Distance Estimation Errors and Why We Need a Rangefinder

I used to consider myself an expert at estimating distances, because as a wildlife biologist we used a common survey method that estimated the distance from the survey points or lines to the wildlife species being surveyed.

Accurate distance estimations were very important to obtain accurate density estimates, so we constantly practiced estimating distances. After range finders became affordable and we were able to measure the distances, we were surprised by how much most observers underestimated distances, especially for distances over 100 meters (109 yards).

My Distance Estimation Test

elk in rangefinder

Estimate at 270, 300, 350 or 400 yards? Or know the exact distance?

As a test, I used one of our survey areas that had a golden eagle nest that could easily be seen from one of the survey points and was recorded by eight different observers over nine years. Since each person should have been standing at the same flagged point and the eagle nest was always at the same place, all of the distance estimates should have been within a few meters of each other.

I measured the distance to the nest with a range finder at 369 meters (403.5 yards), but our distance estimates ranged from 190 to 325 meters (208 – 355 yards) which was between -49% and -12% error. The average estimate was 250 meters (273 yards), which was about 32% short of the actual distance. I am happy to note that my distance estimate was the closest (325 meters), but was still 12% short, which evidently was not good enough for the U.S. Army in 1917 and it is not good enough for shooting at that distance.

Studies on Distance Estimation

Strauss and Carnahan (“Observed Errors in Distance Estimation”), found that observers underestimate distances to objects 21 to 383 feet with an average error of -8.6%  and a median error of – 22%. Individual performance ranged from -96% to +811%. The study obviously included untrained observers, but the huge error range is very surprising especially since their study was limited to fairly short distances since 383 feet is only about 128 yards.

128 yards may be an extreme distance for shooting a bow, but it’s not a big deal for modern muzzleloaders (if the distance estimate is accurate) and 128 yards is no problem at all for center fire rifles. It is longer distances, above 150 yards for modern muzzleloaders and above 250 yards for center fire rifles where distance estimation becomes more and more critical for consistently hitting the kill zone of an elk or deer.

I read an old (1917) U.S. Military training manual (link no longer valid) stating that trained troops should be able to estimate distances out to 600 yards with only a 10% error, but a 1982 U.S. Army document (Download PDF) claims that 20% errors were more common and had become the new standard. So maybe I’m not so bad at estimating distances, but remember 20% error is for the average soldier, not for special forces or trained snipers. Also note that snipers use very good range finders and usually have spotters, so distance estimates would only be used as a back up.

A 2008 U.S. Marine Corps – Range Estimation Lesson Plan (Download Doc) lists several factors that cause shooters to under and over estimate distances to targets (summarized in Table 1 below).

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Table 1. Factors Causing Under Estimation or Over Estimation of Target Distances

Under Estimate Distance Over Estimate Distance
Nature of Target Target with regular or easily visible outline Target with broken or camouflaged outline
High contrast Low contrast/ camouflaged
Clearly exposed Partially hidden
Large targets Small targets
Nature of Terrain Up steep hill from observer Down steep hill from observer
Across body of water, snow, or level plain Across undulating ground
Light Conditions/ Visibility of Target Sun behind observer, object is bright Sun in observer’s eyes, object is dark
When in high altitudes and very clear atmosphere In poor light, haze or fog
Target appears farther from prone position

Basically, any light or visibility conditions that make a target easy to see, causes us to underestimate the distance. Any conditions that make a target hard to see, causes us to overestimate the distance.

So, thinking back to the eagle’s nest, it was a large, clearly visible target, observed high up on a canyon wall, so all of these factors tend to cause observers (including me) to underestimate the distance.

What if that Distance Estimate was for a Shot at an Elk?

It was “eye-opening” to realize I had underestimated the distance by 78 yards. If I had been estimating that distance for a shot a an animal, a 400 yard shot is a long way to shoot. I don’t want to set up a shot like that with any distance error in my calculations.

My first thought was how much would a 78 yard error effect that shot and would I have a chance to hit an elk or mule deer at that distance with that error. And what about the other bad estimates at different distances? Read Effect of Distance Estimation Errors on Bullet Impact.

I am not a guy that needs or wants all the latest and greatest gadgets, but since I learned that my distance estimate at the eagle’s nest was 12% short, I have never hunted without a range finder. Now that there are so many reliable and affordable rangefinders, there is no reason not to have one.

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