DIGITAL TERRAIN MODEL USING LASER SCANNING IN THE NEAPOLITAN VOLCANIC AREA Achilli V. (1), Al-Bayari O. (2), Borgström S. (3), Del Gaudio C. (3), De Martino P. (3), Dubbini M. (2), Fabris M. (1), Marzocchi W. (3), Menin A. (1), Ricciardi G.P. (3), Ricco C. (3), Salemi G. (1), Sepe V. (3), Siniscalchi V. (3), Trevisan R. (1) KEY WORDS: Laser Scanning, GPS, DTM (1) DAUR - University of Padua (2) DISTART - University of Bologna (3) Osservatorio Vesuviano - Naples omar.al-bayari@mail.ing.unibo.it ABSTRACT The Neapolitan volcanic district is one of the most important volcanic areas in Italy; for this reason, it is regularly monitored using different geodetical and surveying methods. The aim of using laser scanning in this area is to evaluate the possibilities to get a good quality DTM (Digital Terrain Model) for different applications. Therefore, some experiments have been carried out over the area. The first experiment was performed on the Phlegrean Fields area, where strong vertical deformations have been measured in the past; the second one was carried out on the northern part of Ischia Island, where small movements have been detected. The DTM and the final results will be hereby presented and discussed; the precision obtained is verified by a comparison with other techniques (GPS measurements and spirit levelling data). 1 INTRODUCTION Having a good quality DTM in Neapolitan area is coming out from the importance of geological morphology and environmental risks of zone. The area is considered one of the more active volcanic zone in Italy and it has monitored for long years. Airborne laser scanning, recently, is one of the best method to obtain a reliable DTM with precision could reach one decimetre. The system mainly consist of two sensor group: the first is Laser Range Finder and its used for measuring the distance from the sensor to the terrain surface. GPS and INS (Inertial Navigation System) constitute the second group and determine the position and altitude of the Laser Range Finder at the time of measurements. Many factors could affect the precision of the laser system used and, consequently, the precision of the derived DTM. The main factors are: 1 - The range accuracy which mainly could affected by: - Non-parallel alignment of the send and receive parts of the sensor. - Inaccuracies in the measurement of the pulse path time. - Variation in the rotation speed or oscillation of the mirror. 2 - Position accuracy which depends on many factors such as satellite configuration, multipath and distance between reference-rover receivers. 3 - Altitude accuracy, which is related to INS quality and could be affected by: - alignment errors. - impurity in the accelerometer. 4 - Time offsets, to obtain a good and accurate three dimensional positioning. Orientation, position and range are required to be taken at the same time in the same coordinate time system (all measurements should be synchronised). 5 - Coordinate system, which depends on the coordinates transformation between WGS84 to National or local system and also on the geoid measurements. Naturally, there are more complex relations among these factors and the flight height, the scan angle, the terrain topography, the land cover and the control points used for coordinates transformation. Due to all above mentioned factors, and new prospect field problems it has been mandatory to perform some experiments at selected areas before performing entire surveying of whole area. The aim of these experiments is to evaluate the DTM precision using data acquired at different heights above the ground and estimate the economic aspect of the method (cost/time factor).
2 LASER SCANNER SYSTEM TopEye laser system has been used during the experiments, the system is mounted on the Helicopter which gives possibility to perform different types of surveying at different altitude with different characteristic such as density of points per meter squared, different scan angles etc. The system scan the ground across the track of the Helicopter and measure the distance to the ground with up to 7000 laser pulse per second. The system could record four echoes for each laser pulses. Recording different echoes for a single pulses help to identify the heights of the objects on the terrain, such as building, trees, power lines...etc. The TopEye system has two working modes, named FLA and FL2. In the FLA the laser collect First echo, Last echo and Amplitudes, in FL2 mode the laser collects first, 2 nd, 3 rd and last echoes plus their amplitudes. It is advantageous to use FLA- mode wherever possible as this gives higher laser pulse frequency and smaller raw data files. The main characteristic of TopEye system could be seen in table 1 and the principle of surveying in figure 1. Laser Range finder: PRF 7000 Hz Returns echoes 4 with 2m object separation Strength: 128 levels Beam divergence 1 or 2 or 4 mrad Swath 20º stabilized 20-340 m 40º Not stabilized 40-680 m Absolute accuracy: 1 σ depending altitude above ground 10-30 cm VideoCameras: Sony Hi 8 PAL or NTCS INS: System H-764 GPS: Trimble 4700 Pilot Guidance System Data Storage: Exabyte Mount External POD and cabinets in cabin Camera digitale Hasslblad phase I Table 1 - Characteristics of TopEye Laser System Speed: 36 90 km/h Altitude: 60 960 m Swath: 20 680 m = 2d d = 0.25 4.00 m Laser footprint diameter: 0.1 3.8 m Fig. 1 The Laser Survey method The TopEye system, additionally, has a digital Hasselblad-camera, calibrated at laboratory, which could be used to produce geocoded and mosaiced photos using altitude of camera and laser data. The pixel size is possible to vary between 2cm to 20cm covering an area of 40x60 m 2 to 400x600 m 2 depending on flight height.
