We present results of cloud catalogs of 12CO, 13CO, and C18O (J = 1–0) in a section of the third Galactic quadrant over (195° < l < 220°, ∣b∣ < 5°) from the Milky Way Imaging Scroll Painting project. The data were acquired with the PMO 13.7 m millimeter telescope with ∼50″angular resolution. We construct three molecular cloud catalogs containing information of 12CO, 13CO, and C18O from the position–position–velocity (PPV) data cubes. The 12CO cloud catalog contains 7069 samples identified based on the DBSCAN algorithm. We develop a new algorithm, the stacking bump algorithm, for identifying 13CO and C18O emission by searching for weak signals in the original spectra of 13CO and C18O within the boundary in PPV space defined by the 12CO cloud. Above the 2σ threshold level, we identified 1197 clouds having 13CO emission and 32 clouds having C18O emission. We test the stacking bump algorithm in the noise-only datacube and find that the 2σ threshold can effectively avoid the possibility of false detection generated by noise. The results proved that the new algorithm has high accuracy and completeness. Statistics of peak intensity, projected angular area, line width, and flux of the clouds show that the power-law indices obtained from different isotopic lines are close to each other.

Noise is a significant part within a millimeter-wave molecular line datacube. Analyzing the noise improves our understanding of noise characteristics, and further contributes to scientific discoveries. We measure the noise level of a single datacube from MWISP and perform statistical analyses. We identified major factors which increase the noise level of a single datacube, including bad channels, edge effects, baseline distortion and line contamination. Cleaning algorithms are applied to remove or reduce these noise components. As a result, we obtained the cleaned datacube in which noise follows a positively skewed normal distribution. We further analyzed the noise structure distribution of a 3D mosaicked datacube in the range l = 40 ⋅ ° 7 to 43 ⋅ ° 3 and b = − 2 ⋅ ° 3 to 0 ⋅ ° 3 and found that noise in the final mosaicked datacube is mainly characterized by noise fluctuation among the cells.