INTEGRATION OF A TERRESTRIAL LASER SCANNER WITH GPS/IMU ORIENTATION SENSORS

Transcription

1 INTEGRATION OF A TERRESTRIAL LASER SCANNER WITH GPS/IMU ORIENTATION SENSORS J.Taaya, R.Aaus, E.Bosch, A.Serra, W.Kornus, A.Baron Institut Cartogràfic de Cataunya (ICC), Parc de Montjuïc, E Barceona taaya@icc.es, raaus@icc.es, eosch@icc.es, aserra@icc.es,wkornus@icc.es, aaron@icc.es Ths 17 Cose-Range Integrated Mapping Systes/Laser Scanning KEY WORDS: Integration, Orientation, GPS/IMU, Terrestria, Laser scanning ABSTRACT: Direct orientation of aser systes has een widey used for airorne aser sensors ut not with the terrestria aser systes. The usua way to operate a terrestria aser is y scanning a scene whie the sensor reains static. In order to cover the whoe scene, different scans can e coined y atching severa coon points, finay the orientation of the scene is perfored y identifying and providing coordinates for a iniu of 3 points. This procedure is very tie consuing eading to a very ow productivity. In order to increase productivity the Institut Cartografic de Cataunya (ICC) has integrated a terrestria aser scanner in a oie vehice with the ai to operating the aser whie the vehice is oving. This paper descries the integration of a terrestria aser scanner with the GPS/IMU orientation sensors of a Land Based Moie Mapping Syste. The aser puses are synchronized to the GPS tie y using a odified PPS signa fro a GPS receiver. In order to transfer the reference frae fro the GPS/IMU sensors to the aser sensor a nuer of cairation scans and contro points are used and the offset and isaignent etween the aser and GPS/IMU sensors is deterined. The resuts of the cairation procedures, as we as accuracies and perforance otained y the integrated GPS/IMU/terrestria aser syste are presented in the paper. 1. INTRODUCTION During the ast years the use of terrestria aser scanners for 3D odeing has widey een expanded. These systes provide direct easureents of couds of points that are iuinated y the aser. A terrestria aser scanner generay coects the data y easuring a copete scene whie the sensor reains static. Thereafter, the scene has to e georeferenced y identifying at east 3 points on the aser scanner iage and giving the ground coordinates. In order to iprove the productivity the ICC decided to directy orient the aser scenes y integrating a terrestria aser scanner with a GPS/IMU (Inertia Measureent Unit) orientation syste in a oie vehice with the ai to operate the aser whie the vehice is oving. That integration aowed a direct orientation of the aser scene reducing draaticay the tie required to otain a geocoded scene ready to extract inforation. 2. GEOMÒBIL In the first stage, the Geoòi (see figure 1) integrated two CCD caeras in a van to capture stereoscopic pairs of digita iages and a the equipent required to georeference directy the coected iages. The van was deveoped as a oduar syste that shoud e easiy upgradeae with new sensors. Hence, the Geoòi is coposed of different susystes: an iage susyste, a synchronization susyste, an orientation susyste, a power suppy susyste, a data storage susyste and a ciatization susyste. The integration of airorne asers with GPS/IMU orientation systes has een widey used since id 90 s providing good resuts and eading to a wide expoit of the airorne idar technique (Lindenerger 1993). However, terrestria asers have not foowed the sae path and they are sedo directy oriented y a GPS/IMU syste. The integration of the terrestria aser with the GPS/IMU sensors was done in the frae of the Geoòi project (Taaya et a. 2004), the oie apping syste deveoped at ICC. Figure 1: Geoòi syste deveoped at ICC 19

