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A preliminary survey is required, in order to understand the site and its surroundings. Due to flight regulament constrains it is important to check with local and national authorities if it is possible to fly drones over the area, and obtain the necessary permits (this may require some times, from weeks up to months, depending from country to country). It is also important to check for near natural or artificial obstacles that will influence the design of the drone's path.

Using well materialized Ground Control Points (GCPs) and using the same set in all the performed flights will require the use of topographic instruments only for the first acquisition, as once measured, the GCPs will be re-used for the subsequents flights. It is important to underline the importance of acquiring a trustworthy set of GCPs to be used for all the surveys, instead of measuring new GCPs every time. It is more important, in fact, not to acquire the most accurate GNSS data, but to use the most precise set of GCPs, assuming for (at least locally) the validity of the first data acquired, cause repeating measures over time may lead to non extremely precise co-registration of the generated models. Instruments required: Total station D-GNSS antenna Action to perform: Defining and measuring a topographic network Materialization of the topographical vertices Materialization of the control points Ground control points displacement around and inside the site Natural points selection and digitization for future surveys Control points measurement and recording (via Total station or GNSS receiver) Softwares to use: Topographic network compensation (e.g. MicroSurvey StarNet)

In UAV photogrammetric acquisition the flight plan is a crucial part for guarantee the success of the subsequent 3D object reconstruction. In this step it is important to decide the level of details you want to reach. The flight plan will influence different parameters of the reconstructed object, such as GSD (influenced by flight altitude), camera orientation (depending on the object conformation). It is also important to consider a sufficient overlap between images (usually 80%-60%). Instruments required: UAV platform DSLR camera (for close range acquisition) Action to perform: Flight Planning Flight Execution Softwares to use: Flight planning software (Pix4D capture, DJI DSPRO, UGCS, etc.)

This step is related to the surveyed object and to the software used for processing the data. Usually it is possible to refer to a traditional photogrammetric workflow: Data loading Tie points extraction (Sparse point cloud) Camera Calibration Relative orientation Absolute orientation (using georef. data) Densification (Dense point cloud) Meshing (Triangulated mesh) Texturing DSM generation 3D model exporting Software to use: Photogrammetric suite (MicMac, AgiSoft Metashape, Pix4D Mapper, etc.)

In this step it is possible to perform the desired analysis, depending on the aim of the research to carry out. This is an illustrative and not limiting list of the possible analysis to perform: Cloud to Cloud (C2C) comparison Radiometric analysis (color change of the surfaces) DSM for surface comparison Soil volume excavated Visual interpretation GCP e CP analysis Software to use: Cloud Compare 3D Reshaper

Alessio Calantropio

Gabriella Carpentiero

Lorenzo Teppati Losè

Aerial photogrammetry is a powerful technique that can be used for recording and analyzing excavation phases of archaeological sites. Based on the authors experience, there are some strategies that can speed up the data acquisition, processing and analysis steps. This works aims to underline best practices and lesson learned in the scenario of UAV data acquisition and management for multitemporal surveys of archaeological sites with a photogrammetric approach.

UAV data acquisition and management for multitemporal surveys of archaeological sites.

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Workflow steps(5)

1 Preliminary survey of the site (and its surroundings): Where to fly and what to avoid.

2 Georeferencing the surveyed data: Using a GNSS receiver for precise positioning of Ground Control Points

3 UAV Data acquisition: Using the drone for acquiring aerial images of the site

4 Data processing: Create a 3D model of the site using the acquired images

5 Data analysis: How to use the generated 3D model for study the site