monument Demonstration [41], in which the rapid and open distribution of information was also sought [42].  
The research work scheme is divided into the following 6 activities (Figure 5), which start from a digital twin generated by a photogrammetric [43] survey with a drone and work their way up to the different configurations of a particular metaverse:  
* Activity I. The first activity of this workflow establishes a starting point prior to the deepening of the metaverse. In this activity, the heritage asset under study is filtered and selected, and the previously generated digital twin is optimized and prepared for its integration in the metaverse.
* Activity II. This activity is based on a general analysis of the metaverse according to the field in which it is active.
* Activity III. An overview is presented that allows us to distinguish between the different types and analyze the different digital resources currently available that can be used as metaverses to host our digital twins.  
Figure 4: Schematization of work milestones.  
* Activity IV. In the development of this activity, the functionalities and capacities of 32 metaverses in which the generated digital twin could be included are listed and analyzed. Based on this detailed analysis, a first screening was carried out according to our requirements (digital infrastructure twins obtained through TLS or SFM [44]).
* Activity V. The 7 metaverses selected in the previous screening were analyzed, and a second filter was applied to select a single metaverse from which the results of the virtualization and integration in the metaverse are shown.
* Activity VI. The functionalities of the selected platform were tested with our working model in order to obtain results compatible with VR (virtual reality) and AR (augmented reality) environments [45].  
As we indicate above, a premise of this work is that the workflow is low cost. Figure 4 shows the main stages of this work. First, \"stage 1: photogrammetry. Generate digital twins\" is proposed using photogrammetry, because it is an affordable system in terms of both equipment and technology. In our specific case, the DJIII Mini 2 UAV equipment and the Agisoft Metashape software were used. Comparatively, this technology is cheaper than traditional scans carried out with TLS (terrestrial laser scanner). The order of magnitude would be an initial investment that is 30 times less in both software and equipment if we compare it with current equipment, such as BLK 360 from the Leica company. This equipment has other drawbacks, such as the ghost zones it generates [35]. In \"stage two: optimization and model re-fining\", the use of three software programs that are common for processing the results obtained from data collection with UAVs such as TLS is proposed: Rhinoceros, Blender and 3D Builder. At this stage, there is no economic improvement from one method to another. Finally, in \"stage 3: VR environment\", as we show throughout the text, we have different alternatives, and a selection criterion is that our environment can be  
Figure 5: Schematization of work activities and milestones.  
used by both the content generator and the content consumer at no cost for said universal accessibility, regardless of economic condition.  
We can affirm that the total cost of the proposal carried out with photogrammetry would be 30 times less than one in which the capture was carried out with TLS equipment. It should be noted that the costs are attributed to the need to capture the reality of the built heritage because we would have free software alternatives such as BLENDER, and a premise for the choice of platform on which the content is hosted is that it is free.