# Towards a Digital Twin Prototype of Alpine Glaciers: Proposal for a Possible Theoretical Framework
## 4 Discussions
The combination of the IRIDE constellations will allow a daily revisit time in Italy with a GSD of about 2 m.  
While waiting for IRIDE, commercial providers such as PlanetScope [88], SkySAT [89] and ICEYE [90] could represent a possible solution. PlanetScope can, in fact, acquire optical data with a daily revisit time (at nadir), GSD ranging between 3.0 and 4.1 m; on the other hand, SkySAT sensors provide optical images with a GSD in the range of 0.57-1.0 m and a revisit time of 4-5 days; ICEYE acquire SAR data with a great variety of combinations in terms of both spatial and temporal resolution. These data are commercial, i.e., not free. Nevertheless, they are also entered into institutional archives, opening to a new hybrid approach that integrates commercial and public services. While waiting for more structured actions from institutional players (i.e., national or international space agencies), where high-resolution 3D data are needed, airborne and UAVs (Unmanned Aerial Vehicles) can support the process at the single glacier level. Acquisitions can be achieved through multispectral digital photogrammetry or Airborne Laser Scanner (ALS) in response to specific actions from single users. Products obtainable from this type of acquisitions (Digital Orthophoto Maps (DOMs), point clouds and Digital Surface Models (DSMs)) make possible, in fact, a detailed monitoring of glacier dynamics [91]. DSMs can eventually be derived also from sub-meter satellite stereo images (Worldview, Pleiades, GeoEye, ongoing Italian Iride mission) partially filling the gap between global DSMs from free satellite missions and very-high-resolution DSMs from aerial/UAV surveys [92]. Especially in the framework of regional (systemic) approaches to glacier DTs, this transitory situation necessitates that these single acquisitions are somehow properly coordinated by accredited players who are called to propose standards and guidelines to make all information comparable. In particular, some legislative actions are expected at the national and international level to formally identify the subject that could be in charge of supervising and managing the coordination process, by firstly defining proper standards for (i) data acquisition and processing, (ii) data representation, (iii) type of information and (iv) suitability conditions under which different information can be used. As far as DT modelling features are concerned, we propose a scalable approach relying on a facet-based logic. These should drive the system, according to user's needs, to operate the generation of a specific output while selecting/flagging the desired options that the system can offer. Options should concern: (i) output spatial resolution; (ii) output time resolution (if dynamic); (iii) type of modelled glacier parameter (or parameters); (iv) level of accuracy of estimates (if applicable); (v) type of model (if single or multi glaciers); and (vi) type of inputs (one should be enabled to adopt inputs from the system or to upload his/her own data). The output type is expected to be \"static\" if fixed at whatever past, present or future (simulated) situations, or \"dynamic\" if proposing a sequence of situations presented in the form of changing 3D models, graphs or tables. Both static and dynamic approaches could rely on actual data or respond to simulations where inputs can be arbitrarily set. Both visual (perceptive) and quantitative outputs should be generated.