In recent decades, GIS tools have been directly applied to photo-interpretative geomorphological and geological mapping. Although these tools are powerful and effective tools in the creation of digital maps, it is often very difficult to obtain a correct recognition of the nature and boundaries of geomorphological landforms using two-dimensional images. In addition, the output typically requires some improvements, usually by means of field verifications or using oblique field photographs. The aim of this paper is to present the ArcGDS™ tool, which allows the direct exploitation, visualization and digitization of stereoscopic digital linear scanned images (e.g. digital image strips, ©swisstopo). Through two case studies, we show how 3-D digital mapping makes it possible to produce Quaternary geological and geomorphological maps with a limited complementary fieldwork approach and to provide a quantitative assessment of surface deformations through the acquisition of precise elevation coordinates. Combined with high-resolution digital elevation models, ArcGDS™ is a powerful tool, particularly over large areas, as well as under forest cover and on very steep slopes.
The general application of geographical information systems (GIS) for digitizing, managing, mapping and updating geographical and geological data has become widely accepted over recent decades (e.g. Bonham-Carter, 1994; Carrara and Guzzetti, 1995; Oleschko Lutkova et al., 2008). With the increasing quality and availability of digital topographic maps, undistorted aerial photographs (orthophotos) and high-resolution (0.5 up to 5.0 m) digital elevation models (DEMs), GIS tools have also been applied to photo-interpretative Quaternary geological and geomorphological mapping (e.g. Gustavsson et al., 2006; Theler et al., 2008; Whitmeyer et al., 2010).
Despite the power of GIS tools for digital mapping, it is often difficult to correctly recognize the nature and the boundaries of geomorphological landforms, unconsolidated sedimentary deposits or landslides on two-dimensional (2-D) images. This applies particularly to steep zones (i.e. rock walls and talus slopes), areas under forest canopies or with very complex topography (as, for example, in rockslide deposit landscapes or in glacier forefields) and in heavily urbanized zones. In many cases, maps represented only by 2-D data must be improved by means of field verifications or, at least, using oblique field photographs (Mihai et al., 2008). On the other hand, our natural perception of the real world is three-dimensional (3-D) and is therefore partially impaired by the application of 2-D visualization techniques (Fig. 1) (Schneider and Otto, 2007; Bozzini et al., 2011).
Comparison between the Sosto (2220.6 m a.s.l.) on an aerial orthophotograph (©swisstopo), the Swiss National Map 1 : 25 000 (©swisstopo) and a classical terrestrial oblique photograph (©C. Scapozza). The mountain symbol of the Blenio Valley (Canton Ticino), with its typical pyramidal form, is directly perceivable only from the terrestrial perspective.
In recent years, high-resolution DEMs have become available in many regions
of the world, making it possible to produce detailed hillshades and
therefore providing a pseudo-3-D visualization of the relief. A new
high-resolution DEM (called swissALTI3-D, ©swisstopo) has been
available in Switzerland since the beginning of 2013, at all elevations,
with an accuracy of
In order to complement the analysis and interpretation of DEMs, in
combination with orthophotographs, which in Switzerland is currently a
standard procedure in the production of geological maps (Wiederkehr and
Möri, 2013), we present here a new technique which allows the direct
exploitation, visualization and digitization of numerical oblique aerial
photographs or digital linear scanned image strips, which in Switzerland
have a surface resolution of between
The distinctive features, potentialities and limits of this technique will
be presented through two Cases Studies of photo-interpretative digital
mapping based on 3-D visualization using ArcGDS™ carried out in
Switzerland. The first example concerns the Quaternary geological
cartography in the framework of GeoCover (©swisstopo), which was
executed in Canton Graubünden in the region of Reichenau (46
Three-dimensional digital mapping is based on the visualization, through a double polarized
screen and polarized glasses, of a pair of digital aerial photographs, which
are perfectly superimposed, thereby making it possible to reproduce a stereo
representation of the topography depicted in both photographs (Fig. 2). In
the case studies presented here, the input data consist of digital strips
captured by a linear scanner (12 000 pixels), which produce images with a
resolution of between 25 and 50 cm pixel
Example of 3-D cartography on the ArcGDS™ extension. On the
left, 3-D cartography window with a stereoscopic visualization of a digital
image strip of Sheet 1195/Reichenau (©swisstopo, 2008); on the
right, the final result mapped in 2-D on a 270
Realization of the digital image of 6 km wide strips collected by
digital airborne pushbroom imagery by the ADS80 airborne
digital sensor of the Swiss Federal Office of Topography swisstopo. The acquisition of the
three different available scenes (backward, nadir and forward scene) in
accordance with the camera angle is also shown. Inspired by a diagram which
can be found on
Digital aerial photographs conversion and orientation procedures, making it possible to edit 3-D features in the ArcGDS™ software.
