José Luis Lerma

Integration of Laser Scanning and Imagery for Photorealistic 3D Architectural Documentation

Documentation of architectural and archaeological sites and monuments is an activity that requires the capture of information from different sources. Metric information is mandatory as the basis for documentation, information management, archiving, analysis, monitoring and dissemination activities, among others. In highly weathered environments, accurate 3D models are required for multi-temporal analysis over time in 4D, altogether with imagery. Experience has shown that it is possible to provide the necessary information with the required accuracy and completeness only by integration of multisource data (Georgopoulos & Ioannidis, 2006). Especially in large and complex monuments parallel use of geodetic and surveying measurements, photogrammetric data acquisition with imagery and terrestrial laser scans has proven to be the ideal combination (Haddad, 2007; Haddad & Ishakat, 2007). Furthermore, the multi-sensor integration of surveying data (not only terrestrial but also aerial) and architectural needs should be combined with a multi-resolution approach, spanning from few centimetres down to millimetres, if necessary, both in geometry and in texture (Guidi et al., 2008; Remondino et al., 2009). A general overview of the typical requirements and solutions for cultural heritage documentation based on purpose, product, methodology or eventual emphasis is presented in Patias (2006). Modern technology has changed matters in documentation radically and promises to keep bringing rapid changes. Photographic and non-photographic (graphic) documentation tools are merging in one process, in which the digital photographic technology is the main base (Haddad, 2010; Haddad & Akasheh, 2005). Until the wide spread used of terrestrial laser scanning (TLS) for cultural heritage documentation, most of the multi-temporal information for documentation and conservation activities was based mainly on graphic documentation: 2D drawings of elevation plans, cross-section and last but not least, images (analogue or digital). However, photographs are easier to interpret and recognize than drawings; they contain information about surface detail and can provide information on the condition of a monument, before, during, and after restoration. Nowadays, the afore-mentioned documents are still requested by architects or building personnel but the analysis is starting

Classification of lidar bare-earth points, buildings, vegetation, and small objects based on region growing and angular classifier

In recent years, light detection and ranging (lidar) systems have been intensively used in different urban applications such as map updating, communication analysis, virtual city modelling, risk assessment, and monitoring. A prerequisite to enhance lidar data content is to differentiate ground (bare earth) points that yield digital terrain models and off-terrain points in order to classify urban objects and vegetation. The increasing demand for a fast and efficient algorithm to extract three-dimensional urban features was the motive behind this work. A new combined approach to extract bare-earth points is proposed, and a novel methodology to automatically classify airborne laser data into different objects in an urban area is presented. In addition, a new concept of angular classification is introduced to differentiate buildings from vegetation and other small objects. The new angular classifier analyses the distribution of bare-earth points around unclassified point clusters to determine whether a cluster can be classified either as building or as vegetation. The experimental results confirm the high accuracy achieved to automatically classify urban objects in flat complex areas.

Integration of TLS Data and Non-metric Imagery to Improve Photo Models and Recording - A Case Study on Djin Block No. 9, Petra (Jordan)

In this paper we present a methodology to accurately derive 3D photo models of a World Heritage monument by means of terrestrial laser scanning (TLS) and close range photogrammetry. TLS will provide 3D point clouds as well as rough photo models that can substantially be improved draping texture with external imagery. The tomb Djin Block No. 9 in Petra is used to compare texture results delivered by the camera of the terrestrial laser scanner versus an external off-the-shelf digital camera. The issue of image resection and calibration of non-metric single imagery is tackled making use of a digital surface model. The presented approach is suitable for off-the-self digital cameras and can cope with single images without overlap. Results show that it is possible, on the one hand, to enhance photo models draping external imagery onto the 3D model, and on the other, to take advantage of single non-metric images to record cultural heritage.

Grey Level and Noise Evaluation of a Foveon X3 Image Sensor: A Statistical and Experimental Approach

Radiometric values on digital imagery are affected by several sources of uncertainty. A practical, comprehensive and flexible procedure to analyze the radiometric values and the uncertainty effects due to the camera sensor system is described in this paper. The procedure is performed on the grey level output signal using image raw units with digital numbers ranging from 0 to 212-1. The procedure is entirely based on statistical and experimental techniques. Design of Experiments (DoE) for Linear Models (LM) are derived to analyze the radiometric values and estimate the uncertainty. The presented linear model integrates all the individual sensor noise sources in one global component and characterizes the radiometric values and the uncertainty effects according to the influential factors such as the scene reflectance, wavelength range and time. The experiments are carried out under laboratory conditions to minimize the rest of uncertainty sources that might affect the radiometric values. It is confirmed the flexibility of the procedure to model and characterize the radiometric values, as well as to determine the behaviour of two phenomena when dealing with image sensors: the noise of a single image and the stability (trend and noise) of a sequence of images.

