## 3 Measurement of CT point spacing in the glass cover  
To verify the accuracy of the glass cover, the confining pressure was set to 0 kPa, and the axial height and radial length of the triaxial model in the glass cover in air and after water injection were tested. The test values in air were taken as the true values, and the test errors were compared. The results after correction using the spatial refraction correction model are shown in Table 6. The upper, middle, and lower parts correspond to the three regions (I, II, and III) divided by the rubber membrane, as shown in Figure 11.  
According to Table 6, the influence of refraction on the radial direction is much greater than that on the axial direction. The maximum relative error before correction is 20.79%. However, the test value in the glass cover after the spatial correction model is close to the true value in the air, with an absolute error of less than 0.05 mm and an average relative error of only 0.18%. This fully demonstrates the effectiveness of the spatial refraction correction model under a fixed medium combination and greatly improves the accuracy of photogrammetry. Therefore, the photogrammetry method after refraction correction meets the accuracy requirements of the glass cover.  
Figure 11: Pasting of CT (Coded Targets) identification points and partition.  
\begin{table}
\begin{tabular}{c c c c c c} \hline
**Direction** & **Survey Number** & \begin{tabular}{c} **Caliper** \\ **/mm** \\ \end{tabular} & \begin{tabular}{c} **Photogrammetry** \\ **/mm** \\ \end{tabular} & \
\begin{tabular}{c} **Absolute Error** \\ **/mm** \\ \end{tabular} &
\begin{tabular}{c} **Relative Error** \\ **/\%** \\ \end{tabular} \\ \hline \multirow{4}{*}{Axial direction} & H-1 & 87.11 & 87.164 & 0.054 & 0.062 \\ \cline{2-6}  & H-2 & 87.10 & 86.995 & \(-\)0.105 & \(-\)0.121 \\ \cline{2-6}  & H-3 & 87.08 & 87.163 & 0.083 & 0.095 \\ \hline \multicolumn{2}{c}{Average} & & & 0.011 & 0.012 \\ \hline \multirow{4}{*}{Radial direction} & D-1 & 34.40 & 34.437 & 0.037 & 0.108 \\ \cline{2-6}  & D-2 & 34.40 & 34.425 & 0.025 & 0.073 \\ \cline{2-6}  & D-3 & 34.41 & 34.439 & 0.029 & 0.084 \\ \hline \multicolumn{2}{c}{Average} & & & 0.030 & 0.088 \\ \hline \end{tabular}
\end{table}
Table 5: Comparative analysis of surfaces using vernier calipers and photogrammetry.  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline
**Direction** & \begin{tabular}{c} **Measuring** \\ **Position** \\ \end{tabular} & \begin{tabular}{c} **Test Value** \\ **in Air/mm** \\ \end{tabular} & \begin{tabular}{c} **Test Value** \\ **in Glas** \\ **Cooverimm** \\ \end{tabular} & \begin{tabular}{c} **Magnification** \\ **Factor** \\ \end{tabular} & \begin{tabular}{c} **Correction** \\ **Value in Rear** \\ **Class Coverimm** \\ \end{tabular} & \begin{tabular}{c} **Correction** \\ **Value in Rear** \\ **Before Correction** \\ \end{tabular} & \
\begin{tabular}{c} **Absolute Error** \\ **/mm** \\ \end{tabular} &
\begin{tabular}{c} **Relative Error** \\ **/\%** \\ \end{tabular} \\ \hline Axial direction & I & 20.03 & 20.09 & 1.003 & 20.05 & 0.06 & 0.02 & 0.30 & 0.10 \\  & II & 39.96 & 40.03 & 1.002 & 0.998 & 39.95 & 0.07 & 0.01 & 0.18 & 0.03 \\  & III & 60.13 & 60.21 & 1.001 & & 60.09 & 0.08 & 0.04 & 0.13 & 0.07 \\  & I & 20.04 & 24.19 & 1.207 & & 20.06 & 4.15 & 0.02 & 20.71 & 0.10 \\ Radial direction & II & 20.06 & 24.23 & 1.208 & 0.829 & 20.09 & 4.17 & 0.03 & 20.79 & 0.17 \\  & III & 20.08 & 24.15 & 1.203 & & 20.03 & 4.07 & 0.05 & 20.27 & 0.26 \\  & & & & & & & & & \\ \multicolumn{10}{c}{Radial corrected mean value} & & & & & & & & \\ \hline \hline \end{tabular}
\end{table}
Table 6: Axial and radial measurement values after spatial refraction correction.