Quaternary geological maps drawn by 3-D digital photo-interpretation
for GeoCover (©swisstopo).
A Google Earth™ image of the region between Domat/Ems and Flims (Canton Graubünden, Switzerland). In red, the Tamins rockslide; in yellow, the Flims rockslide; and in blue, the zone where the “Bonaduz gravel” was sedimented.
In order to exploit the digital aerial photographs for 3-D editing in the
ArcGDS™ software, the images must be converted and oriented so as to
avoid compatibility problems with ArcGDS™ and with the main GIS
software (Fig. 4). This conversion, executed using the ERDAS ER Mapper
software, allows the images to be translated from the geoTIFF format to the
.ECW format (Enhanced Compression Wavelet). The ArcGDS™ software
itself allows the images to be oriented and converts the data from the .ECW
format to the project file with a .GDP extension, allowing it to be edited
in an ESRI® ArcGIS™ environment. This kind of editing
means that it is possible to collect perimeters, surface areas and volumes
of landforms; create and reshape objects; navigate around the stereoscopic
pair of images beginning with a selected point in an ArcMap window; snap to
the ground surface using the image correlation when moving in
In the framework of GeoCover (©swisstopo), in 2012 the Institute of Earth Sciences of the University of Applied Sciences and Arts of Southern Switzerland (SUPSI – Scuola Universitaria Professionale della Svizzera Italiana) developed a 1 : 25 000 digital map of the Quaternary landforms and deposits of Sheet 1195/Reichenau and of parts of sheets 1174/Elm and 1175/Vättis (Fig. 5), located in the northern part of the Canton Graubünden between Flims and Chur (Vorderrhein Valley). This mapping of the Quaternary landforms and deposits was based on the directives of the Swiss Geological Survey (OFEG 2003).
The mapped area is characterized by a complex event stratigraphy, which is
well known in the literature (Pollet and Schneider, 2004; Poschinger,
2006a) and which presents a large number of landslides, such as the famous
Flims rockslide, deep-seated gravitational slope deformations (DSGSDs) and
a range of hillslope and alluvial deposits. This bibliographic knowledge is
very important and represents a basis on which to make a correct
interpretation of the different deposits and landforms visible through the
hillshade of the digital elevation model. In the studied area, different
rockslide deposits characterize the valley floor. The first was the Tamins
rockslide, which blocked the Vorderrhein River and caused the formation of
the Bonaduz palaeo-lake. This first event has not been dated. The Flims rockslide
came next and, according to surface exposure dating of boulder and
bedrock surfaces related directly to the rockslide, is dated at around
A methodological aspect related to the realization of these digital maps was
also to improve the quality of the digitization work by reducing the
presence of typical topological errors, which are often generated by
applying editing tools to polygons (e.g. cutting, reshaping and clipping
tools), where a geological limit is represented by the perimeter of two
adjacent polygons. In order to reduce the number of topological errors, and
to maintain coherence with the principles of geological mapping, the
digitization was performed exclusively by means of polylines (which
represent geological construction lines, such as a geological boundary or
linear geomorphological landforms) and points. Finally, polygons were
generated by a dedicated ESRI® ArcGIS™ tool called
The analysis and interpretation of 3-D digital image strips with ArcGDS™, in combination with orthophotographs and DEMs, makes it possible to perform a detailed digital mapping of Quaternary landforms and deposits, without field verifications. This technique also makes it possible to produce a landform classification with the same accuracy as the legend of the Geological Atlas of Switzerland 1 : 25 000 (OFEG, 2003), which is comprised mainly of field geological mapping. This approach makes it possible to define both kinds of landform and deposit, for large landforms of hillslope instabilities, inside forest canopies and on steep slopes. According to the shape, appearance, morphological characteristics and other identifying elements, it was possible to distinguish six slope and slope foot deposits based on their morphogenetic processes; five glacial and glaciofluvial deposits based on their morphology and age; and five periglacial deposits based on their morphogenetic processes, morphology and age (Table 1).
