I recently extracted coastlines (for my master thesis) for a Bangladesh region for the years 2013 and 2014. Regardless of the method I used, the distance between the 2 coastlines was always the same: 42 m.
This distance is way too big for a one year difference (same time of day, so tides were more or less the same). I realized that the 42 m are exactly the length of a diagonal with 30 m sides (which is exactly the resolution of these Landsat images).
In conclusion, no distances smaller than the pixel resolution can be measured. Does anyone know of any method to improve this, or free higher resolution alternatives to Landsat?