2 The orientation susyste is ainy ased on the integration of a GPS receiver and an IMU sensor. However, due to the arge nuer of GPS outages that occur in a terrestria capaign, it aso incudes the integration of a DMI sensor (Distance Measureent Indicator), which is used to reduce IMU drifts during these outages. The orientation susyste is copeted with a GAMS syste (GPS Aziuth Measureent Syste), an approach fro Appanix ased on the heading deterination using two GPS antennas ounted on the top of the van, which aows a rapid heading correction after GPS outages. In order to correcty transfer the reference frae fro the orientation susyste to the CCD caeras, a rigid structure was designed (Figure 2). The reaction of the structure under different forces was odeed and the atest design showed deforations of ess than 1 in distance and ess than 70 arcseconds in ange. The structure acted as a patfor for integrating the sensors used for the orientation and any other sensor ounted on it. In particuar, the integration of the digita caeras and the GPS/IMU orientation susyste ead to very good resuts, photograetric points are deterined with accuracies etter than 5 c in across track directions and 13 c in aong track direction at an average distance of 18 (Aaús et a, 2004). the distance easured y the aser, φ the rotation ange of the irror and ϕ the aser position ange during the scanning (see figure 3). φ Figure 3: Terrestria aser spherica coordinate frae (courtesy of Rieg LMS GH) The precise orientation coputed y the orientation susyste (GPS/IMU) can e transferred to any sensor (in particuar to the terrestria aser) ounted on the patfor. Thus, the aser data can e directy oriented appying the sae principe used for airorne asers. Figure 4: Static terrestria aser scene coded with a coination of intensity and distance. The rotating irror scans the scene in vertica ines whie the aser rotates around a vertica axis covering the scene in the horizonta direction The transforation fro the aser spherica coordinate frae to a aser cartesian coordinate frae is given y foowing equation: x = r sinφ y = r sinφ sinϕ z = r cosφ Figure 2: Geoòi integration patfor (response to stress) 3. INTEGRATION OF A LASER SCANNING The aser seected for the integration was a Rieg Z-210 that is ae to coect up to points per second. For each point a distance easureent, an intensity vaue and RGB data is coected. The aser has a rotating irror that aows taking vertica profies whie a servootor rotates horizontay the whoe aser for scanning a static scene (see figures 3 and 4). For each aser point aso the ange readings of the irror and the scan encoders are otained. These anguar vaues, together with the distance easureent, are used to ocate the easured point in a oca aser reference frae. The raw data coected y a terrestria aser are usuay paraeterized in a spherica coordinates frae, denoting r Equation 1: Preiinary transforation fro the aser spherica coordinate frae to a aser cartesian coordinate frae By construction the aser axes are not perfecty aigned, the irror rotation axis (φ ange) and the aser scanning axis (ϕ ange) do not intersect, so this difference has to e corrected in order to transfer the initia aser spherica coordinates to a oca aser cartesian coordinates frae. In equation 2 the foruas used y the aser software (Rieg, 2001) to perfor the transforation are descried. Assuing that the origin of the aser cartesian frae ies on the axis defined y the rotating irror (φ ange), r o is an offset on the distance easureent and ϕ d is the isaignent of the scanner rotation axis: 20

