Documentation of Roman Excavations by Laser
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DOCUMENTATION OF ROM AN EXCAVATIONS BY LA SER SCANNING K. Szocs, Z. Kibedy, A.Barsi Department of Photogrammetry and Geoinformatics Budapest University of Technology and Economics H-1111 Budapest, Hungary Commission V, WG V/2 KEY WORDS : Cultural herit age, Terrestrial laser scanning, 3D modeling, Archeology, Visualization, Documentation technology ABSTRACT: Nowadays 3D laserscanned point clouds are used in many fields of sciences. This paper presents a comparison study of the established geodetic tec hnologies and laserscanning. The modern technology allows more details high accuracy, rapid data acquisition, and more realistic new products – such as three dimensional virtual models or textured meshes – or even orthophotos can be easily created. First o f all we studied the required archeologic documentation of various excavations and realized that better results, more realistic and detailed documentation can be reached at the same time as with traditional surveys. We developed a mapping process for arche ologically relevant objects – ground plans, overviews, brickworks – which is preferred in many cases. In case of roman tessellated pavements or brickwork drawings the documentation process can be shortened, and human resource can be saved using ortophotos or post -processing. Although the post -processing requires time, it is productive and accelerates the work in the excavation field. Three dimensional wireframe models or the point cloud itself help us to draw reconstruction plans or in other applications li ke creating chronoscope. In every respect, the most important view was that we wanted to get such a documentation, which can present more information than the traditional methods and aims to create a global archeological informational system. 1. INTRODUCTION on the project, the focal length of the lens can be chosen between 20 or 85 mm . The scanning process allows high - 1.1 General considerations speed 3D measurements. Laserscanners can mea sure millions of points with a laser beam within a short period Laserscanners have found large interest during the past of time, and after the scanning automatic colour few years among archaeologists . We wanted to prove the panoramic scans can be achieved . Table 1 shows the usability of this technology and the efficiency compared main technical parameters. to conventional geodetic methods or orthophotos. Therefore we scanned the same excavated area, and after the measurements the documentation was made in the Range 2 m up to 1000 m office. In most cases, archaeological findings are found Repeatability Up to 4 mm during buil ding operations or any other public services (averaged) that are under constructions. On that score, the Measurement rate Up to 8000 -12000 archaeologist has no time for documentation. Besi des poin ts/sec this, in many cases the reconstruction of the ruins is not Accuracy (<50m) 10 mm possible because of the lack of funds or other various Laser wavelength : Near infrared reasons, so regularly the ruins to be preserved must be Beam divergence 0.25 mrad cover ed after mapping,. We ma de our e xperiences in Vertical scanning range 0° to 80° Budapest, in the area of the Aquincum Civil Town. The core of the Civil Town is located around the Museum in Horizontal scanning 0° to 360° the reconstructed ruin park. In the neighbourhood of the range ruin park, there are different kind of roman findings and Minimum angle step 0.008° ruins. Dur ing our project we co -operated with the width (vertical) archaeologists of the Museum. Because of the Minimum ang le step 0.01° aforementioned reasons, the outside excavated area – not width (horizontal) including the Museum and the ruin park – is usually available only for short time , so that it is very important Table 1. Technical specification for Riegl LMS -Z420i to scan every detail and plan the survey comprehensively. The typical objects are 1-2 meter high wall remains , In many cases the object has to be scanned from more murals, mortal remains, crocks, or mosaics. viewpo in ts because of hidden areas ; so after the scanning process the different scan positions are to be register ed 1.2 Archaeological survey with laserscanning into one project coordinate system. To make it easier, we use artificial control points , the so called reflectors. After The scanning was executed by a Riegl LMS -Z420i registration, the measurements were transformed in to the terrestrial laserscanner . This type of laserscanner can be Hungarian National Coordinate System ( EOV or HD -72). used with a 6 Megapixel Nikon D100 camera. Depending The scanner controller software allows transforming from the scanner coordinate system into the camera’s Before colouring the point cloud the images were coordinate system . T he camera position and mounting are manipulated to homogenous luminosity and d istur bing known during the image acquisition. details were removed . Because both lens are calibrated – Under