Understanding Angle-Compensating Rangefinders

As I mentioned in my last column, the higher the magnification of the rangefinder and the smaller the reticle, the more precise the instrument will be. The reticle is the dot you place on the object to be ranged. It can be red, green or black in color and might be in the shape of a square, circle, crosshairs or dot.

When using some of the higher power rangefinders, you could potentially range an antler tip sticking above the grass at 40 yards if you are able to hold the device steady enough. I prefer to use optics with the highest power magnification and the smallest possible reticle. This leads us to our next topic, the importance of understanding angle-compensating rangefinders.

The Math and Science

As most of you know, when shooting uphill or downhill you shouldn’t shoot using the line-of-sight distance to the target. If you do, your arrow will hit high. For most reasonable archery shots — say 40 yards and under — you actually need to shoot using the horizontal distance to the target.

So, if you will be hunting anywhere other than on flat ground, you need to use an angle-compensating rangefinder that calculates the horizontal distance to the target. When the line-of-sight distance and the upward or downward angle is known, the horizontal distance can be calculated using basic trigonometry (remember high school math?). Angle-compensated rangefinders have an internal computer that makes this calculation for you.

The yardage calculated using this method will work very well for short archery shots, but it won’t work very well on longer shots. The further away the target, and the steeper the angle, the less this formula will match up with how your bow actually shoots.

At longer distances, arrows shot at an upward angle will hit below where this formula predicts they should hit, while arrows shot downward will hit much closer to the predicted impact point using the horizontal distance formula calculated by most angle-compensating rangefinders. This is because gravity helps the arrow shot at a downward angle maintain its velocity, while gravity slows down the arrow shot at an upward angle. Arrows shot uphill lose speed more quickly than arrows shot downhill because of the pull of gravity. So, the arrow shot uphill will hit lower than the arrow shot downhill.

How They Work

Not all angle-compensating rangefinders are created equal. In my experience, rangefinders vary much more in the tilt-compensation department than they do in line-of-sight measurements. I assume this is because the accelerometer they use to calculate the angles are not all that accurate or not properly calibrated.

It is much more difficult to test your rangefinder’s angle-compensating accuracy than it is to check it for line-of-sight accuracy. Line-of-sight, or flat-ground accuracy, can easily be tested by using a long steel measuring tape to measure the exact distance to the target and then see what your rangefinder shows. Most rangefinders are a little off, but that is not a big deal as long as you always sight in using the rangefinder’s yardage rather than the marked yardage at the range, because you will be shooting using your rangefinder’s yardage while hunting.

A few angle-compensating rangefinders incorporate a program that calculates both the horizontal distance and the actual "shoot-for" distance. In other words, the rangefinder calculates the horizontal distance and then adds a fudge factor depending on whether the shot is uphill or downhill. This fudge factor is arrived at via a complex formula such as the Archer’s Advantage program. It does this by using the arrow’s weight, velocity, bow geometry and type of fletching, as well as the arrow’s diameter.

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Angle-compensating rangefinders without this feature will show the shoot-for distance to be exactly the same as the horizontal distance whether you are shooting uphill or downhill. Rangefinders that add the fudge factor will give you a range that is much closer to the actual, in-the-field shoot-for distance.

No matter which rangefinder you buy, you need to actually test it using the bow and arrows you plan to hunt with, and you must use it on steep slopes and long distances, both uphill and downhill.

Practice, and Take Notes

I have a target just outside my archery shop in Colorado that is at a 37-degree angle uphill, at a considerably long range. I shoot arrows up at this target and then climb up to the target, pull my arrows and shoot those same arrows back down at a target set at the bottom of the hill. I stand right next to each target when shooting, so I know the horizontal distance for both uphill and downhill shots is exactly the same.

With every angle-compensating rangefinder I have ever used, the shoot-for distance the rangefinder provides does not give me a reading that matches where my arrows hit. I write on the top of the rangefinder with a Sharpie marker something like “add 2 yards at 60 yards at a 37-degree angle.” This allows me to adjust my shoot-for distance according to how my bow actually shoots in the mountains.

Another thing to keep in mind is that your form changes when shooting uphill and downhill. So, even if your rangefinder is perfect, you may not be. Many people — I’m talking about right-handed shooters — will hit left on uphill targets and right on downhill targets. It’s important to make a note of your particular idiosyncrasies and try to remember to compensate accordingly. To be continued…