Vol49,4_5,2006 1067 ANNALS OF GEOPHYSICS, VOL. 49, N. 4/5, August/October 2006 Key words photogrammetry – DTMs generation – archival photographs – surface representation – land- slide analysis 1. Introduction The historical aerial photos are of fundamen- tal importance not only for qualitative analysis of the territory (traditionally conducted by photoint- erpretation of stereoscopic models), but for a quantitative assessment as well. Appropriate com- parison of photogrammetric surveys of a zone re- alised in different years allows identification of geometric changes occurring during the time in- terval in question (differential photogrammetry, Anzidei et al., 2000; Baldi et al., 2002, 2005). This technique has many fields of application: for example, the study of changes in urban areas, farming activities and evolution of coastlines (Dolan et al., 1980; Overton and Fisher, 1996), checking glaciers (Kääb and Funk, 1999) and monitoring unstable slopes (Kääb, 2000). Stereoscopic interpretation of two or more sets of aerial photographs taken over a sufficient- Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis Antonio Zanutta (1), Paolo Baldi (2), Gabriele Bitelli (1), Mauro Cardinali (3) and Alberto Carrara (4) (1) Dipartimento di Ingegneria delle Strutture, dei Trasporti, delle Acque, del Rilevamento, del Territorio (DISTART), Università degli Studi di Bologna, Italy (2) Dipartimento di Fisica, Università di Studi di Bologna, Italy (3) Istituto di Elettronica e di Ingegneria dell’Informazione e delle Telecomunicazioni (IEIIT), CNR, Bologna, Italy (4) Istituto di Ricerca per la Protezione Idrogeologica (IRPI), CNR, Perugia, Italy Abstract The results of two survey methods, geological photointerpretation and historical photogrammetry, are compared in order to evaluate the temporal evolution of a unstable slope located in the Tuscan-Emilian Apennines (Italy). Historical aerial photos of the area, derived from photogrammetric surveys conducted in 1954 (scale 1:60 000), in 1971 (scale 1:20 000), and in 1976 (scale 1:17 000) were available. A photogrammetric flight was further con- ducted in 2000, at a scale of 1:4400, with a traditional GPS ground survey support. First, the results of photo- graphic analysis with the photointerpretation method are presented: the landslides are described from a geolog- ical point of view, showing its temporal evolution. To quantitatively assess the landslide movements, Digital Ter- rain Models were generated by means of an analytical plotter and a digital photogrammetric workstation, with semi-automatic and automatic procedures. To generate these products, it was necessary to solve problems relat- ed to a lack of data concerning the aerial cameras used for the historical flights (internal orientation) and the dif- ficulty identifying control points on the photos in order to define the external orientation. An unconventional photogrammetric methodology, based on identification of homologous points in zones considered outside the landslide area, has been there developed and tested to insert the various surveys into a single reference system. Mailing address: Dr. Antonio Zanutta, Dipartimento di Ingegneria delle Strutture, dei Trasporti, delle Acque, del Rilevamento, del Territorio (DISTART), Università de- gli Studi di Bologna, Viale Risorgimento 2, 40136 Bologna, Italy; e-mail: antonio.zanutta@mail.ing.unibo.it. 1068 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara ly long time span (20-30 years) is the most effec- tive and popular technique for the identification and morphological characterisation of landslides (multi-temporal landslide mapping) (van Westen and Getahun, 2003). This procedure assumes particular theoretical and applicative importance due to the fact that most landslide motion tends to take place within or very close to pre-existing landslide deposits. Therefore, knowledge of the location, type and distribution of landslides oc- curring over time in a territory, is an essential tool for forecasting future events. It must nevertheless be pointed out that the reliability of stereoscopic interpretation strong- ly depends on the experience of the photo-inter- preter. Thus, when studying landslide phenom- ena, it becomes important to utilise more quan- titative techniques, such as analytic or digital photogrammetry, in association with photo-in- terpretation, which is intrinsically subjective and qualitative (Brunsden and Chandler, 1996; Lane et al., 1998). Applicability of the two methods obviously depends on the availability of aerial photos of the territory, covering a sufficiently long time span. In this regard, it should be remembered that the first Italian national scale photogram- metric coverage, with classic pseudo-vertical photos (23×23 cm), was conducted in 1954- 1955 by the Istituto Geografico Militare (IGM). These photos, even though at a scale (1:33 000- 1:60 000), may be a fundamental source of knowledge for the study of the temporal evolu- tion