Quite a compliment, Balaji, from a guy whose sketches/paintings I absolutely love!
But to me this is no more than a crude quick sketch. Yes, waterproof ink, as most of the WC work I do is line and wash technique. Quick faint outline in pencil erased, overlaid and improved with indelible ink line and detail, washed in with WC. I recognize a kindred spirit.
Working that way with stuff in my lap usually allows me to capture a scene and a feeling very quickly, often just 20-30 minutes per painting. And those I DO consider finished works, as opposed to this partcular one.
Just my peculiar way, but I exceedingly rarely actually do a reference sketch with intention to make a finished work later. Prefer everything alla prima, one and done, to capture what I feel as well as see at that moment. I don't even seem to use my own photos as references. Not a value statement, just a preference.
When I started painting (drawing all my life) 47 years ago I primarily worked from my head, my imagination, and prolifically. Now I'm just the opposite. The meditative aspect of painting in situ from inspiration right in front of me is what turns me on and has turned me into a plein air painter. Sure, I can still paint inside or from memory or imagination, but it's not my current practice.
Here's one where Covid forced me to break my own pattern. Basically locked to my home, I referred to a small sketch made previously of a pond's edge that inspired me, sat outdoors in my back yard, and created a more vivid and expansive vision in pastel, and I still consider it one of my best (as does my skeptical wife). Is that techincally en plein air? Who cares.