LiDAR Surveys Research Articles

Shoreline and Sand Storage Dynamics from Annual Airborne LIDAR Surveys, Texas Gulf Coast

ABSTRACT Paine, J.G.; Caudle, T.L., and Andrews, J.R., 2017. Shoreline and sand storage dynamics from annual airborne LIDAR surveys, Texas Gulf Coast. Annual airborne LIDAR surveys were conducted along the Texas Gulf of Mexico shoreline between 2010 and 2012 to map shoreline position, determine shoreline movement and its historical context, and quantify beach and dune morphology by determining elevation threshold area (ETA) relationships for Holocene barrier islands, strandplains, and fluvial and deltaic headlands and marshes. Historical (1800s to 2007) movement is erosional for all major Texas shoreline segments, averaging 1.3 m/y of retreat. Shorelines retreated between 2007 and 2010 at a higher average rate of 2.8 m/y because of erosion and partial recovery from Hurricanes Ike (2008), Humberto (2007), and Dolly (2008). Despite the erosional context, airborne LIDAR surveys show that the shoreline advanced at 75% of 11,783 monitoring sites between 2010 and 2011 and moved an average of 6.5 m seaward durin...

Influence of the 18.6-year lunar nodal tidal cycle on tidal flats: Mont-Saint-Michel Bay, France

The influence of the 18.6-year nodal tidal cycle on estuarine accretion, and indirectly on channel migration patterns, in Mont-Saint-Michel Bay, a UNESCO World Heritage Site famous for its monastery, is explored over three tidal cycles from a 68-year dataset that combines aerial photographs (1947–2007), one DGPS survey (1997), and nine LiDAR Digital Elevation Models (2002–2015). Sand-flat elevation and sediment-budget changes in this megatidal environment were further determined over the last nodal cycle using DGPS and LiDAR surveys. We infer that the changes observed are linked to variations in tidal prism and current velocities in this shallow, large tide-range setting. Increasing (rising tidal range) or decreasing (falling tidal range) flood-dominated asymmetry are deemed to lead, respectively, to waxing and waning of sediment import, in the form of large sand banks, into the bay over the 18.6-year nodal tidal cycle that, in turn, force channel migration. Coriolis acceleration and centrifugal forces are also considered to contribute to the observed channel migration.

Erosion and channel changes due to extreme flooding in the Fourmile Creek catchment, Colorado

Infrequent, large magnitude geomorphic events generate quantifiable change on geologically short timescales and are crucial to understanding landscape evolution. Airborne lidar surveys and field measurements were used to investigate floodplain erosion and deposition along a 19.5km reach of Fourmile Creek, Colorado that was devastated by severe flooding in 2013 that followed a 2010 wildfire. >350mm of rain fell on the Fourmile catchment from September 9–15, 2013, generating discharge that exceeded bankfull for >120h at the Orodell gage, with local unit stream power>300Wm−2 throughout the study reach. Debris flows occurred on steep hillslopes and tributary channels in the most intensely burned areas. Lidar difference measurements and field studies highlight zones of local deposition along the study reach, but demonstrate overall net erosion of ~0.25m for the 19.5km reach of Fourmile floodplain, mainly by channel widening. Tributary junctions where debris-flow sediment entered the floodplain and local decreases in unit stream power controlled some zones of deposition. Overall, mass balance calculations show that a total sediment loss of ~91,000–161,000m3 from the Fourmile Creek floodplain and hillslopes, which is broadly consistent with channel sediment flux estimates of 71,000–111,000m3. Measurements from the Fourmile catchment demonstrate that floodplain erosion was a major source of sediment in the 2013 flood and demonstrate that infrequent events marked by long-duration flooding and high total energy expenditure can account for a large fraction of total sediment transport in mountain streams.