of slope instability (there are also other old IGM surveys, at different scales, taken of indi- vidual portions of the territory). Starting from 1970, in order to produce large scale maps (technical regional maps) and also in the context of specific projects (for example, monitoring of river valleys, glacier faces, slope landslides, etc.), many public administrations managed the realization of local photogrammet- ric surveys. To assess the potential offered by the integra- tion of qualitative and quantitative photogram- metric techniques in the study of landslides, we identified as test area an unstable slope located in the territory of the Municipality of Vergato, near the city of Bologna (Tuscan-Emilian Apennines, Italy) where in recent years a warning system was implemented installing inclinometers and piezometers, measuring pore water pressure and ground displacements by means of wire exten- someters, GPS surveys, laser scanning and digi- tal photogrammetric techniques (Mora et al., 2003; Bitelli et al., 2004; Pesci et al., 2004; Si- moni et al., 2004; Dubbini and Zanutta, 2005). This work presents the results of photo-in- terpretive analysis of four sets of aerial photos. Such analysis characterises the slope geo-mor- phologically, pointing out its temporal evolu- tion based on morphological changes identified in the available stereoscopic models. A quantitative analysis of the landslide movements was performed comparing multi- temporal DTMs produced with an analytical plotter (DIGICART 40, Officine Galileo), and a digital photogrammetric workstation (Helava System). 2. Photo interpretation investigations of the landslide phenomena The territory of the Municipality of Vergato contains numerous slopes affected by land- slides, many of which are currently active. Due to its geological-geomorphologic structure and types of land use and settlement, this area may be considered highly representative of the mountainous territory of the Emilia-Romagna Region, for which recent studies claim that of more than 32 000 landslides identified and mapped, approximately 26% are to be consid- ered «active» (Bertolini and Pellegrini, 2001). Table I. Main characteristics of aerial photos utilised for production of multi-temporal map of landslides in instudy area. Agency Photo scale Type Year Month GAI, IGMI 1:60 000 B&W 1954 June Emilia-Romagna 1:20 000 B&W 1971 - Region Emilia-Romagna 1:17 000 Colour 1976 May Region Alifoto Srl 1:4400 B&W 2000 April for ENEL Hydro Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis A multi-temporal landslide map was pro- duced for this area by means of a detailed inter- pretive study of photos taken during four pho- togrammetric flights, from 1954 to 2000, with photographic scale ranging from 1:60 000 to ap- proximately 1:4400 (table I; fig. 1a-d). The multi-temporal landslide map was pro- duced by means of careful photo-interpretive examination of all the available aerial photos, 1069 Fig. 1a-d. Multitemporal aerial photos details concerning landslide bodies (Municipality of Vergato, Bologna, Tuscan-Emilian Apennines, Italy; φ = 44°17l25mN, ω = 11°07l16mE): a) 1954; b) 1971; c) 1976; d) 2000. This work examines the instability phenom- ena affecting the slope on the right bank of the Reno River, below the Vergato-Piandisetta road in the vicinity of Cà di Malta and Cà del Bosco (Bologna). This slope is composed of terrains pertaining to the Scaly Clay Complex: their poor mechanical properties make it one of the most unstable geological complexes in the en- tire Apennine territory. a b c d 1070 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara using a stereoscope with continuous 2-20X zoom. Interpretation began with an analysis of the 1954 aerial photos, in which it was possible to identify both large-scale landslides (mostly old and relicts) and smaller landslides, general- ly more recent. The aerial photos from the oth- er flights were then analysed, separately and in combination with those from 1954, comparing similarities and differences in morphology in each portion of the slope under study. The land- slides were classified on the basis of estimated depth, type of movement (slip-flow and flow) and relative age (old or recent). This last param- eter was deduced from the degree of morpho- logical «freshness» that the instability present- ed in each set of photos. The photo-interpreted landslides were first redrawn on map at 1:4400 scale and then digi- tised and stored in a GIS for data display and analysis. Figure 2 shows the distribution of the land- slides identifiable on the 1954 photos. The old- est landslides may be classified as slip and slip- flow within which (as described below) numer- ous landslides identified in the flights conduct- ed from 1954 to 2000 have developed. The main landslide is a deep slip, mostly relict, with a surface area of about 160 000 m2 and average slope of 15°, extending from 320 m a.s.l, below the Vergato-Piandisetta road, until the Reno River to 185 m a.s.l. Over time, this landslide underwent partial reactivations with Fig. 2. Distribution of old or very old landslide bodies identified in 1954 aerial photos, classifiable as slip and slip-flow, with elements of activity. Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis movements of various magnitude that masked or cancelled some of the features of the main land- slide, in some cases making it difficult to identi- fy. The alimentation area, shown by a marked concavity on the slope, is characterized by a main scarp whose arched shape is recognisable at various points, and continues for more than a kilometre. Locally, the slope’s concavity is more complex, marked by later landslides that created sudden changes in dip. The accumulation zone shows general longitudinal and transverse con- vexity to the slope, which becomes more accen- tuated at the foot, where the Reno River was slightly deviated toward the west for more than 350 m. Two old slip-flows, with a characteristic narrow, elongated accumulation shape which then fans out at the foot of the landslide, have de- veloped at the sides of this relict landslide. The phenomenon that has developed on the right side of the main slip (hereafter called «Cà del Bosco») presents a complex and deep scarp area that reduces almost to a funnel around 225 m of high. On the other hand, the deposit pres- ents a sharply lobed convex shape on which several generations or pulsations of landslides may be noted, superimposed on the accumula- tion of the old main slip. The landslide that developed on the left side of the main slip (hereafter called «Cà di Mal- ta») presents less evident morphological char- acteristics. Its scarp area is not very accentuat- ed and is highlighted by a large concavity, while the deposit presents a marked lobe at the foot of the landslide that further reduces the Reno River channel by a few meters. Figure 2 also shows distribution of the land- slides classified as slip and slip-flow, recent and/or active, traceable to around 1954. In the photos, these landslides show clear evidence of morphological «freshness», demonstrating move- ments in evolution. These movements are of low or medium magnitude, with extensions ranging between 1000 and 15000 m2. They are located al- most exclusively in the scarp areas of the Cà del Bosco and Cà di Malta landslides, without ever pushing down to the base of the slope. The landslides which occurred during the pe- riod 1954-1971, are essentially two deep slip- flows extending from 40 000 m2 to 70 000 m2, classifiable as reactivations of the previous Cà del Bosco and Cà di Malta landslides, within and at the borders of which small landslides no larger than 5000 m2 may be seen. In the period 1954-1971, at least two gener- ations of movement may be seen for the Cà del Bosco landslide. The first involved the upper- middle part of the previous instability, without reaching the base of the slope, whose foot ap- pears changed at several points, the left side more convex downhill and advanced by about 20 m compared to its appearance in the 1954 photo. The second was a reactivation of the pre- vious landslide. The scarp has receded by about 100 m, reaching almost near the Vergato- Piandisetta road, while the deposit has reached the base of the slope, very probably blocking the valley road. The activity of this movement is demonstrated by an irregular surface with mounds, landslips troughs, and numerous ten- sion fractures open both longitudinally and transversely. The medium-high section of the flow (from high 250 to 220 m) is winding, with a sharply depressed and concave transverse pro- file developing within the centre of the valley. As evidence of the rapid flow of material, along the sides of this section of the flow one may clearly see levees that form crests a few meters high, extending for over 200 m in length. The foot of the flow is clearly convex, with an irreg- ular surface lobed by pronounced bumps. In the Cà di Malta landslide area, the insta- bilities occurring in the period 1954-1971 reac- tivated the entire old body with slip-flow move- ments, considerably reducing the scarp and ad- vancing its foot by about 20 m to within the Reno River bed. Specifically, the border has re- ceded by over 50 m, touching the head of the Cà del Bosco landslide. The main scarp has steps of more than 10 m and extends continu- ously, mainly along the left side of the landslide for about 200 m. The deposit has an irregular surface, with a generally convex shape that has been remodelled and regularised in the centre section as the result of drainage works with channels and surface drains running rectilinear or transverse to the slope. As shown in the 1971 photos, the foot of the landslide is limited by blocks aligned in the Reno River bed, seen for the entire width of the landslide front at about 20 m from the bank, which precisely in this sec- 1071 1072 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara tion