**1. Introduction**

The evaluation of deformations in civil infrastructures or natural environments is normally assessed by the realization of very accurate three-dimensional (3D) models by means of a range of complementary geomatics techniques such as total stations, global navigation satellite systems (GNSS), photogrammetry, or laser scanning [1]. Nonetheless, each geomatic technique is a ffected by its own instrumental and physical limitations, especially when the monitored area is large and has a complex topography [2]. For instance, atmospheric refraction can severely limit the achievable accuracy in the distance and angular measurements, also in laser and image scanning [3]. Therefore, the integration of data, collected by di fferent techniques, in a unique coordinate reference frame becomes crucial to producing consistent 3D models able to be used for overtime deformation monitoring purposes [4].

In recent decades, advanced monitoring technologies have spread and increasingly been used for the study and managemen<sup>t</sup> of geological hazard and risk, which may compromise the state of preservation of civil infrastructure [5]. Monitoring actions are necessary to guarantee health and safety conditions by controlling the evolution of deformation patterns or detecting significant instabilities. In terms of spatial and temporal resolution, the improvement of geomatics techniques represents a significant achievement. These methods provide innovative tools in supporting mapping products and geological analysis required for assessment and evaluation. Accurate and fully geo-referenced 3D datasets can be used to characterize in detail structural and geological settings, as well as the geomorphology of a studied area. Geological applications, for example, geo-hydrological risk assessment, rockfall runout modellings, or slope stability analysis, can have a grea<sup>t</sup> benefit through non-destructive investigation.

Where a topographic survey is based on limited distances (e.g., tens to hundreds of meters), it has historically been carried out with total stations. Although such method provides high accuracy and precision for the measurement of individual points, significant time is required to collect a su fficient density of data to produce rough landscape Digital Elevation Models (DEMs) [6]. Laser Scanning (LS) and Close-Range Photogrammetry (CRP) are state of the art techniques for acquiring dense and precise topographic data at the output detail, for accurate volume measurements or modeling.

Accurate mapping and monitoring of lakeside reservoirs, as well as coastal areas and fluvial processes, are critical tasks to which several techniques have been used, from aerial photographs, remote sensing, land surveying, CRP and, more recently Terrestrial Laser Scanning (TLS) and Mobile Laser Scanning (MLS). These latter technologies are in principle advantageous because of their good accuracy, easiness of use and lower time of response [7].

Most of the existing geomatics techniques are sometimes una ffordable, and there is not an all-in-one solution able to provide spatial information with suitable accuracy and temporal frequency. For the completion of existing surveys in particular, MLS was considered the most suitable alternative for the challenges established in the project requirements, concerning productivity, sample density and final costs. In particular, Mobile Mapping System (MMS) technology enables users to reach complex and enclosed spaces, either scanning by hand or by attaching a scanner to a trolley, drone, or mounting on a pole. As a result, the variety of di fficult-to-survey environments becomes wider. This solves the problem linked to GNNS-based systems where it does not work well in complex contexts, for example, woods, where tree canopies block signals, as occurred in this case study. With no reliance on remote data, MMS are a priori a truly go-anywhere technology.

In this paper, a comparison of a long-range handheld MMS with the CRP has been evaluated that offers particular promise for site-scale topographic surveys. Experiments were conducted exploiting the case study of Cortes de Pallas [2], where three-year period monitoring surveys have been undertaken with sub-millimetric electronic distance meter (EDM) techniques and CRP. The result of the survey consists of a 3D model where the combination of these techniques ensures a complete mapping of the site, avoiding the creation of gaps in the point clouds thanks to the compensation of one technique on the other and vice versa. This 3D model provides a basis for the analysis of the rocky landslide deformation monitoring. The main contribution of this manuscript is to demonstrate how MMS can complement the 3D mapping of a challenging environment, by integrating multi-source data in a unique reference frame and with an accuracy comparable with other state of art methods. Several tests are presented, providing the research community with guidelines that will be useful for other similar settings, presenting the MMS as an alternative approach when other geomatic methods fail.

This article is structured as follows: after a first state-of-the-art review in using MLS for mapping and monitoring purposes, Section 3 describes the geomatics techniques used for the data acquisition in the selected case study. Section 4 focuses on the data processing and the integration of georeferenced point clouds from MMS with the CRP. In Section 5 the results of this comparison are evaluated to assess the accuracy achieved in the combination of these data. The discussion and conclusion are finally presented in Sections 6 and 7.