3 x = r cosϕ e e o y = r sinϕ o X = r sinφ + xe + ye sinϕ Y = r sinφ sinϕ + xe sinϕ ye Z = r cosφ Equation 2: Fina transforation fro the aser spherica coordinate frae to a aser cartesian coordinate frae If the aser reains static whie coecting a scene it woud e easy to deterine the transforation to a apping reference frae y easuring soe contro points. Once a goa transation (X T,Y T,Z T ) and a rotation atrix deterined every aser point (X,Y,Z ) on the scene can e transfored y appying the sae function (see equation 3). X Y Z X = Y Z T T T transation d d + M X Y Z Equation 3: Transforation etween the aser cartesian coordinate frae to a apping cartesian coordinate frae (static case) This transforation can aso e deterined directy y the GPS/IMU susyste using the foruas descried in equation 4. According to the expanations in the previous paragraphs, the aser was rigidy ounted on the integration patfor (figure 5). Assuing that the GPS/IMU systes are capae to deterine the orientation of the integration patfor at any oent, the transfer of the reference frae fro the GPS/IMU to the aser can e done as ong as the aser data is synchronized with the GPS/IMU oservations and the spatia transforation etween the GPS/IMU frae and the aser frae is known, i.e. deterined in a cairation procedure. Laser M are interna cock can e reset y an externa TTL signa. By using this possiiity a odified PPS signa is pseudorandoy sent to the aser. After the survey, at the office, a software synchronizes the aser interna tie syste to a goa GPS tie syste y coparing the pseudorandoy resets of the aser interna cock to the previousy stored tie at which the TTL signas were generated. Knowing the GPS tie at the start of each ine, the GPS tie of every aser point is coputed y adding the aser repetition period to the tie of the previous aser point. The orientation susyste of the Geoòi aows a continuous deterination of the transforation etween the IMU reference frae to the apping reference frae. Once every aser point is tie aeed in GPS tie the transforation etween the aser reference frae to the apping reference frae (e.g. WGS84) can e done according to equation 4 in two steps: a) transforation fro the aser to the IMU reference frae and ) transforation fro the IMU to the apping reference frae. The first step coprises the offset deterination etween the aser and the IMU reference frae ( V in equation 4) and the isaignent atrix etween aser and the IMU reference frae ( M in equation 4). The offset and the isaignent atrix ( V and M ) reain constant as ong as oth systes (IMU and aser) are rigidy ounted on the integration patfor and the patfor does not have any distortion due to the stress. As expained in the next section these constant vaues are deterined in a cairation survey. In a second step the rotation atrix ( M in equation 4) and the transation vector (X GPS/IMU, Y GPS/IMU, Z GPS/IMU in equation 4) are deterined using the integration of the GPS/IMU oservations. Notice that the rotation and transation atrix appied in the second step are not constant and keep varying during the survey. Therefore, for each aser point a different transation and rotation atrix wi e derived fro the coputed trajectory. X Y Z X = Y Z GPS / IMU GPS / IMU GPS / IMU + M X M Y + V Z Equation 4: Transforation etween the aser cartesian coordinate frae to a apping cartesian coordinate frae (kineatic case) The use of integrated GPS/IMU data to directy orient aser data has the advantage that the transforation etween the oca aser reference frae and the apping reference frae is known at any oent (as far as the aser is synchronized), independenty if the aser is coecting data in a static ode or in kineatic ode. Therefore, the aser can e used as a pushroo sensor whie fixing the scan ange and sweeping the scene with profies whie the vehice is in oveent. Figure 5: Terrestria Laser integrated in the Geoòi The aser aes the eginning of each ine with a precise interna cock. The synchronization is perfored y reating the aser tie syste to the GPS tie syste. The aser 3.1 Cairation The offset and the isaignent atrix are deterined y appying equation 4 to a set of static aser scenes where 21

4 severa ground contro points have een oserved and y a fina adjustent. A nuer of previousy surveyed points were signaised with refecting ateria, which aows a good identification of the ground contro points in the aser intensity iage (see second iage on figure 6). Severa scenes were captured whie the van was stationary and the GPS/IMU sensors were coecting data. Afterwards, an adjustent was done for coputing the offset and the isaignent atrix. The transation and rotation atrix (X GPS/IMU, Y GPS/IMU, Z GPS/IMU ) and M were derived for the GPS/IMU trajectory and used as oservations in the adjustent. The easureents of the signaised points in the intensity iage were aso used as oservations. In order to iprove the deterination of the paraeters and to decoupe their correation four scenes taken with different aziuth anges were used in the adjustent. Figure 7 shows the distriution of the adjustent with the red eipses indicating the position and fora error of the IMU orientation, the ue eipses indicating the position and fora error of the aser orientation and the very sa ack eipses showing the position and fora error of the 15 signaised points used (notice that soe of the points differ ony in the vertica coordinates and therefore, are shown superiposed in figure 7). The ack ines ink the aser position with the ground contro points and syoise the aser iage easureents of the signaised points. The resuts and the fora accuracies of the adjustent are presented in tae 1. vaue σ offset Misaignent anges X Y Z ω ϕ κ Figure 6: Cairation scene. Coination of intensity and distance iage (aove), intensity iage (centre) and RGB iage (eow) (signaised points are easiy identified in the intensity iage) Tae 1: Deterination of the offset and isaignent atrix in the adjustent 4. DYNAMIC LASER SCANNING Once the aser was synchronized to GPS tie and the constant offset and isaignent atrix coputed y using the ethod descried in the previous section, it was possie to use the terrestria aser as a pushroo sensor y fixing the scan ange and sweeping the scene with profies whie the vehice is in oveent. Every singe point easured y the aser is transfored to the apping reference frae y using its corresponding orientation and appying the foruas descried in equation 4. d = d 2d 3d Figure 7: Laser ounting cairation: adjustent of four aser static scenes. Red: position of the GPS/IMU sensors. Bue: position of the terrestria aser. Back: contro point easured in the aser scenes Figures 8 and 9 show the aser RGB iages coected in a kineatic ode. The aser was verticay ounted on the integration patfor and the rotating irror was perforing vertica scans ooking at the right side of the van whie it was oving. In figure 8 the exterior uidings of a roundaout 22