is eroded and receded due to lateral under- mining. In addition, the road’s path has been al- tered along this section of the landslide, making it straighter than it was in 1954. The movements which occurred in the period 1971-1976, identifiable in the colour aerial pho- tos taken in 1976, are mainly due to small flows or slip-flows occurring in the centre channel of the Cà del Bosco flow or in the scarp area of the Cà di Malta landslide. On the border of this lat- ter landslide, it is interesting to note the presence of an anthropic intervention that caused about 20 m of stripping and the creation of a rise of more than 1000 m2 halfway up the hill. It is important to point out that the most extensive reactivation began precisely at the site of such work, as seen on the 2000 photos. Lastly, the landslides occurring between 1976 and 2000 were essentially a total or partial reactivations of previous Cà del Bosco and Cà di Malta landslides, with deep slip-flows measuring between 0.5 and 4.0 ha and small flows with sur- face area not exceeding 0.5 ha. The most evident reactivations might be traceable to movements occurring in October 1996 following long, heavy precipitation, or in November 1998 in the ab- sence of exceptional rains. These photos show the Cà del Bosco land- slide reactivated at its top, with the formation of a few slip-flows, the most evident of which is the one formed from the right side of the scarp at 290 m high near a few farm annexes of Cà di Sotto homes. The alimentation area of this land- slide is characterised by a sharp depression, bounded by a main scarp of about 20 m and by a series of secondary scarps and tiers. The de- posit is channelled in the medium-high section of the old 1971 flow, which is topographically depressed and concave, filling it with more than 10 m of sediment (estimated thickness). The Fig. 3. Three-dimensional perspective view of southern portion of slope under study, obtained by projecting the 2000 digital orthophoto onto the corresponding 2 × 2 m DTM (heigh exaltation: 30%). The image represents a vector/raster result which has metric characteristics, coming from photogrammetric restitution. The border of the Cà di Malta landslide is indicated with a yellow line; red lines show local roads. Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis flow stops at 225 high, where it forms a topo- graphically pronounced and prominent lobe. In the area of the Cà di Malta landslide (fig. 3), the instabilities occurring in the period 1976- 2000 reactivated most of the old 1971 body, with slip-flow and flow movements, considerably pulling back the scarp precisely at the site of the anthropic intervention observed during the 1976 flight. At this point, the border has receded by about 100 m, pushed back as far as the Vergato- Piandisetta road and creating, once again, a sharp depression with a main scarp of over 20 m. The other movements noted in the scarp area or at the borders of the Cà di Malta landslide also show a distribution of retrogressive activity, but with more limited retrograding of about 20-30 m. These movements are clearly seen in the medi- um-high part of the slope, where they all meet at around 200 m high. On the other hand, the medi- um-low sector of the slope has been heavily re- modelled by work done to reinforce the land- slide, with drainage and reprofiling of the slope. In these cases, man’s cancellation of shapes makes it very difficult, if not impossible, to de- fine and map the real extent of the movement. Nevertheless, careful examination and compari- son of the last two aerial photo flights has pro- vided elements useful for defining and extending the landslide up to the Reno River. The hypothe- sis that the landslide may have reached the valley is supported by bibliographic information (Mora et al., 2003) reporting partial obstruction of the waterway following the reactivation taking place in October 1996. 3. Photogrammetric investigation of the landslide phenomena In order to define the shape and position of objects or portions of land by means of pho- togrammetry, it is necessary to reconstruct the geometric relationships on which the images were taken. The cameras used in photogrammetry pro- duce photos, which, with sufficient approxima- tion, may be considered central perspectives of the photographed object, that is all the rays go- ing from the image to the object pass through the perspective centre. The basic mathematical model used to con- vert from a two-dimensional system (photo) to a three-dimensional system (object) is ex- pressed by well-known colinearity equations (Kraus, 1997) expressing that the perspective centre, the object points, and the image points are lying on the same projective straight lines. To transform the image coordinates in the corresponding object coordinates, at least two photos of the same object and the nine orienta- tion parameters internal (IOP), and external (EOP) are needed. As known, in traditional