영주댐 운영 전 내성천에서 하도 형태의 단기 변화

내성천은 산지를 사행하는 모래하천으로 오랫동안 그 하천지형학적 특성을 유지해왔다. 그러나 최근 식생의 활착으로 종래의 경관을 상실해 가고 있다. 본 연구에서는 내성천 장기조사 연구의 일환으로 내성천의 56.8 km 구간을 대상으로 지난 2012 - 2016년 기간에 발생한 지형변화를 분석하였다. 항공 라이다 및 하천측량이 시행되었고 수문자료와 현장 조사자료가 함께 활용되었다. 주요 4개 지점중에서 만곡부 지점에서는 만곡사주가 확장되었고, 직선구간에서는 하중 사주가 발달하였다. 한 두 번의 유사 퇴적으로 인해 발생한 하중 사주의 고도 변화는 0.6 - 1.4 m에 달하였으나 하상저하는 분명하지 않았다. 이러한 변화는 2014 - 2015년의 갈수기에 식생이 크게 활착하였다가 2016년에 홍수로 인해 퇴적이 일어난데 기인하는 것으로 보인다. 이러한 과정은 이전에 내성천 중하류 구간에 형성되어 있는 하중도의 주된 형성과정으로 판단된다. The Naeseong Stream is a meandering sand-bed stream flowing through mountains and has so long maintained its geomorphological uniqueness characterized by extensive braided bare bars. Recently, its long-lasting landscape has been changed due to encroachment of vegetation. In this study being a part of long-term monitoring research morphological changes of the 56.8 km long study reach of the Naeseong Stream, which occurred during the period of 2012 - 2016 were analyzed. Airborne LiDAR and terrestrial cross-section surveys were carried out. Hydrological and on-site investigation data were also collected. Among the main four sites, two bend reaches showed point bars enlarged, while along the other two straight reaches mid-channel bars were either newly formed or increased in area and height. At the highest deposition point of each bar, vertical changes which were caused by one or two times of sediment deposition amounted to 0.6 - 1.4 m. On the contrary channel bed degradation was not obvious. Overall morphological changes in the study reach were attributed to deposition of sediment which occurred during the flood in July 2016 on the bar surfaces vegetated during the precedent dry seasons. These kind of geomorphological processes are thought to be the same as those related to the existing mid-channel islands along the mid- and downstream reach of the Naeseong Stream.

Assessment of NASA airborne laser altimetry data using ground-based GPS data near Summit Station, Greenland

Abstract. A series of NASA airborne lidars have been used in support of satellite laser altimetry missions. These airborne laser altimeters have been deployed for satellite instrument development, for spaceborne data validation, and to bridge the data gap between satellite missions. We used data from ground-based Global Positioning System (GPS) surveys of an 11 km long track near Summit Station, Greenland, to assess the surface–elevation bias and measurement precision of three airborne laser altimeters including the Airborne Topographic Mapper (ATM), the Land, Vegetation, and Ice Sensor (LVIS), and the Multiple Altimeter Beam Experimental Lidar (MABEL). Ground-based GPS data from the monthly ground-based traverses, which commenced in 2006, allowed for the assessment of nine airborne lidar surveys associated with ATM and LVIS between 2007 and 2016. Surface–elevation biases for these altimeters – over the flat, ice-sheet interior – are less than 0.12 m, while assessments of measurement precision are 0.09 m or better. Ground-based GPS positions determined both with and without differential post-processing techniques provided internally consistent solutions. Results from the analyses of ground-based and airborne data provide validation strategy guidance for the Ice, Cloud, and land Elevation Satellite 2 (ICESat-2) elevation and elevation-change data products.

Influence of digital terrain model on mapping of slope deposits depth

In this work the influence of the characteristics of different Digital Terrain Models (DTMs) on Slope Deposits depth (dSD) maps is evaluated. dSD data collected during fieldwork have been mapped through GPS. Two different DTMs have been used for the analysis: one derived from LiDAR survey (1 m pixel size), the other extracted by the topographic maps of Regione Toscana (10 m pixel size). dSD maps have been extracted by integrating cluster analysis of morphometric variables with dSD data. In order to investigate the effects of spatial resolution, the LiDAR DTM has been down-sampled to both 5 m and 10 m. The weight of GPS positioning uncertainty has been assessed by performing a statistical analysis on the morphometric clusters falling into buffer areas of sampling points. Comparisons among outputs have been performed through map accuracy estimations, as well as by spatial distribution visual check and extension assessment of dSD classes. Results show that by increasing spatial resolution of input DTMs the statistical quality of the dSD maps generally reduces. Concerning the effect of buffer size around sampling points, as a general rule, the larger the buffer, the lower the map accuracy. Only the map obtained by 1 m LiDAR DTM is insensitive to buffer size.

