全景相机因其360°大视场、旋转不变性等优点,逐渐被用于构建车载移动测图系统.标定是保证系统获取精确地理信息数据的重要前提.本文针对全景相机和定位定姿系统(POS)集成的车载移动测图系统,提出一种外方位元素标定的方法.首先,在实际场景中布设高精度已知控制点.其次,构建全景球面模型,将全景影像通过球面投影反变换投影到该球面上,从球面上选择控制点而不是直接从存在扭曲的全景影像上选择控制点并得到其球面坐标.在建立点的相关性之后,结合地理参考绝对定位方程和坐标变换,求得全景相机相对POS的平移与旋转参数.最后,采用本文提出的标定方法,分别选择北京航天城和天津滨海新区进行试验.试验表明,GPS信号良好时,点的绝对定位中误差可达平面10.3 cm、高程16.5 cm;GPS信号不好时,点的绝对定位中误差为平面35.4 cm、高程54.8 cm;在较短距离范围内(<3 km),距离量测相对误差最大为5 cm左右,GPS信号对相对量测没有明显影响.
Having the advantage of 360° imaging and rotation invariance, panoramic camera has gradually been used in mobile mapping systems(MMS). Calibration is an essential requirement to make sure that MMS can get high quality geo-information. This paper presents a way to address the extrinsic calibration for vehicle-based MMS composed of panoramic camera and Position and Orientation System (POS). Firstly, control points in the natural scene are set up, whose spatial coordinates are measured with high precision. Secondly, a panoramic spherical model is constructed and panoramic image can be projected to this model by means of spherical reverse transformation projection. Then, localize and select the control points in 3D spherical panoramic view but not in panoramic distorted image directly, the spherical coordinates of control points in panoramic image are gotten. After points correspondence is established, make use of direct geo-reference positioning equation and coordinate transformation, the translation and rotation parameters of panoramic camera relative to POS are computed. Experiments are conducted separately in space city calibration site located in Beijing and the Binhai New Area in Tianjin using our approach. Test results are listed as follows. When the GPS signal are of good quality, absolute positioning mean square error of a point is 10.3 cm in two-dimension plane and 16.5 cm in height direction; Otherwise, it is 35.4 cm in two-dimension plane and 54.8 cm in height direction. The max relative error of distance measurement is about 5 cm over a short distance (distance<3 km), which is not obviously affected by the GPS signal quality.
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