Oh boy, I wish I had a good answer to this! I haven't figured out why bikers and walkers are such hard-line scolders, worse than drivers-- but there it is.
A long time ago I tried continuing my ride with "excuse me", or "sorry, I'm measuring a course", etc. What was I thinking? That they would immediately understand and move over or whatever? It ain't happening. Now I have 2 approaches: 1, try to measure when they're not out there, like super-early maybe; 2, when I'm on the wrong side and approaching someone, I pretty much come to a stop and try to stay out of the way, wait until they go past. Often I'm lucky and folks will notice that I'm waiting and just move over to their left if there is room, so I get going and I thank them. The more people on a bike path, the more time I have to figure on for the measurement.
No more explaining for me. Except that "Official Measurer" on my back sometimes starts some more positive conversations, that's a good thing.