Données LiDAR aériennes: pertinence de l'interprétation visuelle pour la foresterie

Airborne LiDAR data: relevance of visual interpretation for forestry Airborne LiDAR surveys are particularly well adapted to map, study and manage large forest extents. Products derived from this technology are increasingly used by managers to establish a general diagnosis of the condition of forests. Less common is the use of these products to conduct detailed analyses on small areas; for example creating detailed reference maps like inventories or timber marking to support field operations. In this context, the use of direct visual interpretation is interesting, because it is much easier to implement than automatic algorithms and allows a quick and reliable identification of zonal (e.g. forest edge, deciduous/persistent ratio), structural (stratification) and point (e.g. tree/stem position and height) features. This article examines three important points which determine the relevance of visual interpretation: acquisition parameters, interactive representation and identification of forest characteristics. It is shown that the use of thematic color maps within interactive 3D point cloud and/or cross-sections makes it possible to establish (for all strata) detailed and accurate maps of a parcel at the individual tree scale.

Multi-temporal monitoring of a regional riparian buffer network (>12,000 km) with LiDAR and photogrammetric point clouds

Riparian buffers are of major concern for land and water resource managers despite their relatively low spatial coverage. In Europe, this concern has been acknowledged by different environmental directives which recommend multi-scale monitoring (from local to regional scales). Remote sensing methods could be a cost-effective alternative to field-based monitoring, to build replicable “wall-to-wall” monitoring strategies of large river networks and associated riparian buffers. The main goal of our study is to extract and analyze various parameters of the riparian buffers of up to 12,000 km of river in southern Belgium (Wallonia) from three-dimensional (3D) point clouds based on LiDAR and photogrammetric surveys to i) map riparian buffers parameters on different scales, ii) interpret the regional patterns of the riparian buffers and iii) propose new riparian buffer management indicators. We propose different strategies to synthesize and visualize relevant information at different spatial scales ranging from local (<10 km) to regional scale (>12,000 km). Our results showed that the selected parameters had a clear regional pattern. The reaches of Ardenne ecoregion have channels with the highest flow widths and shallowest depths. In contrast, the reaches of the Loam ecoregion have the narrowest and deepest flow channels. Regional variability in channel width and depth is used to locate management units potentially affected by human impact. Riparian forest of the Loam ecoregion is characterized by the lowest longitudinal continuity and mean tree height, underlining significant human disturbance. As the availability of 3D point clouds at the regional scale is constantly growing, our study proposes reproducible methods which can be integrated into regional monitoring by land managers. With LiDAR still being relatively expensive to acquire, the use of photogrammetric point clouds combined with LiDAR data is a cost-effective means to update the characterization of the riparian forest conditions.

The application of airborne mapping LiDAR for the documentation of ancient cities and regions in tropical regions

It is a conundrum of the 21st Century that there is much left to discover and yet never before has our cultural and ecological patrimony been so threatened. This is especially true in tropical regions where heavy vegetation, inaccessibility, and rugged topography hamper investigation. Here we present two case studies that add to a growing body of literature demonstrating the utility of airborne mapping LiDAR (a.k.a. Airborne Laser Scanning) for rapid archaeological assessments in poorly documented regions. The first outlines a program of LiDAR scanning to better understand the urban center of Angamuco in the Mexican State of Michoacán. This work shows that (1) large urban centers with complex spatial organization were present centuries prior to the formation of the Purépecha Empire; (2) the settlement incorporates gardens and other landscape features within and around the settlement demonstrating a high degree of human environmental modification; and (3) current models for the evolution of social complexity in the region cannot account for the presence of Angamuco. The second presents the results of a LiDAR survey of a remote valley in the Mosquitia tropical wilderness of Honduras which has seen little archaeological research. Here we demonstrate that (1) though today the valley is a wilderness it was densely inhabited in the past; (2) this population was organized into a three-tiered system composed of 19 settlements dominated by a city; and (3) this occupation was embedded within a human engineered landscape. For both, LiDAR data fundamentally changed the understanding of coupled human/natural systems in these areas while providing critical baseline data for conservation and management.