The classification for slope and slope foot deposits (Fig. 5a), and in particular for cones, was based on the slope angles (e.g. Bertran, 2004). This made it possible to distinguish fluvial alluvial fans (constituted by fluvial deposits) from torrential alluvial fans (mainly constituted by debris flow deposits) and from slope deposits (rockfall to rockslide deposits). The shape (concave or convex), the ground surface granulometry and the presence of morphological indicators such as debris flow gullies, remnants of avalanche cones or permafrost creep lobes (e.g. Francou, 1988; Scapozza, 2013) make it possible to differentiate slope deposits between talus and scree slope deposits, and between avalanche, mixed cones and rockfall deposits s.l. (deposits due to mass-wasting processes, including coarse-scree slopes).
In terms of glacial deposits (Fig. 5b), it was possible to distinguish their chronostratigraphical situation on the basis of the moraine ridge morphology, the regional situation and an estimation of the depression of the equilibrium line altitude of the glaciers (ELA), corresponding to the zone separating the accumulation zone of a glacier from the ablation zone (see also case study 2) (e.g. Ivy-Ochs et al., 2006). Postglacial deposits are therefore often represented by moraines which are related to the Little Ice Age (AD 1350–1850) or, at least, to the late Holocene (in particular during the Subatlantic, 2.6–0 ka cal BP). They are situated close to the current glaciers, present well-defined moraine ridges consisting of fresh material and are normally un-vegetated. Glacial deposits related to the last glaciation (and deglaciation) include moraines from the Last Glacial Maximum (LGM, ca. 28.0–23.0 ka cal BP), which are not present in the study area, to the end of the Oldest Dryas (ca. 19.0–14.5 ka cal BP). The last glacial stadial before the Bølling–Allerød interstadial (ca. 14.5–12.9 ka cal BP) is known as the Daun Stadial, and it is normally characterized by landform features with well-defined but smoothed moraine ridges with relatively few large boulders, and often overprinted by solifluction processes, which were active during the Younger Dryas (ca. 12.9–11.6 ka cal BP) (Ivy-Ochs et al., 2006). Moraine ridges located just outside the Postglacial deposits and characterized by multi-walled (normally three) sharp crests, often blocky, were considered to belong to the Egesen Stadial, which is normally characterized by three phases of re-advancement of valley and cirque glaciers, dating from the Younger Dryas and from the early Holocene (Ivy-Ochs et al., 2007).
Periglacial deposits, in particular rock glaciers (Fig. 5c), were finally
differentiated into intact (i.e. active and inactive rock glaciers; sensu Basch,
1996) and relict landforms on the basis of their shape, surface topography,
elevation and slope orientation, in accordance with the morphological
characteristics listed by Scapozza and Fontana (2009). The
morphostratigraphical relationships with glacial and slope deposits also
makes it possible to differentiate the origin of the deposits constituting
the rock glacier, in line with the classification into
Extract from the Quaternary geological map of Sheet 1195/Reichenau
(GeoCover, ©swisstopo).
The right flank of Val Bedretto is characterized by a series of morphostructures (mainly counterscarps) with displacements of up to 40 m parallel to the steep-subvertical foliation of the Mesozoic lithologies (Alpine cover units) of the Bedretto zone. Deep-seated gravitational slope deformations (Sackung) and rockslides affect both valley flanks and may play an important role in the development of these morphostructures, which are probably the visible expression of slope tectonics processes (e.g. Jaboyedoff et al., 2011). Three-dimensional digital stereoscopic photogrammetry, together with quantitative geomorphological analysis, will make it possible to assess the genesis of these morphostructures.
List of slope, slope foot, glacial, glaciofluvial and periglacial deposits differentiated exclusively by 3-D digital photo-interpretation in the framework of Quaternary geological mapping of Sheet 1195/Reichenau and part of the sheets 1174/Elm and 1175/Vättis of GeoCover (©swisstopo).
Location of Bedretto Valley and detailed geomorphological map of moraine ridges (see Table 1 for their classification), main rockslides, deep-seated gravitational slope deformations (DSGSDs) and morphostructures, and displacements calculated using 3-D digital photogrammetry. Base map: contour lines calculated on the basis of the swissALTI3-D 2 m pixel resolution DEM (©swisstopo, 2012).
In terms of the genesis of these structures, several studies have focused on
postglacial uplift and how this interplays with slope deformation (e.g.