A new methodology to estimate the discrete-return point density on airborne lidar surveys

The distribution of the discrete-return point density in airborne lidar flights obtained from an oscillating mirror laser scanner is analysed and alternative formulations to determine its value are presented. The point density in a lidar swath varies and can best be fitted with a potential function. This study confirms that calculating the overall point density with traditional statistical parameters yields biased results owing to the abnormally high densities of the swath boundaries. New formulas for calculating the representative mean are proposed: a weighted arithmetic mean (WAM) based on a potential function; geometric mean (GM); and harmonic mean (HM). All three means give more weight to the central sectors across the strip and less to the boundary sectors where extreme data redundancy exists. The WAM based on a potential function yields equivalent estimates as the HM; the GM yields slightly higher estimates. The results obtained improve the mean estimation and, more importantly, allow users to estimate better the mean point density on airborne lidar surveys, which are usually overestimated approximately by 15%.

Empirical study of variation in lidar point density over different land covers

Point density in airborne lidar surveys is one of the key parameters that influence not only the accuracy of generated DSM/DEM but also processing and costs. Point density variations occur (independently of keeping constant flight parameters) throughout the survey depending on the topography, the land cover, and the laser scanning mechanism. In this article, variations in point density across different land covers are analysed with an airborne oscillating mirror laser scanner. A wide group of samples from the different land covers is taken from single flight strips over level ground in order to minimize the effect of topography. The influence of the oscillating mirror laser scanner system is also minimized considering points along the central swath area. Mean values for each land cover are established regarding ground point density and first pulse returns. Significant differences in both raw points as well as on-the-ground points are registered, mainly due to the presence of features over the ground and the degree of opacity thereof. Regarding ground points, the relative differences between the two software packages used are 5% approximately. Significant point density differences can be found among the six analysed land covers. Furthermore, extrapolated pulse rate increments are presented to fulfil lidar specifications that neglect land cover as an input parameter to satisfy ground point density values, namely in non-overlapping areas.

Exploitation of thermal imagery for the detection of pathologies in monuments

Documentation of monuments includes not only geometric analysis but also the detection of alterations and pathologies to define proper actions for protection and preservation. Several pathologies can be found on facades such as moisture, detachments, leaks and replacements. Multispectral techniques can help users to detect and determine these phenomena. Thermography is a non-intrusive imaging technique that allows the inspection of different materials and related issues based on their temperature. It has the advantage of reaching unapproachable areas onto and below the surface. However, there are still many issues in thermography when applied to cultural heritage surveys. Depending on the application, multitemporal images can be captured and combined with conventional images to analyse its state of conservation. This paper presents an approach based on multitemporal thermal imagery to detect alterations on building facades. Methods and tools of multispectral analysis targeting the detection of alterations and pathologies are presented and evaluated.

Inertial Navigation System Data Filtering Prior to GPS/INS Integration

In the integration of Global Positioning System (GPS) and Inertial Navigation System (INS), the commonly used Kalman filter provides satisfactory results if both sources of information are continuously available. However, GPS outages provoke a fast degradation of precision, especially in low dynamic trajectories such as a mobile platform device held by a human operator. To deal with this problem we propose a data-filtering scheme to apply to INS raw data prior to the integration with GPS. The proposed technique proves to be very valuable for mitigating the high short-term instability of raw INS data during the walking movement and is also capable of eliminating the induced undesirable human operator vibrations. Final imposed corrections adapted to the particular dynamical response of the INS sensor provide comparably accurate results and often better than those achieved in similar works with the use of the Kalman filter.

Calibration and Direct Georeferencing Analysis of a Multi-Sensor System for Cultural Heritage Recording

The authors would like to thank for the support provided by the Spanish Ministry of Science and Innovation to the project HAR2010-18620. Special thanks need to be expressed to Dr. LUIS GARCIA-ASENJO and Mr. PASCUAL GARRIGUES from the Polytechnic University of Valencia for their contributions.

Documentation of weathered architectural heritage with visible, near infrared, thermal and laser scanning data

Documentation of cultural heritage requires simple, quick and easy to use multi-sensor approaches to determine the state of conservation of monuments and sites. The documentation of a highly weathered architectural heritage such as the Obelisk Tomb is a good example to test the performance integrating multispectral imagery and laser scanning data. The Obelisk Tomb is the first important facade that a visitor sees while entering to the archaeological site of Petra in Jordan. The rich architectural formations carry Egyptian, Hellenistic and Nabataean influences. The damage that was inflicted on this unique monument led us to study it applying a number of modern digital techniques including 3D scanning, multispectral photography with visible and near infrared images, and thermography. All the multiband content is initially registered onto different multispectral bands. The multispectral information is enhanced and eventually draped onto the 3D laser scanning model in order to improve documentation and analys...