Camera system considerations for geomorphic applications of SfM photogrammetry

AbstractThe availability of high‐resolution, multi‐temporal, remotely sensed topographic data is revolutionizing geomorphic analysis. Three‐dimensional topographic point measurements acquired from structure‐from‐motion (SfM) photogrammetry have been shown to be highly accurate and cost‐effective compared to laser‐based alternatives in some environments. Use of consumer‐grade digital cameras to generate terrain models and derivatives is becoming prevalent within the geomorphic community despite the details of these instruments being largely overlooked in current SfM literature.A practical discussion of camera system selection, configuration, and image acquisition is presented. The hypothesis that optimizing source imagery can increase digital terrain model (DTM) accuracy is tested by evaluating accuracies of four SfM datasets conducted over multiple years of a gravel bed river floodplain using independent ground check points with the purpose of comparing morphological sediment budgets computed from SfM‐ and LiDAR‐derived DTMs. Case study results are compared to existing SfM validation studies in an attempt to deconstruct the principle components of an SfM error budget.Greater information capacity of source imagery was found to increase pixel matching quality, which produced eight times greater point density and six times greater accuracy. When propagated through volumetric change analysis, individual DTM accuracy (6–37 cm) was sufficient to detect moderate geomorphic change (order 100 000 m3) on an unvegetated fluvial surface; change detection determined from repeat LiDAR and SfM surveys differed by about 10%. Simple camera selection criteria increased accuracy by 64%; configuration settings or image post‐processing techniques increased point density by 5–25% and decreased processing time by 10–30%.Regression analysis of 67 reviewed datasets revealed that the best explanatory variable to predict accuracy of SfM data is photographic scale. Despite the prevalent use of object distance ratios to describe scale, nominal ground sample distance is shown to be a superior metric, explaining 68% of the variability in mean absolute vertical error. Published 2016. This article is a U.S. Government work and is in the public domain in the USA

Remote Sensing of a Shallow, Fringing Reef Platform for Analysis of Island Sector Susceptibility and Development of a Coastal Vulnerability Index

Moffitt, D.L. and Kumar, L., 2018. Remote sensing of a shallow, fringing reef platform for analysis of island sector susceptibility and development of a coastal vulnerability index.Like many islands in middle latitudes, Pacific islands are often fringed by protective coral reefs that attenuate wave energy impacting the coastline. Measurement of the extent and distribution of shallow reef platforms by remote sensing of bathymetry enables an assessment of the relative exposure or protection of each part of an island. Fringing reefs are one of a number of physical variables relevant to the coastline's susceptibility to erosion and inundation resulting from extreme weather and conditions exacerbated by climate change and rising sea levels. In this research the extent of shallow reef platform around Lifuka Island, Tonga, was determined using a modified Stumpf band ratio method applied to Landsat 8 and high-resolution Quickbird satellite imagery, with aerial LIDAR data available for ground-truth purposes. The results indicate that both sets of satellite imagery provide a means of measuring the bathymetry to at least 15 m with a correlation (R2) value of 0.87 for Landsat 8 and to 15 m with a correlation (R2) value of 0.86 for Quickbird, when compared with the LIDAR survey. This article forms part of the research into the preparation of a coastal vulnerability index to inform decision-making and risk assessment for Pacific Island nations at the subisland scale. The mapping of the fringing reef by remote sensing allows its spatial distribution to be considered in the assessment of risk to coastal regions to inundation and erosion processes. Further, it demonstrates the value of freely available Landsat data for planning adaptation to climate change in developing nations at a level pertinent to local governance of villages and communities.

