Several good papers about the monocular depth estimation. Supervised Depth Estimator:

Using depth ground truth as label to compute pixel wise loss.

Unsupervised Depth Estimator: Photometric (Unsupervised Learning of Monocular Depth Estimation and Visual Odometry with Deep Feature Reconstruction)[https://paperswithcode.com/paper/unsupervised-learning-of-monocular-depth-1] CVPR2018

Our proposal: Use depth estimator to initialize Visual Odometry, and use visual odometry to refine the depth estimated by the depth estimator.