3 EXPERIMENTS AND DATA PROCESSING The experiments have been performed at two areas: the first survey was at the Phlegrean Fields area (figure 2) with a flight height of 195m above ground; this area has had large vertical movements during last years. The second experiment has been done at Ischia Island (figure 3), with performing survey at different height above ground (750m and 195 m), which means different sensor settings for the strip wide and the density of captured points. The selected areas have very different characteristics (morphology, vegetation, building, versant, ). Fig. 2 - The Geodetic Surveillance Network at the Phlegrean Fields area Fig. 3 - Ischia Island 3.1 Planning of Surveying The survey with TopEye system should be planned using the specific programm created by Spectra Precision terrasat GmbH for the TopEye system. The coordinate of flight line should be in WGS84 system and the positioning comes out from GPS satellites in real time during the survey. But, the designed project is in national coordinate system, therefore the coordinates should be transformed from national system to WGS84 system. TopEye Mission Planning Software (MPS) has the possibility to transform the coordinates from Italian national system to WGS84 system, but with low precision, which could cause flaying far-off the desired area more than hundred meters. To overcome this problem, the IGM95 transformation parameters, published by IGM (Military Geographical Institute), or common known points
between two systems could be used. In figure 4 is shown the planning of the project at Phlegrean Fields area, with 10 flight lines planned and 10% lateral coverage. Fig. 4 - Planned flight lines at the Phlegrean Fields area 3.2 Data Processing The first step of elaboration laser data is the processing of GPS data to obtain the helicopter trajectory; the software used was the GeoGenius. The second step is the laser data processing using the TopEye software. This programm uses the GPS solution (position of helicopter at 1 Hz), the INS data (altitude at 50Hz), the laser range and mirror scan angles to calculate the coordinates of the laser foot print on the ground for each laser pulse. The final position is given in WGS84 geocentric coordinates. 4 ANALYSIS OF RESULTS During the surveying, the laser system receives the echoes of laser pulse reflected from any object in its path, without any distinction among objects on the ground or above the ground; so, the initial data points set (DTM) will indicate all ground and non-ground objects together. The Terrascan software by Terra Solid was used to classify the captured data. 4.1 Reference Stations From previous experiences related to the reference stations number used in the airborne laser scanning surveys, its mandatory to use at least two reference stations for each project, if the surveyed area it isn t so wide. But if the flight line extend more than 25km the reference stations used should cover the project area, taking in consideration that the distance between reference stations should not exceed 25km. As a test in these experiments (figure 5), five reference stations have been used: MORT and MEZZ at Ischia Island, RITE and POSO at Phlegrean Fields, and then all four stations have been connected to ACEA permanent station which has known coordinates in WGS84 system. The four reference stations have been processed in static mode respect to ACEA permanent station to obtain a good set of coordinates in WGS84 system. The helicopter trajectory has been determined by GeoGenius software respect to all reference stations. We didn t found differences in kinematic results respect to stations in each area. But some differences have been found in trajectory results respect to stations in different areas (figure 6). According to these results, the distance between the reference station and the flight line should not exceed 25km, since the ambiguity resolution could be influenced by the ionosphere, the satellite geometry and the multipath.
Fig. 5 - Reference stations at surveyed areas, and the flight lines of the Helicopter trajectory during perform the survey. (distance between two areas is about 25 km) (m) 0.15 0.1 0.05 0-0.05-0.1-0.15-0.2 1 398 795 1192 diff. of Height 1589 1986 2383 2780 3177 3574 epoches Fig. 6 - Differences in the height component between two kinematic GPS solutions. The elaboration is related at two reference stations with an inter-distance of about 20km. 3971 4368 4765 5162 5559 5956 4.2 Lateral Coverage Problem The control of captured data is an important step to identify lateral gaps between some strips, considering that 10% lateral coverage parameter was used in the mission planning (figure 7). The result is the 10% lateral coverage it isn t enough to cover the gap between strips due to the helicopter tilt (roll) could be more than 4 during survey, depending on the weather conditions. So, with 4 inclination angle the lateral coverage should be more than 15%.