ACCURACY ASSESSMENT OF UAV LIDAR SURVEY FOR RIVER MORPHOLOGICAL MAPPING

ACCURACY ASSESSMENT OF UAV LIDAR SURVEY FOR RIVER MORPHOLOGICAL MAPPING

Multi-temporal images and 3D dense models for archaeological site monitoring in Hierapolis of Phrygia (TK)

The archaeological areas are one of the fields in which the contribution of Geomatics techniques for Heritage documentation were employed since long time and are now getting popular. In recent times, TLS and RPAS systems become more and more interesting to be studied in excavations sites for monitoring purposes and solving large scale and comprehensive mapping matters both from terrestrial and aerial point of view. The paper will focuses on these issue and on the great challenge of monitoring sites during time, integrating and conforming multiple data coming from previous metric survey projects or image data collected in the past for different purposes. The test-sites is the complex archaeological landscape of the ancient city of Hierapolis in Phrygia on which the MAIER – Italian Archaeological Mission of Hierapolis operates from the 60s of the 20 th century and where Politecnico di Torino conducted several documentation survey campaigns. Some case studies in Hierapolis monumental areas about 3D modelling from image and range data represent nowadays an interesting attempt to answer these tasks. In particular, the presented cases involve many multi-temporal datasets that have been acquired by different university staffs in subsequent campaigns, in 1997, 2007, 2012, 2015, by employing various sensors following the evolution of the acquisition techniques offers by geomatics in the archaeological field surveys. 3D models are generated by terrestrial LiDAR and by terrestrial and aerial photogrammetric surveys; these dense models are intended to be compared and integrated with newer models generated by the UAV systems employed in last 2015 mission. In the Sacred Area of the Apollo Temple, two different datasets allow to perform investigation and comparisons before and after archaeological accommodation of the sepulchre of Apollo. The 3D model was created from images captured by a curbed balloon and further by another particular nadir point of view (a man harnessed and suspended from a crane collected photos using a Rollei semi-metric film camera); an UAV flight was performed lastly in 2015 by a EBee fix wing drone. On the other hand, in the massive complex of the Bath-Church the deeply damaged East wall was kept under observation during years until the securing intervention with a castle of tubes has been achieved: till that moment the pipes structure masks almost completely the wall, even during the last Lidar survey performed in 2012. The integration with previous images datasets collected before the securing intervention and used to generate a 3D model, want to overcome this limitation in a multi-temporal integration. In such perspective, 3D data derived from digital acquisition and modelling constantly contribute to build a knowledge on the site and help to improve their communication. Furthermore the combination of digital models and direct observations of the excavations results can improve the enhancement of the dissemination aims.

From remote sensing to a serious game: Digital reconstruction of an abandoned medieval village in Southern Italy

Abstract The digital reconstruction of the history of a buried medieval village is the main focus of this paper. The study, based on remote sensing and historical sources, is the starting point of the development of a serious game aimed at educational purposes and exploitation of remote sensing data in the field of edutainment. The selected historical scenario is Yrsum, a village in Basilicata (South of Italy), founded in the 11th century and abandoned in the 14th century. A LiDAR survey along with satellite multispectral data (suitably elaborated for feature extraction) as well as the historical sources and archaeological records provided useful information on the ‘forma urbis’ of the medieval settlement from its foundation to its abandonment. The extraction of the archaeological features and the analysis of urban pattern put in evidence similarities with some medieval settlements based on “motte and bailey” typology that spread in Southern Italy, France and England from the 11th to the 13th century. After the virtual reconstruction, an interactive application articulated both on bi-dimensional and three-dimensional elements have been developed. The major novelty compared to most common video games has been the possibility to derive the game from rigorously scientific data. The player enjoys and learns within a logic of an edutainment game (a combination of education and entertainment), which has become by now a well-established concept but still rarely applied in the field of cultural heritage.