pho- togrammetry, IOPs are determined in laboratory by means of camera calibration procedures; EOPs can be determined by spatial resection by using known Ground Control Points (GCPs), visible on the photos, whose coordinates are measured in the image reference systems. Orientation parameters defining the camera station coordinates and the orientation angles are often unavailable for historical aerial pho- tos. Therefore, to produce the 3D model of the area, it is necessary to adopt an unconventional approach that gives results less precise than the rigorous one. The aerial photos can be consid- ered taken by non metric cameras, and a self- calibration procedure can then be performed (self-calibrating bundle block adjustment), based on the availability of a sufficient number of GCPs. Generally this method is not applica- ble because it is difficult to find enough GCPs measured at the same time of the flight. Since large-scale maps of the zone are avail- able, one solution may be to detect the GCPs from them (Chandler et al., 1988a,b; Chandler and Brunsden, 1995), even if a variety of prob- lems may be encountered: the maps may have been drawn from aerial photogrammetric sur- veys prior or subsequent to the one being analysed; it is difficult to assess the real error to be assigned to the coordinates of the points measured on the map (for example, altitudes de- rived by interpolation from level curves); it is often very difficult to identify equivalent points, i.e. the search for map GCPs on stereoscopic models. The method adopted in this paper to solve the orientation problem and to co-register the multi-temporal three-dimensional models, is based on identification of a large number of 1073 1074 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara points, located outside the landslide bodies, and visible on multi-temporal stereoscopic models. As mentioned above, three sets of historical aerial photos (from 1954, 1971 and 1976) plus the large-scale photos obtained in 2000 with a calibrated aerial camera, are available for the area under study. Simultaneously with the 2000 flight, a large number of ground points (GCPs), well distributed in the zone, were measured in the frame of a GPS campaign (Mora et al., 2003). The procedure consisted of four steps: i) Orientation of the 2000 photos. ii) Identification of natural points on the 2000 stereoscopic model and editing of precise monographs. iii) Photogrammetric measurement of 3D coordinates of such points in the 2000 reference system and identification of the same points on the old photos. iv) Computation of EOPs of various histor- ical models using GCPs derived from the 2000 survey. The GCPs were selected outside the land- slide bodies, giving the preference to artificial elements (for example, buildings), homoge- neously distributed in the area. The GCPs selection procedure was based on assessment of space resection program for the historical images, rejecting, with an iterative approach, points showing high external orienta- tion residuals respect to a theoretical value coming from the scale of the images. Obvious- ly, the ability to correctly identify points unus- able for support the orientation is greater if a large number of points is examined. At the end of the procedure, all of the «his- torical» stereoscopic models were oriented into an absolute reference system derived from the 2000 survey, making it possible to perform com- parisons between the DTMs, as later discussed. The orientation and plotting steps for the stereoscopic models were first performed with a DIGICART 40 (Officine Galileo) analytical plot- ter. Because no calibration certificates were available for the cameras used on the historical flights (1954, 1971, 1976), the internal orienta- tion and image coordinates refinement procedure was applied using the focal length pressed on the films, ignoring effects due to lens distortion, non-planarity of the film, and non alignment of the principal point with its theoretical position. Correction was made for the earth curvature. Relative orientation was performed using a very large number of points distributed uni- formly through the entire area of the models covering the landslide zone. Absolute orienta- tion was performed with the procedure de- scribed above. Once the orientation parameters were known, it was possible to acquire three-di- mensional data for the slope under study. DTMs of the zone with grids of different size (2 × 2 m, 4 × 4 m, 8 × 8 m) were created from the four models in order to reveal any mass move- ments and to simultaneously determine the best cell size for describing morphological details. In assessing the degree of uncertainty of the results, it has to be considered that an uncon- ventional method was used to solve the orienta- tion problem, and that the available images are characterized by different scale, ranging from 1:60 000 to 1:4400. This fact created many problems: 1) Errors of a 1:60 000 scale model are greater than those of a 1:4400 scale model. 