these circumstances it was a good opportunity to the distortion parameters are known – the software can try orthophoto creation. In order to improve the results, it automatically create distor tion -free images for was useful to acquire more se ries of images with different orthophotos. settings ; although the scanne r can rotate 360 deg, photo s Due to the accuracy, the standard deviation of the against the sun couldn’t be take n. Preferably the measured point cloud i s at least 5 mm; consequently the orthophotos and the colour pointclouds must be thickness of the pointcloud is at leas t 1 cm. Applying esthetic al ly homogenous . more scan position, this value grows. In spite of this, the During the measurement’s planning, we took notice s of averaging leads to good results. The orthophoto creation many aspects . In case of nearly orthogonal scanning with resampling and pointcloud thinning method can get position, the multiple projections don’t really effect. better images. The difference appears mainly in edges. Therefore these positions are favourable; contain more In order to compute the true orthophoto Mat lab was used . details and less hidde n area s. After registration , the removal of the disturbing objects In every respect t he scanning resolution depend s on the e.g. pedestrians from the point cloud helps the modelling . scanned object and the distance from the scanner. The By the limitation of the examined field, the used database overview is executed by lower resolution , while the can be reduced and post -processing requires less archaeological ly important parts, e.g. brickworks by computer memory. higher one . In a roman era date d excavation the findings The modelling was created mostly in Cyclone and are wall ruins, bones, mor tal remains, murals. In case of CloudWorks software. detailed areas the importance of image resolution grows. In optimal case we can measure with 5 mm accuracy but some object s require more details. Due to the high 2. ARCHAEOLOGICAL DOCUMENTATION resolution of the acquired images, an orthophoto can FROM LASERSCANNING contain a lot of information. Making orthophoto is recommended for mapping little object s but it can’t work During evaluating , the surrou ndings can be represented in every cases . In the orthophoto some parts have by more details, not only the main characteristics. different luminosity. Sometimes this can lead to false Depending on the ground, triangulated surface model s are conclusion . Actually our researches focus on the easiest way s to create contour maps beside ground -plan described terrestrial laserscanner ; for higher accuracy and overviews. The goal of the survey in the ruin park close -range (object) scanners are suggested . was to comp lete the previous maps with more details and In our first project in the Aquincum Ruin Park, the aim precisely, as well as documenting the actual condition of was to map the wide -ranging reconstructed area . On e of the monuments. the most important features of the surveyed field is having antique wall ruins mostly under the ground . Therefore we can only mapping, check the reconstruction, complete the missing parts and improve the accuracy .. The possibility of panoramic scan ha s been proved to be useful, reducing t he time of measurement, and survey the whole neighbourhood . .3 Archaeological documentations As laserscanning became popular in many fields of scienc e, the archaeological view of the documentation has been examined . The traditional geodetic documentation contain s the following drawings: - Overview of the excavation (scale: M=1:100, 200, 500 etc. ), - Detailed ground plan (scale: M=1:10, 1:20) Figure 1: Ground -plan detail of the Ruin Park - Brickwork drawings ( scale 1:10 ). Budapest, Aquincum Museum The drawings had to be transforme d into EOV system ,. This transformation allowed the comparison between the The scanning procedure lasted 2 working days and during different mapping methods. For the map generalization this time 15 scan positions were used . The whole process we used the same legend . The documentation can be of evaluation including registration and pre-processing replaced by an orthophoto. Due to laserscanning, the took about 10 work day s. The main characteristic of the orthophoto is base d on the surface model, s o this process survey was that the ruins are located in a small field, and results true orthophoto, contrary to traditional there were many hidden parts. B rickwork drawings aren’t orthophotos that only eliminate the perspective distortion needed, because the original wall remains are usually of the images. under the ground . Although the visible ruins are mostly reconstructed, both the upper and the bottom edges was 1.4 Software for data p rocessing and modelling drown . In the middle of Figure 1 there is a sanctuary with origina l remains. The crumbled par ts are also evaluated. The control of the measurements and the pre-processing The top of the original wall ruins were signed to was practically made by the scanner software .