Numerical modeling of the morphodynamic response of a low-lying barrier island beach and foredune system inundated during Hurricane Ike using XBeach and CSHORE

Follet's Island (FI) is a sediment-starved barrier island located on the Upper Texas Coast; a stretch of coastline along the Gulf of Mexico that experiences on average four hurricanes and four tropical storms per decade. During Hurricane Ike, water levels and wave heights at FI exceeded the 100-year and 40-year return values, respectively, leading to significant overtopping and morphology changes of this low-lying barrier island. The physical processes governing the real-time morphodynamic response of the beach and dune system during 96h of hurricane impact were modeled using XBeach (2D) and CSHORE (1D). Hydrodynamic boundary conditions were obtained from ADCIRC/SWAN model runs validated with measured buoy and wave gauge data while LiDAR surveys provided pre- and post-storm measured topography.XBeach displayed a decent model skill of 0.34 and provided numerical outputs of the entire 2D domain such as topography, suspended sediment load and bed load which was very useful in visualizing erosion and deposition patterns. CSHORE also displayed a decent model skill of 0.33 and was able to accurately predict the post-storm beach slope and shoreline, but was less effective at simulating the foredune morphology. Modeling results show that the complete morphodynamic response of FI to Hurricane Ike was governed by a sequence of impact regimes, including swash, collision, overwash, inundation, and storm surge ebb.

A Study on In Situ Calibration of an Off-The-Shelf Digital Camera Integrated to a Lidar System

Photogrammetric procedures using the integration of imagery and light detection and ranging (Lidar) datasets are becoming common in mapping companies nowadays. When photogrammetric and Lidar surveys are performed simultaneously while having the camera connected to the Lidar system, the image's exterior orientation parameters computed from the Lidar system can be used to perform photogrammetric applications. To implement such approach, camera calibration procedures should be applied to obtain accurate interior orientation parameters. This paper shows the performed study and experimental results from an in situ self-calibration using simultaneously collected imagery and Lidar datasets. The experimental study is conducted to devise a methodology for the in situ self-calibration that uses a set of vertical control points, extracted from Lidar point cloud, as well as the three-dimensional coordinates of the camera exposure stations, computed from the Lidar trajectory. For the small photogrammetric block used in this study, the results from the performed experiments demonstrated that the in situ self-calibration is required to obtain horizontal accuracies of approximately one image pixel on the ground in bundle adjustment assisted by direct measured of the coordinates of the camera exposure stations.

Optimization of Data Collection and Refinement of Post-processing Techniques for Maritime Canada's First Shallow Water Topographic-bathymetric Lidar Survey

ABSTRACT Webster, T.; McGuigan, K.; Crowell, N.; Collins, K., and MacDonald, C., 2016. Optimization of data collection and refinement of post-processing techniques for Maritime Canada's first shallow water topographic-bathymetric lidar survey. In: Brock, J.C.; Gesch, D.B.; Parrish, C.E.; Rogers, J.N., and Wright, C.W. (eds.), Advances in Topobathymetric Mapping, Models, and Applications. Journal of Coastal Research, Special Issue, No. 76, pp. 31–43. Coconut Creek (Florida), ISSN 0749-0208. An airborne topographic-bathymetric lidar survey was conducted for five coastal study sites in Maritime Canada in fall 2014 using the shallow water Leica AHAB Chiroptera II sensor. The sensor utilizes near-infrared (NIR) and green lasers to map topography, water surface, and bathymetry, and is equipped with a 60 MPIX camera, which results in 5-cm resolution color and NIR orthophotos. Depth penetration of the lidar sensor is limited by water clarity, and because the coastal zone is vulnerable to reduced water clarity/inc...