2) It is difficult to identify homologous points to use as reference (especially between 1954 and 2000), due to the different resolution of the photos and to the lack of useful elements in the territory. The search procedure was con- ducted by dividing the common zone into sub- areas in which a large number of homologous points were then catalogued. Analysis of resid- uals deriving from the external orientation pro- cedure revealed, at any step, the points that could be considered useful for this purpose, be- cause assumed not subject to movements. 3) In the 1:4400 photos points cover the en- tire observed area, whereas in the lower-scale images they are concentrated in a small zone of the model. In the absence of data from other sources, simple approaches such as computation of vari- ance propagation and empirical assessments of the quality of results were utilised to estimate the precision of DTMs acquired in stereoscopy and therefore to evaluate the significance of the measured deformations. Assuming the so called Normal Case, in or- der to simplify the colinearity equation (the camera axes are considered parallel to each oth- Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis er and perpendicular to the base line) and ap- plying the law of variance propagation, consid- ering both the error assigned to measurement of horizontal parallax (assuming σParallax=±10 µm) and, only for the pre-2000 surveys, the camera focal length (assuming σCamera=±10 µm), we derived a theoretical mean square deviation val- ue expected for Z coordinate (elevation); if B is the distance between the camera’s centres rela- tive to two stereo pairs, as known, for usual base-ratios (B/Z > 0.5) this is generally greater than the planimetric ones. Table II shows the main flight data and the value of theoretical σZ. Table II shows a clear correspondence be- tween the mean standard deviations and the scale factors of the photos: the 1954 survey shows theoretical errors of a higher magnitude than those from 2000. Moreover 1971 presents theoretical errors that are slightly lower than those from 1976 data; this is due to the base-ra- tios values. Obviously, the error related to un- certainty in collimation of natural points must be added to the σZ theoretical one. A measurement repeatability test was per- formed: DTMs on sample area were generated in different periods, by the same operator, with the same analytical instrument, in semi-auto- matic mode. The comparison of such results (see table III) shows good repeatability of the measurements and confirms the close correla- tion between image scale factor and errors. Considering the above observations as a whole and the values listed in table II, and in or- der to adopt a simple and homogeneous criteri- on, we have chosen 1 m as the significant range of movement in DTMs comparisons, from 1971 up to 2000. The point differences lying within such interval have therefore been considered non-indicative of deformation due to landslide movement. If the 1954 DTM is used for com- parisons, the significant range assumed is 2 m. To guarantee the quality of data to be used in this research, the above-described photogram- metric method was performed using both an an- alytical plotter and semi-automatic or manual vector procedures, with a significant amount of work of an expert operator, and the digital pho- togrammetry technique for the automatic gener- ation of terrain models and orthophotos. In par- ticular, in addition to the photogrammetric process performed by analytical stereoplotter, the 2000 survey images were used to generate a DTM with a high-level digital photogrammetric workstation (Helava System), adopting semi-au- tomatic and automatic procedures. The films were scanned with a RasterMas- ter (Wehrli & Associates Inc., NY, U.S.A.) pho- togrammetric scanner at a 1000 dpi resolution, 1075 Table II. Characteristics of available photogrammet- ric surveys and theoretical assessment of level error, as- suming σPξ = Standard Deviation (SD) in measurement of horizontal parallax = ±10 µm and, for pre-2000 sur- veys, σc = SD of focal length of camera = ±10 µm. Val- ues expressed in meters. Year 1954 1971 1976 2000 Mean flight 9509.9 3376.8 2973.9 1011.3 heigh (m) Nominal 0.154 0.152 0.153 0.152 focal length Base line 5729.2 1818.5 1137.7 365.6 Mean topographic 300.00 300.00 300.00 300.00 heigh (m) Mean 60000 20000 17500 4700 scale factor Mean B / Z 0.62 0.59 0.43 0.51 σZ ± 1.45 ± 0.58 ± 0.72 ± 0.18 Table III. Measurement repeatability test. 1971 1976 2000 No. points 214 214 850 Standard deviation (m) 0.68 0.59 0.36 Table IV. Characteristics of images acquired with photogrammetric scanner. Year of the survey 2000 Photo scale 1:4.400 Panchromatic film Colour Image resolution ( µm) 24 Ground resolution (cm) 12 File image dimension (Mb) 240 1076 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara Fig. 4. Comparison between the reference DTM (DIGICART 2 × 2 m) and DTM automatically generated with Helava system; colour classified differences are shown overlapped the shaded relief model. Fig. 5. High differences obtained from 1976 and 1954 surveys. Differences (in meters) superimposed on 1976 ortho-photo are indicated by different colours. Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis sufficient to guarantee a high level of detail (ground pixel size about 12 cm; table IV). The DTM was firstly generated automatical- ly, with post-editing by the operator to correct er- rors deriving from the correlation procedure; ed- iting consisted mainly of manual insertion of breaklines into incorrect zones, with consequent local recalculation of the surface area. The DTM automatically generated by digi- tal photogrammetry was then compared to the one generated by the analytical plotter; fig. 4 shows the differences between the two models. It may be seen that for 96.5% of the points, the difference between the two models is within an interval of ± 1 m, and that the largest differences are localized mainly in zones with complex morphology and shadowed areas. In addition, together with DTM and classic vector products, a digital orthophoto has been produced, to give a photographic representa- tion, with metric characteristics, of the situation in a landslide area at a defined time. Modern digital photogrammetry and GIS programs can also generate highly effective three-dimension- al views that combine the expressive potentials of ortho-photos and three-dimensional data of DTM. Figure 3, generated with ErMapper soft- ware, refers to the 2000 orthophoto. The comparison of multitemporal DTMs, performed without the use of interpolators, al- lowed us to quantify the geometric variations oc- curring as a result of evolution of the landslide. The two following figs. 5 and 6 graphically show the results of comparisons of the 1954, 1976 and 1077 Fig. 6. High differences between the 8 × 8 m DTMs obtained from 2000 and 1976 surveys. Differences (in me- ters) superimposed on 2000 orthophoto are indicated by different colours. 1078 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara 2000 surveys; the movement determined for in- dividual cells is highlighted in colour. For better legibility of the figures, the digital orthophotos generated for the 1976 flight and the 2000 flight, respectively, have been used. Figure 5 shows the result of the comparison between DTMs from 1954 and 1976, with 8×8 m grid resolution. Figure 6 refers to differences rel- ative to the period 1976-2000, for which a his- togram of heigh differences is shown in fig. 7. Other comparisons, for example those in- volving the 1971 model, are not discussed in this paper, due to the lack of further information with respect to the presented data. 4. Landslide map and differential DTM comparison Comparative analysis of the multi temporal DTMs indicate that the landslide reactivated in the time interval 1954-1971, while in the fol- lowing period (1971-1976) the slope was al- most inactive. The movements involved both the Cà di Bosco and Cà di Malta landslide bod- ies (fig. 5). The alimentation area is character- ized by a main scarp with arched shape, locally complex, that is recognisable at various points. The movements observed in this area confirm a distribution of a moderate retrogressive activity along the left side of the landslide area, as de- tected by means of the photo interpretation in- vestigations. The amount of material moved is less than assumed by qualitative inspection. The time interval 1976-2000 was character- ized by a more important reactivation of the landslide in 1996, after a long period of relative inactivity (Mora et al., 2003). Subsequently, in 1998, a small rotational slide involved the upper part of the Cà di Malta main body. Part of the effects of these movements, principally at the foot of the landslide, were remodelled by hu- man activity in the frame of consolidation and drainage works. The results of natural and anthropic activi- ties are shown in fig. 6. The largest mass move- ments occurred at the scarp areas of the Cà di Malta and Cà del Bosco landslides, where nega- tive values ranging between 2 and 8 m are pres- ent in fairly limited areas. On the other hand, the most significant uplifts are also localized in very restricted areas, mainly at the foot of the land- slides, and they do not exceed 4.5 m. Over an area of about 15 × 104 m2 the volumetric analysis gives a negative variation of 75×10 3 m3 and an accumulation of about 25×103 m3. Observing the statistic of the residual con- cerning areas assumed to be stable (differences <1.00 m) and assuming systematic errors due to model orientations of about 20 cm for the 1997- Fig. 7. Statistic of differences between 2000 and 1976 DTMs. 1079 Qualitative and quantitative photogrammetric techniques for multi-temporal landslide analysis 2000 data, the estimation of the volumes is giv- en with the precision of 20%. The difference between the