I've never done it. But I think I'd stand under the edge of the crown and measure from there to the trunk. Or (to avoid having to look up, which often makes me dizzy) I'd stand a distance away and figure out where the edge of the crown is, then go to that spot and measure the distance to the trunk. Or if I had a helper, I'd have that person stand under the edge of the crown and then I'd measure from the trunk to where the helper is standing.

Now I'm sure some experienced tree-crown-measurer will answer and make me feel like a fool....

Walk out to the drip edge of the tree, then measure the distance from you to the trunk, multiply x 2 for the diameter. This is close enough for your purposes. Being a landscape contractor I am constantly having to measure properties and I have discovered that Google Earth comes in very handy for this. Zoom into your property, then use the measurement tool for tree canopy diameters property dimensions etc. Of course it depends on how well satellite imagery is for your area.

Bob

I've done it by standing under the edge of the tree canopy and marking my position relative to other things on the property whose location I know, not the tree trunk whose location I don't know as precisely. For example, when the neighbour's tree extended half way across my yard, I knew that because it reached to my front sidewalk, which goes right down the middle.

Having said that, it may not matter that much exactly where the canopy is. You can plant under it, and if you are trying to give clearance to a new tree, well, by the time the new one grows in, the old tree will have expanded!

KarinL

Remember that the crown of the tree is a "soft measurement" in that if you are off by a couple of feet it really does not have a big impact because you are unlikely to have a use for that level of precision up there like you would down on the ground with a tree trunk. You can measure shade on the ground much more accurately than trying to calculate it from crown diameter, heiht of the crown, and sun position. Some measurements can be qualitative others need to be quantitative.

I do not often measure tree crowns.

Thanks everyone! At least I know there isn't some magic way to do it with a laser level and 3 colors of string ;) Look up, take a rough guess, and call it close enough seems to be the consensus & that works for me. Being an engineer I thought, "Surely there must be some calculation that is accurate to 0.015' that I should be using."

Now I feel better about guesstimating the crown size.

Somewhere (I wish I could remember where) I saw a diagram for this. Take your plot plan and make two reference lines along two perpendicular sides, say the front and the west side. Standing on the reference line, move left or right until you can run a sightline (if that's what it's called) at a right angle to the reference line out to the object you've chosen and mark that location on the reference line. By taking the measurements of those markers along the reference line, you can fairly accurately plot any object (including tree canopies ... which do change) on the property.

I hope my description hasn't lost you completely. I tried making a diagram, but upon previewing the message, I saw that all the spaces had been deleted, leaving useless strings of symbols. If I run across the original diagram I saw, I'll come back and post it.

The circles are pretty, and may alert you that a crown is or is about to encroach on your roof or the power lines, and also have some utility as they remind you that there is a root zone as well as a trunk, but as laag points out, you have other "cones of shade" to draw that may affect what else you plant where. It also depends on whether the tree is limbed up high.