Lidar-based mapping of flood control levees in South Louisiana

ABSTRACTFlood protection in south Louisiana is largely dependent on earthen levees, and in the aftermath of Hurricane Katrina the state’s levee system has received intense scrutiny. Accurate elevation data along the levees are critical to local levee district managers responsible for monitoring and maintaining the extensive system of non-federal levees in coastal Louisiana. In 2012, high resolution airborne lidar data were acquired over levees in Lafourche Parish, Louisiana, and a mobile terrestrial lidar survey was conducted for selected levee segments using a terrestrial lidar scanner mounted on a truck. The mobile terrestrial lidar data were collected to test the feasibility of using this relatively new technology to map flood control levees and to compare the accuracy of the terrestrial and airborne lidar. Metrics assessing levee geometry derived from the two lidar surveys are also presented as an efficient, comprehensive method to quantify levee height and stability. The vertical root mean square error values of the terrestrial lidar and airborne lidar digital-derived digital terrain models were 0.038 m and 0.055 m, respectively. The comparison of levee metrics derived from the airborne and terrestrial lidar-based digital terrain models showed that both types of lidar yielded similar results, indicating that either or both surveying techniques could be used to monitor geomorphic change over time. Because airborne lidar is costly, many parts of the USA and other countries have never been mapped with airborne lidar, and repeat surveys are often not available for change detection studies. Terrestrial lidar provides a practical option for conducting repeat surveys of levees and other terrain features that cover a relatively small area, such as eroding cliffs or stream banks, and dunes.

An evaluation of the effectiveness of low-cost UAVs and structure from motion for geomorphic change detection

The measurement of topography and of topographic change is essential for the study of many geomorphic processes. In recent years, structure from motion (SfM) techniques applied to photographs taken by camera-equipped unmanned aerial vehicles (UAVs) has become a powerful new tool for the generation of high resolution topography. The variety of available UAV systems continues to increase rapidly, but it is not clear whether increased UAV sophistication translates into improved quality of the calculated topography. To evaluate the lower end of the UAV spectrum, a simple low cost UAV was deployed to calculate high resolution topography in the Daan River gorge in western Taiwan, a site with a complicated 3D morphology and a wide range of surface types, making it a challenging site for topographic measurement. Terrestrial lidar surveys were conducted in parallel with UAV surveys in both June and November 2014, enabling an assessment of the reliability of the UAV survey to detect geomorphic changes in the range of 30cm to several meters. A further UAV survey was conducted in June 2015 in order to quantify changes resulting from the 2015 spring monsoon. To evaluate the accuracy of the UAV derived topography, it was compared to terrestrial lidar data collected during the same survey period using the cloud-to-cloud comparison algorithm M3C2. The UAV-generated point clouds match the lidar point clouds well, with RMS errors of 30–40cm; however, the accuracy of the SfM point clouds depends strongly on the characteristics of the surface being considered, with vegetation, water, and small scale texture causing inaccuracies. The lidar and SfM data yield similar maps of change from June to November 2014, with the same areas of geomorphic change detected by both methods. The SfM-generated change map for November 2014 to June 2015 indicates that the 2015 spring monsoon caused erosion throughout the gorge and highlights the importance of event-driven erosion in the Daan River. The results suggest that even very basic UAVs can yield data suitable for measuring geomorphic change on the scale of a channel reach.

Using an unmanned aerial vehicle for topography mapping of the fault zone based on structure from motion photogrammetry

ABSTRACTHigh-precision and high-resolution topography is the basis of the quantitative study of active faults. Light detection and ranging (lidar) is currently the most popular method for obtaining such data, but its relatively high cost greatly limits its use in many geoscience applications. Recently, with the rapid development of computer vision science and the growing application of small unmanned aerial vehicles (UAVs), Structure from Motion (SfM) photogrammetry shows great potential for providing topographic information of comparable resolution and precision to lidar surveys, but at significantly lower cost. In this study, we examined the applicability of SfM photogrammetry in modelling the topography of the fault zone using images acquired with a low-cost digital camera mounted on a UAV over the Haiyuan fault. The resolution and accuracy of the SfM-derived topography were evaluated in detail using existing airborne lidar data as a benchmark. The results show that the density of the point cloud generated by SfM photogrammetry is nearly 70 times higher than that from the airborne lidar. Furthermore, considering the errors in the lidar data itself, the precision of the SfM point cloud is comparable to that of the lidar point cloud. Overall, our results demonstrate that the UAV-based SfM photogrammetry method can provide an inexpensive, effective, and flexible alternative to airborne lidar for the topography mapping of the fault zone.