Ustaszewski and Pfiffner, 2008; Ustaszewski et al., 2008); other authors
have focused on a genesis primarily related to active tectonic extension
(e.g. Allanic and Gumiaux, 2013). The displacement of several moraine ridges
in the area was first observed by Renner (1982), who determined a mean
displacement rate based on attributing the moraine ridges to the main
stadials of glacier retreat during the Lateglacial. More specifically,
Renner (1982) calculated a mean displacement rate of
Three-dimensional digital photogrammetry (which makes it possible to obtain new instability
and geomorphological maps, and to quantify the absolute displacement of the
structures), and the age revision and calibration of the glacial (moraines)
and periglacial (rock glaciers) landforms of the area, are used to propose a
re-evaluation of the role of genetical processes since the end of the Oldest
Dryas. In particular, this assessment is based on calculating the
displacement rate since the middle of the Lateglacial by merging spatial
(absolute displacement of selected morphostructures) and temporal data (age
of the deposits merged with the morphostructures). Temporal data were
obtained by classifying all the mapped moraine ridges in terms of glacial
stadial affiliation. Definition of the glacial stadials in the Bedretto
Valley was based on calculating the depression of the equilibrium line
altitude of the glaciers (DELA) for all the main local stadials, which were
subsequently grouped into regional stadials (Table 2). The ELA was
calculated using the accumulation area ratio (AAR) hypsometric method, based
on a 0.67 ratio for the accumulation surface/total surface of a glacier
(Kerschner, 1976; Gross et al., 1977), corresponding to a ratio of
Three-dimensional visualization of the right side of the Bedretto Valley, focusing on the moraines and morphostructures of Val d'Olgia and Val Cavagnolo (location in Fig. 8). Basemap: hillshade calculated on the swissALTI3-D 2 m pixel resolution DEM (©swisstopo, 2012).
Slope structures displacement rates in Bedretto Valley, calculated
using 3-D digital photogrammetry.
Definition of the reference values of the DELA for the Ticino glacier in Bedretto Valley during the Lateglacial. The reference values (with the reference positions C, M, etc. defined by Renner, 1982) are based on the glacial positions occurring on the valley bottom (section “Val Bedretto”). The DELA values for the lateral cirques of the entire Bedretto Valley are also shown.
Correlation of the reference regional stadials for the Ticino glacier in Bedretto Valley with the Eastern Alps model, and proposal for the calibrated minimum age based on the radiocarbon dating compilation and calibration performed by Scapozza et al. (2014).
On the basis of the geomorphological mapping performed using a 3-D digital
photo-interpretation (Figs. 8 and 9), it is possible to observe that
morphostructures are (1) laterally continuous at regional scale parallel to
the geological setting, (2) not conditioned by topography and (3) involved
in deep slope instabilities such as rockslides and DSGSDs. Digital
photogrammetry was used to measure the absolute displacement of 41
morphostructures, with values lying between 0.9 and 32.6 m, and with an
accuracy of
Lateglacial data showing a displacement significantly higher than the
Holocene mean value present an exponential increase with age (with a
determination coefficient of
Comparison between the potentialities of hillshaded DEMs and orthophotographs (2-D mapping) and 3-D digital aerial image strips for the Quaternary geological and geomorphological mapping. The 3-D digital image strips are a unique method making it possible to observe the real shape of the landforms and the extent to which this shape is effectively evident.
In conclusion, therefore, the displacement rate values calculated using 3-D digital photogrammetry make it possible to formulate some preliminary propositions regarding the origin of the morphostructures observed in the Bedretto Valley and the evolution of their movements. Most of the observed morphostructures originated from postglacial uplift during the late Pleistocene and from the (neo)tectonic movement during the Holocene along pre-existing geologically steep planes. However, higher displacement rates suggest a significant slope deformation factor with regard to the formation of the observed morphostructures.
As presented and discussed in the above two case studies, 3-D digital mapping using ArcGDS™ represents a powerful method, not only for producing Quaternary geological or geomorphological maps but also for making a quantitative assessment of surface deformations through the acquisition of precise elevation coordinates on stereoscopic digital image strips.
