**2. Close-range remote sensing techniques**

The study of the Planpincieux and Grandes Jorasses glacier surface deformations has been conducted following different approaches: (i) volumetric changes have been evaluated with point clouds and digital elevation models (DEMs). Such measurements have been obtained with laser scanners or structure from motion (SFM) processing. (ii) Surface kinematics maps of specific displacement components, which have been provided by monoscopic time-lapse camera and terrestrial interferometric radars. (iii) 3D displacements measured in specific points with a robotised total station (RTS). Furthermore, helicopter-borne ground-penetrating radar (GPR) campaigns have been conducted to investigate the glacier internal structure and thickness (**Table 1**).

#### **2.1 Point clouds for surface generation**

Three-dimensional point clouds are crucial tools in glacier monitoring; the main survey techniques to obtain them are LiDAR [13], terrestrial laser scanner (TLS) [14] and aerial and terrestrial photogrammetry, particularly structure from motion (SfM) approach [15]. LiDAR and TLS are based on a sensor, terrestrial or airborne, capable of emitting laser pulses at high frequency and measure their 'time of flight' in order to compute the position of each echo. The absolute position of each point is calculated from the emitter centre, geocoded by a GNSS coupled with an inertial measurement unit [16]. Besides its coordinates, each point can be characterised by the intensity of the echo in order to detect the nature of the target [17]. By the exploitation of laser beam divergence, it is also possible to discriminate and analyse multiple echoes or even the full waveform, thus obtaining multiple measurements of different object hit by the same pulse [18].

Concerning SfM, it is a technique originating from computer vision, which, by processing multiple images from different points of view of the same target object, generates a three-dimensional point cloud. The algorithm matches common


**Table 1.**

*List of the surveys conducted in the Planpincieux and Grandes Jorasses glaciers since 2010.*

features in the images and reconstructs the three-dimensional coordinates of the matching points and of the cameras. Resulting points are then collected in the cloud [19]. Images can be captured by various kinds of sensors including cameras, smartphones and drones [20].

### **2.2 Punctual topographic displacement measurements**

Robotised total station (RTS) is a topographic apparatus that measures the sensor-to-target range and the azimuth and zenith angles, which allow determining the target position in a 3D coordinate system whose centre corresponds to the

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*Close-Range Sensing of Alpine Glaciers DOI: http://dx.doi.org/10.5772/intechopen.92841*

volcanos [23] and glaciers [11, 12, 24].

**2.3 Glacier surface kinematics maps**

*2.3.1 Digital image correlation*

corrections.

RTS itself. Typical measurement sensibility of best-quality RTS is of 1.5 mm and 0.5 arcsec, depending on the distance. The RTS is composed of a laser rangefinder and an electronic theodolite that measures respectively distance and angles. The RTS targets retroreflector prisms installed both in and outside the moving area. The latter ones serve as control points for measurement calibration and data

Since it is required to install prisms within the investigated area, the RTS cannot be considered a remote sensing device in a strict sense. Nevertheless, such installation is needed just once; thereafter, the RTS provides measurements from remote, strongly reducing human and financial costs for accessing the surveyed area. This holds especially when the RTS works in automatic target recognition (ATR) mode, with which it carries out autonomously the measurements. In geosciences, the RTS is widely used for gravitational slope phenomena, such as landslides [21, 22],

Spatially distributed data are a relevant tool in glaciological studies because they allow to analyse the surface kinematic patterns and to identify possible different kinematic sectors. In the Planpincieux-Grandes Jorasses glacial complex, two main remote sensing systems have been applied to measure surface kinematics maps: digital image correlation (DIC) and terrestrial radar interferometry (TRI).

With the advent of digital cameras, time-lapse imagery has become popular since the beginning of the 2000s in glaciology, where it has been applied to survey

Fourier CC is computationally efficient but it is more prone to outliers.

DIC is an image analysis technique that is applied to a pair of images to obtain spatially distributed maps of the two displacement components orthogonal to the line-of-sight (LOS). In classical DIC processing, a reference template out of the master image is searched for in an investigated larger area of the slave image. The cross-correlation (CC) is calculated for every possible template of the investigated area and the position of the maximum correlation coefficient corresponds to the displacement of the master template. Alternatively, the CC can be calculated in the Fourier domain according to the convolution theorem.

The main DIC advantages concern the low-cost hardware and its high portability even in harsh environments. Nevertheless, it suffers adverse meteorology and it

In the last two decades, TRI revealed to be a valuable tool to monitor glaciers [9, 34–42]. TRI concerns the analysis of the phase difference between two radar acquisitions, which is directly related to the target displacement component parallel to the LOS. Typical radar apparatuses can provide spatially distributed displacement data in an area of several square kilometres with an operative range of a few kilometres. Radars are active sensors, as such, TRI can be applied during the night and severe meteorological conditions. Moreover, TRI measurements have sub-millimetre sensibility in an optimal context. However, the processing is not trivial and it requires high computational costs. Particularly complicated is the phase wrapping solution, which depends on the phase 2 periodicity and which is

polar ice flow [25–28] and mountain glaciers [6, 8, 29–33].

strongly depends on the visibility conditions.

*2.3.2 Terrestrial radar interferometry*

#### *Close-Range Sensing of Alpine Glaciers DOI: http://dx.doi.org/10.5772/intechopen.92841*

*Glaciers and the Polar Environment*

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smartphones and drones [20].

Jorasses

**Table 1.**

**2.2 Punctual topographic displacement measurements**

features in the images and reconstructs the three-dimensional coordinates of the matching points and of the cameras. Resulting points are then collected in the cloud [19]. Images can be captured by various kinds of sensors including cameras,

borne SFM

*List of the surveys conducted in the Planpincieux and Grandes Jorasses glaciers since 2010.*

**Glacier Survey Dates References**

LiDAR 9/6/2014 TLS 2/10/2015 Helicopter- 2017 borne SFM 28/10/2018

Drone SFM 24/7/2019 Grandes RTS 2010-in course [11, 12]

Helicopter- July 2010

Drone SFM July 2019

DIC 2016-in course GPR 4/6/2010

2019 DIC August 2013-in [8] course TRI 9/8/2013- [9, 42] 10/8/2013 7/8/2014- 8/8/2014 1/9/2015- 14/10/2015 13/6/2016- 19/6/2019 26/9/2019-in course

> 20/9/2019 1/10/2019 5/11/2019

2/4/2013

Planpincieux GPR 2/4/2013,

Robotised total station (RTS) is a topographic apparatus that measures the sensor-to-target range and the azimuth and zenith angles, which allow determining the target position in a 3D coordinate system whose centre corresponds to the RTS itself. Typical measurement sensibility of best-quality RTS is of 1.5 mm and 0.5 arcsec, depending on the distance. The RTS is composed of a laser rangefinder and an electronic theodolite that measures respectively distance and angles. The RTS targets retroreflector prisms installed both in and outside the moving area. The latter ones serve as control points for measurement calibration and data corrections.

Since it is required to install prisms within the investigated area, the RTS cannot be considered a remote sensing device in a strict sense. Nevertheless, such installation is needed just once; thereafter, the RTS provides measurements from remote, strongly reducing human and financial costs for accessing the surveyed area. This holds especially when the RTS works in automatic target recognition (ATR) mode, with which it carries out autonomously the measurements. In geosciences, the RTS is widely used for gravitational slope phenomena, such as landslides [21, 22], volcanos [23] and glaciers [11, 12, 24].
