Significant steps for pre-processing

Hi everyone,is there any way to determine which step important to be considered in order to perform image pre-processing regardless an iterative experiment.

For example, I have this input data, and need to tidy up the imperfection of the drawing. I know that morphological operations is a must (dilation) as I need to get the skeleton of the image. Perhaps, I missed some step in here because somehow the thinned image is not satisfied the next phase requirement (Corner detection).

Thanks

Input Image

Dilated image

Thinned / Skeleton image

Corner detected (Not at the exact edges)