This is all sort of greek to me also. I have tried to look it up but couldn't find anything that I could understand. I do agree with Duane after reading his remarks that if you put the GPS coordinates on a map you should use the datum used (even though the few times I have put it on my maps I didn't do it put I will in the future). I wrote an email to Garmin to find out what datum my Forerunner 201 uses.
My question would be how many measurers put the coordinates on there maps. My guess is not very many do that. I use a Garmin forerunner 201 and from my research on it I do not know what datum it uses and there is nothing on the unit itself that will let you change the datum. I bought this about 10 to 12 years ago and I can not hook it up to my mac (that I bought in 2012) so I may have to upgrade my watch also. This way when I ride the course I can get an elevation chart. Or does anyone have any ideas on the elevation chart. When you use Google earth and you go over a bridge you still get a big dip because it follows the land and not the bridge.