Fig. 7 - Lateral gaps between some strips due to helicopter tilt (roll) during survey, with a 10% lateral coverage. 4.3 Cost/Time Factor and Precision The cost/time factor of airborne laser survey depends on many factors like the flight height, the scan angle, the helicopter speed, the helicopter transfer time to the project area, the manoeuvre time and so on. All these factors affect the strip wide and consequently the density of captured points. Large strip mean a minor survey time with a minor cost, but, of course, a minor precision in the final DTM due to large foot print on the ground with a low points density. As example of survey time, the Phlegrean area (figure 4) was laser scanned with a time of 41 minutes to perform 10 flight lines with their manoeuvre time. But it needs 16 minutes to perform the survey over the same area at 750m flight height, strip wide 540 and same helicopter speed. To verify the difference of acquired laser data at different heights of the helicopter, a common flat area (wide roof building) has been selected and two laser strips at different altitudes have performed. Using the Terrascan software a side view section has been made and the difference between two results is computed as about 10 cm (figure 8). Fig. 8 - Difference in the height component between two different flight heights as show in a view section in Terrascan at selected flat area. Regarding the precision obtained by airborne laser system, the interest has been focused on the height component, by comparing the laser captured points in a flat area to some known points measured by GPS. The precision obtained
could be considered as an index of the method reliability vs. other surveying technique. In table 2 it has shown the difference between the known GPS points and the average of the height component of the laser point strikes around the GPS vertices. GPS points Laser Points at Height 195m dh (m) dh (m) North East Height North East Height 195m 750m _47.406 _24.960 102.776 _47.29 _25.00 102.76 0.02 0.09 _47.30 24.82 102.81-0.03 0.02 _47.43 _24.80 102.80-0.02 0.10 _14.584 _82.637 103.702 _14.63 _82.86 103.74-0.04-0.04 _14.43 _82.86 103.73-0.03-0.06 _14.52 _82.19 103.73-0.03-0.09 Table 2 - Differences in the height component between GPS points and laser strikes around them, with surveying at different flight heights (195m and 750m). 4.4 Classification and Transformation Finally the laser data has been loaded in Terrascan programm, consequently, points have been filtered and classified as ground and non-ground points. The steps for the classification could be summarised as following : - acquiring laser points; - eliminating error points using low points classification tool; - classifying ground (depending on the terrain morphology); - classifying points by height from ground; - reducing the number of points with thinning tool to create the surface model. In figures 9 and 10 are shown the surface model created using the Terra Modeler program after strips classification. Fig. 9 - Surface rendering at Ischia Island using some strips with the Terrascan program. The main problem encountered in this work is the comparison between the points transformed to the local system to points with known orthometric heights. The discrepancy is more than 20cm in some cases. This fact could be coming out from the transformation problem between the WGS84 system to local system, the insufficient number of known points and the not regularly distribution around the surveyed area, the planimetric coordinates difference, whereas known points doesn't match exactly the laser strikes, and finally the discarding the irregular geoid undulation in the area.
Fig. 10 - Contour lines at Ischia Island as computed by Terra Modeller programm. 5 DISCUSSION In this work some experiments performed by airborne laser scanner at Neapolitan area have been discussed. Consequently some important points have been noted: - the lateral coverage used in laser strips should be taken in consideration to be at least 15%, due to the wind condition during the helicopter surveying; - the precision of 10-20 cm of the obtained DTM is good enough for many engineering applications and it depends on the flight height; - the performed surveying at 750 m with 540 m wide strip and 1point/m 2, reduces the time survey and consequently the total cost. - the transformation of the ellipsoidal height to orthometric height should be examined carefully taking in consideration all factors that could affect the result such as the distribution of control points around the interested area and the geoid undulation. ACKNOWLEDGMENT Thanks to AUSELDA AED GROUP for Support and help of this work. REFERENCES Al-Bayari, O. (2000): Some Problems in Kinematic Airborne Laser Survey, Reports on Geodesy 6 th Millennium Meeting Poland-Italy. Geodetic Axelsson, P. (2000): DEM Generation from Laser Scanner data using Adaptive TIN Models, International Archives of Photogrammetry and Remot sensing. Vol. XXXIII, Part B4. Amsterdam 2000. Kraus K., Pfeiefer N. (1998): Determination of Terrain Models in Wooded Areas with Airborne laser Scanner data, ISPRS Jornal of Photogrammetry and Remote Sensing, Vol 53 No 4, pp 193-203. Kilian J., Haala N, Englich M.1(996): Capture and Evaluation of airborne laser scanner data, Int. Arch. Photogramm. Remote Sensing XXXI, 383-388, part B3. http://www.combitech.se/survey/topeye/ http://www.terrasolid.fi - Documentation of terra Applications for Environmental Engineering.