Our WDI reports specifically say that the drawing doesn't need to be to scale. The drawing you posted is more descriptive than anything I've ever put on a report. Usually, it's just a rough sketch with some circles and arrows.

If it's for some other purpose or if you must know the exact dimensions the only way to do it is to break it down to shapes/sections with known formulas to calculate area.

Or, we could just break out the calculus and integrate