5 can e oserved whie in figure 9 soe cars can e identified parked on the street. These iages show the oriented raw aser data represented in a apping reference frae. In order to otain usefu inforation these data have to e fitered and edited e.g. for extracting and odeing the uidings represented in the iages. Figure 10: Intensity iage fro a train overhead power cae (aser ooking up) Figure 8: RGB aser points coected in a kineatic survey (aser ooking to the right side of the van) It is we known that the deterination of the trajectory of a van inside a city is very difficut due to the constant GPS outages. Therefore, the coparison was done on those parts of the trajectory where an acceptae orientation was coputed. Fro a aser point coud potted together with the 1:1000 ine ap ten we defined check points were easured. Figure 11 iustrates the seection of one we defined point at a uiding roof corner. The coordinates of the aser points were copared to the point ap coordinates. Tae 2 shows the coparison of the ten check points identified in the 1:1000 ine ap and in the aser point coud. Figure 9: RGB aser points coected in a kineatic survey (aser ooking to the right side of the van) The aser can aso e ounted horizontay. In this configuration the rotation irror perfors horizonta scans perpendicuar to the direction of the van. This configuration has two ain appications: a) whie the scanner is ooking down it can e used to ode the road surface and ) whie the scanner is ooking up it can e used to ode aeria infrastructures. Figure 10 shows an exape of an horizonta scanner. In that case the scanner was ooking up whie the van was ounted on a train and the train overhead power cae was surveyed. 4.1 Coparison with 1:1000 city ap An uran survey was carried out in kineatic ode with the aser verticay ounted (as shown in figure 5) and perforing vertica scans of the uidings façades. After orienting the aser points using the ethod descried in this paper, the point couds were potted together with the avaiae city ap that have an accuracy of 20 c (1.64 σ) per coponent. Point-ID de dn dh [] MIN. MEAN MAX. RANGE RMS. σ [] de dn dh Tae 2: Differences of coordinates at the 10 check points (aser coordinates ap coordinates) As can e oserved the resuts are on the eve of 0.18 in Easting, 0.35 in Northing and 0.13 in the vertica coponent. As the path studied is not very ong the systeatic difference oserved in the North direction is assued to e caused y a reaining error in the trajectory deterination. 23

6 The direct orientation of a terrestria aser opens the possiiity to use the sensor as a pushroo sensor that is ae to scan crossroad sections and uidings façades whie the vehice is oving. Assuing a good GPS coverage and a good orientation deterination, the dynaic aser survey showed RMS differences of 0.18 in Easting, 0.35 in Northing and 0.13 in the vertica coponent copared to 1:1000 city aps. In order to fuy expoit the integration of a terrestria aser scanner in a oie apping frae further deveopents are necessary incuding uiding reconstruction y processing the aser point coud and iproving the orientation accuracy in uran areas. REFERENCES Figure 11: Coparison of 1:1000 ap with the aser points. in RGB (aove) and coour coded eevation representation (eow) 5. SUMMARY This report descries the integration of a terrestria aser scanner on the Geoòi oie apping syste deveoped at ICC. The critica points of the integration are descried, incuding the aser synchronization and the syste cairation. The cairation procedure descried in the paper aowed the deterination of the IMU/aser offset with a fora accuracy of and the deterination of the isaignent atrix with a fora accuracy of arcseconds. Aaús,R., Baron,A., Bosch,E., Casacuerta,J., Miranda,J., Pa,M., Sànchez,S., Serra,A., Taaya,J., On the accuray and perforance of the Geoòi syste. XXth ISPRS Congress Geo-Iagery Bridging Continents, Juy Istanu, Turkey. Lindenerger,J., Laser-Profiessung zur topographischen Geändeaufnahe. PhD dissertation, Deutsche Geodätische Koission ei der Bayerischen Akadeie der Wissenschaften, Reihe C, Heft Nr Rieg Laser Measureents Systes GH Laser Mirror Scanner LMS-Z210 Technica docuentation. and User s Instructions. Taaya,J. Bosch,E., Aaus,R., Serra,A., Baron,A., Geoòi: the Moie Mapping Syste fro the ICC 4 th Internationa Syposiu on Moie Mapping Technoogy (MMT 2004) March, Kuning, China. 24