Why ‘making trees’ might land you in the woods
Aug 17 2012 in Navigation by Jeffrey Smith
If a boat is “eating trees” — i.e. moving in relationship to its background — are you really safe from collision?
Recently I’ve heard a lot of people use the “Making Trees” system to figure out if they are on a collision course with another boat. At first I thought it was kind of neat. I’d never heard of it. Something about it didn’t seem right though.
The basic idea of “making trees” is that if you see another boat that appears to be moving forward in relation to the trees behind it – “making” them – then it should pass in front of you with no risk of collision. When you first hear it, it sounds like an amazing short cut. You don’t have to spend a long time monitoring and checking on a target to determine risk, you just watch it for a few seconds. Making trees, and you don’t have to worry about again. Not making trees and you know it’s a possible collision.
The trouble is: It doesn’t work! I’m using a high standard here where the rule must give the correct answer every time, because it does occasionally look like it works, but for me, as a professional mariner, a rule of thumb has to work all the time, in every case, or it’s useless and not safe to use.
Below are two cases where making trees gives you an erroneous answer and they are shown side by side with the same case using the correct rule of thumb “Constant Bearing – Decreasing Range” which does work and always gives an accurate picture of the risk. (You can get lots of info about using Constant Bearing – Decreasing Range on the web.) In each situation, the top shows a view out the pilot house windows, with the bow of your yellow boat in front of you and the black vertical post representing the center line of your boat. The lower part of each frame show the situation as viewed from above.
The lower 2 cases show “making trees” giving a correct picture of risk, but how would you know when you were experiencing one of those cases? I’ve put them here to show that using Constant Bearing – Decreasing Range works in these cases too.
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Using “Making Trees” |
Situation |
Using Constant Bearing - |
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Incorrect The Target is “making trees” so the rule says you’re fine right? But look out, you’re actually on a collision course.
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Correct The target is on a constant bearing (see how it’s in the same place in the Pilot House windows) but the range is decreasing (see how it’s getting bigger) therefore you’re on a
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Incorrect The target isn’t “making trees” so the rule says you’re on a collision course. But the rule is wrong. You’re not on a collision course. If you alter course for this target, you’ve wasted your time and you could end up putting yourself in a close quarters situation with it.
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Correct The target is not on a constant bearing (it’s moving right in relation to your view), therefore no risk of collision exists. |
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Correct The Target is “making trees” and no risk of collision exists. This is what the rule is supposed to show, but how could you be sure you’re not in the first example? |
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Correct The bearing is changing so no risk of collision exists. |
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Correct The target is not making trees, and a risk of collision does exist. |
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Correct The bearing is constant, and the range is decreasing so risk of collision does exist. |
Someone out there is going to point out that sometimes CB-DR doesn’t work, and the usual example is the huge tanker. In that case the bow appears to have a changing bearing but at the same time a risk of collision exists with the stern. Its a special case, and in that case you need to treat a very large target as several targets. Ask yourself “am I going to hit the bow of that tanker?” and “am I going to hit the stern?” Remember, just because you’re not going to hit North America at Los Angeles doesn’t mean you’re not headed right at Shilshole.
There are lots of decisions that you have to make as a mariner and there are a lot of shortcuts that you can use to make some of those decisions easier. When you’re trying to determine if you’re on a collision course, you should use the method that gives you the correct information — that method is the Constant Bearing – Decreasing Range method.
Mike Brough said on August 27, 2012
Never did like the Trees method – never quite worked for me – always use the relative bearing method and as Bill pointed out a hand bearing compass helps a bunch – now there are fancy compasses that will remember bearings for several objects
Bill Haimes said on August 18, 2012
The part concerning “Will I hit the bow….Will I hit the stern? is what the Navy calls checking the “Danger end.” Take bearings on both ends and maneuver for the CBDR end if necessary. This is a great use for a hand-bearing compass, particlarly in a smaller vessel where the course steered may not be precise, maiking relative bearings less accurate.
Marty McOmber said on August 18, 2012
Hi folks,
The original post has been updated with graphics that are animated to better illustrate Capt. Jeffrey’s point.
Sorry for the confusion. It was entirely our fault for uploading the wrong version of the graphics.
The new graphics show both the overhead view and the view from the cabin. This should make his point on the importance of using the right means to tell if a danger of collision exists.
s/v Eolian said on August 18, 2012
Thanks.
I’ve never trusted the “making trees” solution to collision avoidance because I’ve never been comfortable with it – something always seemed to be off – like it was not correctly handling “edge cases” (yeah, I write software).
bob
s/v Eolian
Seattle
seaweedsoup said on August 17, 2012
As informational graphics those diagrams are very confusing and unclear. I tend to agree with the other two responses. —GS
Marty McOmber said on August 18, 2012
Hi Seaweedsoup. Have a look at the new graphics on the post about making trees. Should make things much clearer.
Danny B. said on August 17, 2012
I must agree with A. The examples are very unclear and don’t make a clear point. I do belive that the general point could be that “making trees” is not fool proof. It should be used in conjunction with other methods. But many would contend that it is a good “fat thumb” measure of “Constant Bearing – Decreasing Range.”
Danny
Asheim said on August 17, 2012
Sorry, but the figures are not really clear, I dont get it, and Im not sure you got the whole point, sir.
The point of “making trees” is to determine wheter the bearing changes. It’s easy to show with some simple geometry, that, if you assume that the trees are far away compared to the distance between the boats, the bearing is not changing if the other boat is stationary in relation to the background. If you understand this, then the limitations follow. I think it’s more correct to say:
1) If the background is close, making trees will not work.
2) If the background is distant, making trees is quicker and more precise than looking at bearings.
3) Since making trees is all about bearings, the question of distance comes next. A boat stationary against the background could also be moving away from you.
And finally, if you or the other boat changes speed or heading, no method will work. Both making trees and CBDR should be used as approximations, not an absolute. And sometimes a quicker approximation is better.
Cheers,
A
Marty McOmber said on August 18, 2012
Hi A,
Have another look at the post making trees. We updated the graphics so they are animated now and better illustrate the author’s point.
Asheim said on August 20, 2012
Thanks, the figures are much clearer. And it’s quite clear that you missed the crucial part. As I wrote above, the trees need to be in the distance for this method to work. Try to do the math.
cheers,
A