depletion and accumulation volumes is principally due to the removal of material along the river and above the road at the foot of the slope. For more precise analysis of the relation- ships between the high residuals and the 1976 and 2000 landslides map, fig. 6 shows (with coloured rectangles and letters, points A to E) the areas in which the most significant surface lowering or uplifts occurred. The significant topographic changes may be seen at the slip- flow that develops along the left side of the Cà di Malta landslide. Point A corresponds to an area of about 4000 m2, with the largest negative variations, and co- incides with the landslide alimentation area, quantitatively defining the extent of the depres- sion and the distribution of the slopes of the scarp area. Degradation values increase rapidly toward the centre of the area. This depressed area has arched borders located precisely at the main scarp and at the head of the landslide. At point B, the scarp is clearly shown in an area of the differentials map in which major lev- el degradations may be observed. The values presenting the most significant variations (be- tween 2 and 4 m) correspond to the alimentation area of the 2000 slip-flow, which caused a sharp depression bordered by a distinct main scarp and repeated secondary scarps and terracings. Point D shows local uplifts. This zone coin- cides with the foot of the landslide, where the accumulation is elongated and slightly lobed. Other significant topographic changes may be seen at points B and E, along the slip-flow that develops starting from the right side of the Cà del Bosco landslide. Point E shows an area in which the differen- tial map presents a distinct uplift of about 4 m. This area corresponds to the point where the landslide stopped, generating a lobe-shaped ac- cumulation at the foot. Significant topographic changes may also be seen at point C where the slope was remodelled by man for reinforcement and drainage works. Lastly in the area at the foot of the landslide the topographic changes shown on the residuals map are not traceable to landslides, but rather to anthropic activity. In particular, the degradations are due to construction of a short section of road leading to the Reno River, while the uplifts are caused by levelling of the slope by man to re- claim the foot of the Cà di Malta landslide. 5. Conclusions Stereoscopic analysis of four sets of aerial photos, taken over a time span of almost 50 years, allowed reconstruction of the complex sequence of landslide movements occurring on the slope under study. The 1954 photos, despite the low-scale (1:60 000), provided significant evidence of a slip-flow-type landslide body af- fecting the entire slope. This event may be con- sidered the primary cause of all of the subse- quent movements, which are therefore to be un- derstood as partial reactivations of such land- slide. Of these, the Cà di Bosco and Cà di Mal- ta landslides are the most extensive phenomena. The event cannot be dated on the basis of available data. Nevertheless, studies conducted in the Apennine area have demonstrated that most of the large landslides are hundreds or even thousands of years old (Bertolini and Pellegrini, 2001). A quantitative analysis of the territory has been obtained with an appropriate comparison of photogrammetric surveys of the zone realised in different years, allowing identification of geo- metric changes occurring during the time inter- val in question. The absence of fundamental in- formation on geometric parameters of the im- ages has been overcome by adopting an uncon- ventional method to co-register all the datasets in the same reference frame, based on the detection of homologous points in the multi temporal models. The areas and the amount of masses in- volved in the reactivations of the landslides have been estimated, giving a contribution to the mor- phological interpretation of the landslide defor- mation phenomena in the last years. The availability of historical stereoscopic im- ages assumes particular importance in the study of long time span phenomena, which produce surface deformations of the Earth. Furthermore the possibility of adopting the digital photogram- metry approaches will yield faster and higher 1080 Antonio Zanutta, Paolo Baldi, Gabriele Bitelli, Mauro Cardinali and Alberto Carrara resolution results, at lower costs with respect to the analytical analysis. The availability of high- resolution satellite images which may become competitive with small-medium scale aerial pho- tos will lead to the low-cost creation of image archives with repetition intervals that depend on the temporal resolution of the satellite. Acknowledgements The work was partially financed by the PRIN2003 Italian National Research Project «Tecnologie innovative per la previsione, il controllo e la mitigazione dell’impatto delle emergenze ambientali» (Nat. Resp. Prof. Sergio Dequal). 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