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A Visibility Estimation Method Based on Digital Total-sky Images
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摘要: 基于全视野数字图像的能见度估算方法是将全视野图像大气透射率与大气能见度通过曲线拟合方法建立联系，得到估算模型进行能见度估算。首先通过历史全视野图像和对应时刻的大气能见度建立样本数据集，再利用暗原色先验理论获取全视野图像大气透射率，通过多项式拟合方法建立离散样本数据集中大气透射率与对应时刻大气能见度的关系，即全视野图像能见度估算模型，最终将所需估算时刻的数字图像输入估算模型进行能见度估算，得到大气能见度数值。结果表明：建立的基于全视野数字图像能见度估算模型在环境光照均匀时大气能见度估算值与能见度实测值有较好的一致性，可作为前向散射仪的补充观测方法，弥补其采样存在的局限性。Abstract: The proposed visibility estimation method is a curve fitting algorithm, which establishes a relation between the image's atmospheric transmittance and atmospheric visibility. Firstly, the total-sky image is captured by the digital total-sky image visibility experimental platform. The core unit of this platform is a digital camera equipped with a fisheye lens, and the camera is placed vertically towards the sky. The platform can collect a total-sky image at specified time interval, and then the original image is transferred to a computer for image processing. In particular, the total-sky image needs to be converted into a panoramic image using an image calibration algorithm, and the panoramic image contains most of the near-surface image information. Next, the panoramic image is used to compute the visibility. Some visibility-related image features are extracted from the panoramic image firstly. The image's atmospheric transmittance can be calculated using dark channel prior theory. The relationship between the atmospheric transmittance and atmospheric visibility can be established by curve fitting method, and the initial visibility estimate model based on total-sky images is achieved. The model can be improved by combining a number of field experiments. Finally, the retrieved visibility is calculated by importing the real-time total-sky image into the model.Results show that the basic trend of visibility data from total-sky visibility estimation model is consistent with that of the forward scattering visibility meter through the comparative test and calculating correlation coefficients. The trend is most noticeable in low or medium visibility. However, as the visibility increases, the consistency decreases because of more fluctuation. As the forward scattering visibility meter used to establish the model whose measuring range is from 0 to 35 km, estimate model results are generally less than the measurement of forward scattering visibility meter especially when the visibility is high. In general, the basic trend of visibility data of total-sky visibility estimation model is consistent with that of the forward scattering visibility meter when the global atmospheric light is well-distributed and there is no underexposure or overexpose. The correlation coefficient between results of two methods is close to 1, which also means that the consistency between the two methods is good. In addition, the image features used in this method do not depend on a certain point in the image, nor are they limited to a certain range of visual distance. At the same time, there is no need to use manually set target or to fix a particular building, which makes it easier for observers to select the appropriate direction to measure visibility accurately. The proposed method has advantages of high measurement accuracy and large sampling range and can be used as a supplementary observation method of the traditional forward scattering visibility meter.
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Key words:
- total-sky image;
- dark channel prior;
- digital image processing;
- visibility
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图 7 图像大气透射率与大气能见度拟合结果
Fig. 7 Fitting result of image transmission and atmospheric visibility
图 8 2016年7—10月能见度估算值与实测值对比
(a)所有结果对比, (b)低能见度结果对比, (c)中能见度结果对比, (d)高能见度结果对比, (e)夏季结果对比, (f)秋季结果对比
Fig. 8 Comparison diagram between estimated value and measured value from Jul 2016 to Oct 2016
(a)comparison diagram of all results, (b)low visibility comparison diagram, (c)medium visibility comparison diagram, (d)high visibility comparison diagram, (e)comparison diagram of results in summer, (f)comparison diagram of results in autumn
表 1 能见度估算值与实测值对比结果
Table 1 Comparison between estimated value and measured value
参数 所有时段 低能见度 中能见度 高能见度 夏季对比 秋季对比 平均偏差/m -615.1 130.8 -311.3 -2230.7 -636.8 -570.7 相对平均偏差/% -2.58 11.65 -6 -8.34 -3.25 -1.22 均方根误差/m 2564.8 798.2 1417.6 5029.2 2497.1 2697.7 均方根相对误差/% 36.98 58.83 29.38 22.90 41.06 26.78 相关系数 0.8736 0.3670 0.7717 0.8466 0.8587 0.8862 注：表中相关系数均达到0.001显著性水平。 
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