In terms of digital geological mapping, in addition to the standard procedure combining the analysis and interpretation of orthophotographs and DEMs (Wiederkehr and Möri, 2013), 3-D digital mapping makes it possible to observe both the real shape of the landforms and the extent to which this shape was effectively evident (Table 4). This is particularly appropriate below the timberline (subalpine regions), where forest is present. Landforms lying under forest cover are often masked by vegetation on orthophotographs. On DEMs, elevation data do not show the same accuracy as those recorded in un-vegetated areas because of the difficulty encountered in correctly defining the ground surface elevation. Three-dimensional stereoscopic vision is also suitable for steep slopes, where orthophotographs present shadows depending on the slope orientation and where it is not possible to avoid uneven and abrupt transitions in the event of major elevation differences on DEMs, particularly those above 2000 m a.s.l. for the swissALTI3-D (Wiederkehr and Möri, 2013). Since data for the third dimension (elevation) is also collected (with ArcGDS™), it is possible to understand the geometric relationships between different landforms, and also potentially reconstruct a historical sequence of the main events. A relative-dated event stratigraphy can therefore be defined, based on the morphostratigraphy principles (continuity, superposition and cross cutting of the landforms). This approach was adopted for dating the glacial, glaciofluvial and periglacial deposits mapped in the Quaternary geological cartography framework performed in the GeoCover (©swisstopo) (case study 1). Taking in account these three characteristics (suitability under forest cover, on steep slopes and in the definition of geometric relationships between landforms), 3-D digital mapping on image strips is particularly appropriate, for example, for the compilation of landslide inventory maps on a regional scale. Documents of this type are particularly significant when making landslide hazard assessments with regard to making prudent decisions in terms of sustainable territorial planning. This was the case for the Bedretto Valley example presented here (case study 2), where landslides perimeters were mapped and their activity assessed. The 3-D photo-interpretation methodology can provide information on the landslide type, three-dimensional extension (i.e. volume), relative activity, geometry, involved rock or soil material mass. Landslides can be easily recognized by means of this approach, and geomorphological features associated with mass movements – such as scarps, counterscarps, trenches, debris flows, rockfalls and debris fans – can also be mapped. Based on these morphological characteristics, it will be also possible to define a qualitative state of activity (not based directly on their velocity), leading to the definition of landslide intensity (low, medium and high), in accordance with the landslide hazard degree calculation guidelines developed in Switzerland (Lateltin et al., 2005).
Concerning the quantitative assessment of slope deformations performed to
evaluate the slope tectonic activity in the Bedretto Valley (case study 2),
the level of precision achieved by 3-D digital photogrammetry on the
elevation definition on image strips (accuracy of
Photo-interpretation of digital strips is based on the visual analysis of RGB images. This made it possible to resolve interpretation ambiguities of hillshaded DEMs, which depend on illumination altitude and azimuth, and on the kind of land cover (soil, unconsolidated deposits or bedrock). These characteristics are, indeed, fundamental for the correct interpretation of accumulation landforms such as moraines, talus slopes or rockfall deposits. Despite the many positive aspects related to the application of the ArcGDS™ tool for 3-D digital mapping, it is also important to mention some limitations associated with this method. Firstly, 3-D digital mapping is time-consuming because it is necessary to focus the two images (stereo-correlation) in order to obtain a correct definition of the elevation for every collected data point. Combined with the use of polarized glasses, this makes 3-D mapping very tiring on the eyes, and therefore limits the number of daily working hours of one person. This problem can be overcome by combining the use of the 3-D stereoscopic digital images with hillshaded DEMs, leading to the verification or acquisition of three-dimensional features only when strictly necessary. Examples of such cases would be when interpreting deposit types, when mapping features below the forest cover, when delimiting perimeters and calculating the volume of hillslope instabilities or when defining the high-precision elevation data needed to calculate the displacement or movement of material.
In this communication, we highlight the use and potential of 3-D digital mapping for drawing detailed Quaternary geological and geomorphological maps with limited complementary fieldwork. Combined with the high-resolution swissALTI3-D DEM, ArcGDS™ represents a powerful tool for digital mapping, particularly when the mapped regions are very extensive or difficult to access for logistical reasons (absence of roads, footpaths, bridges, etc.), or due to particular topographic characteristics, such as the presence of steep slopes, gorges, dense forest cover, etc. Photo-interpretative maps, compiled in accordance with the methodology proposed here, constitute the base document making it possible to define the sediment storage units and the main depositional process involved in their creation. This is fundamental in order to explain, by application of morphostratigrapic principles, the relationships between the different sediment storage units (concept of the sediment cascades) and the relative ages of the mapped landforms.
We would like to thank Claudio Castelletti and Linda Soma of the Institute of Earth Sciences SUPSI, and Pauline Baland and Andreas Möri of the Swiss Geological Survey, Federal Office of Topography swisstopo. Special thanks go to the handling editor, Philip (Phil) Greenwood, as well as Jan Hardie for proofreading the English. Edited by: P. Greenwood Reviewed by: two anonymous referees