Very surprised to not find this in docs, I swear this was easy in former versions... Whatever, here we go:
Say you have a rivers layer of x,y,m polylines. Each feature is a named river. Each feature was created in the direction of flow, so that the first vertex of each feature is the river's headwater, and the last is its drain (probably where it flows into the next river). I want to assign measures to each vertex of a given feature as the accumulated length of the feature from its drain to that point.
I've only found the 'CreateRoutes' ArcToolbox tool, which not only does the unnecessary work to aggregate features and output a new layer, but it has a wonky idea of 'coordinate priority' to define each feature's measures start point, being that closest to a corner of its bounding box... I want to operate on my layer in-place, and I simply want to start the measures from the start (or end, in my case) of each feature.
One possibility with that tool might be to use its 'TWO_FIELDS' Measure Source, where I use a pair of columns calc'ed with zeros (first column) and feature length (second column)...
I thought I'd check first if anyone else has already been down this path?