# How helpful is slope compensation in a rangefinder (hunting mountains)

sam700

March 30, 2009, 11:53 PM

So I've been looking to get my first rangefinder for hunting and have pretty much settled on a Nikon. I've been looking at either the 800 (good to 800 yards) or the rifleman (good to 550 yards and capable of compensating for slope).

I do hunt some mountainous areas. I would be using the rangefinder in Ak for Caribou and black bear and this year I'd like to get out to WY for Antelope. The farthest I would shoot game if I knew the range would be 400 yds (I do practice at that range 1-2 times a week although, I've never actually shot game past 350).

Would I be better off with the model that compensates for slope or the one that is good to 800 yds. I don't intend to shoot game that far. On the other hand, it seems like since rangefinders are tested under ideal conditions, you can cut the manufactures claims in half in which case the 800 yard model would be about right and the 550 would struggle a bit if I got a 400 yard shot. On the other hand, I do sometimes shoot on steep slopes.

Basically, is the slope compensation useful enough to sacrifice some range. What would you guys recommend?

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~z

March 31, 2009, 10:19 AM

I'd use the longer range rangefinder. You can do the math in your head for the slope once you know the range.

~z

Art Eatman

March 31, 2009, 11:21 AM

Get a 30/60 right triangle. Sorta fix it in your mind about how much is 30 degrees. Heck, get a 45-degree right triangle. Carry them out with you a time or two. Helps you get past the, "Well, lessee, I think that's maybe, er, uh, well..." stuff

The cosine of 30 degrees is 0.866. That means that at 400 yards, the horizontal distance is about 345 yards. The difference in trajectiory between your 200-yard zero and 345 is what, maybe nine inches? One flat land, the drop from a 200-yard zero to 300 is about six inches for most deer cartridges and 21 to 24 inches at 400.

IOW, if your rangefinder says 400 and the slope is no more than around 30 degrees, you just allow a little bit for it. No big deal. Instead of two feet of holdover, you do about nine inches. Roughly. :)

You have to get steeper than 25 to 30 degrees for the angle to really affect hits to a kill zone. 45 degrees, e.g., means a multiplier of 0.7071, so 0.7 is plenty good.

One of the reasons I recommend the Sierra reloading handbook is that it has the best appendices of any for this sort of information--along with other stuff. The loose-leaf binding means you can make copies of the more pertinent info and carry it along with you as a refresher until it gets committed to memory...

MCgunner

March 31, 2009, 11:26 AM

Only folks that have to worry about it around here are bow hunters sitting in trees. :D A HILL, what's a hill?

Thanks, Art. I've never really sat down and thought about it. I'd have had to dust off my trig knowledge to do it at any rate. That gives me more knowledge on the subject if I ever get out there in the mountains again. Pretty simple when you think a little about it, eh? :D

Ghost Tracker

March 31, 2009, 11:39 AM

You can do the math in your head for the slope once you know the range

Hey ~z, Where are the "high plains" in Texas?

I've never hunted in Wyoming but have chased around Alaska quite a bit. The ranges are sometimes l-o-n-g & the angles are sometimes s-t-e-e-p. So I understand your indecision. In my experience, you'll realistically have a higher percentage of shots <400 yards at >40 degrees than the other way around. With all my other pre-shot concerns (wind, placement, breath, squeeze) I personally don't want to add cosines & multiplication to the quiz. I would get the range-finder WITH slope compensation. Good Luck!

Dr. Tad Hussein Winslow

March 31, 2009, 11:55 AM

I'm waiting for Leupold or someone to come out with a quality COMBO bino/rangefinder with the angle correction, in about a 7x40/42 or 8x40/42. Or have they?

MCgunner

March 31, 2009, 12:37 PM

I think that new Burris range finding rifle scope compensates for angle. That's the one I'd pick up on if I still went to the mountains a lot. It's about 800 bucks and around here it's really not necessary. Neat scope for the long range shooter that spends a lot of time in rough country, though.

~z

March 31, 2009, 02:53 PM

The high plains are the area… well here: http://en.wikipedia.org/wiki/High_Plains_(United_States)

Once you have the cosines of the pertinent angles in your mind (or taped to your stock) the math is pretty simple, however I carry a calculator in my field bag. One nice thing about the long shots is you rarely get extreme angles mixed in with long ranges as the distance flattens out the angle.

~z

rcmodel

March 31, 2009, 03:08 PM

You are much better off getting the one with the longest advertised range.

Then cut that in half for deer and other critters that don't reflect light like a stop sign.

The max range they list is off of a highly reflective target, which is a pretty rare find in the hunting fields.

Shooting uphill / downhill with a flat shooting rifle ain't no big thang once you understand the concept.

rc

ArmedBear

March 31, 2009, 03:51 PM

Under 300 yards, it's not going to make a hell of a lot of difference.

Over 300 yards, I'm not sure that a rangefinder will make up for a lack of practice in similar conditions.

That's the problem, really: finding a place where you can practice shots up and down slopes, at 400+ yards. Most shooting ranges seem to be pretty flat...:)

~z

March 31, 2009, 04:15 PM

Get off the bench! Practice where you hunt, dinging a steel in the area where you expect to shoot a deer in 6 months will surely help.

~z

ArmedBear

March 31, 2009, 04:18 PM

Absolutely!

Unfortunately, it's not always legal to do that.

I'm lucky; there are places here where it is. But it's not true everywhere.:(

Sadly, some of that is the fault of shooters and shooters alone. Many areas where open target shooting was allowed have been shut down not due to fears about safety -- hunting is still allowed there -- but because some wild lands were becoming de facto garbage dumps.

~z

March 31, 2009, 04:59 PM

Too true, I always try to pack out more than I pack in and encourage others to do the same.

Sorry, back on topic...range finder, really depends on where you intend to be hunting and what distances you are proficient with. If you WONT take a shot beyond 300yds, you prolly will not need long ranging capabitity. Just a thought.

~z

danweasel

March 31, 2009, 05:38 PM

Yes, As far as I understand slope does not really start effecting bullet drop unless you are shooting really far and or shooting off of a mountain (really steep angle).

Or... Memorize all the cosines of the angles and learn to calculate the slope by sight and you will be fine. Ha!

sam700

March 31, 2009, 07:05 PM

ArmedBear makes a good point in that many people, myself included don't practice at steep angles. I get out for long range pratice a couple times a week and 90% of that practice is either shooting off my pack, bipod and 5% is standing, kneeling etc. Haven't shot off a bench in almost a year as I moved to Mn and the new club I joined only has benches at the 1000 yard line. My gun starts to keyhole around 900 or so so I'm stuck shooting in the grass somewhere short of that. Probably the best thing for me though:)

As far as the distance, I'm comfortable shooting deer sized game to 400 if I know the range. Previously, I did this by scouting the area with a GPS and drawing a diagram from the perspective of where I expected to sit.

If I have to guess at the range, I won't shoot beyond an estimated 300 yard range.

Lucky

April 1, 2009, 12:15 AM

How does the angle affect your shot, and why?

sam700

April 1, 2009, 12:35 AM

Basically, the closer to vertical you are shooting, the less gravity affects the trajectory of the bullet. An extreme example would be shooting strait down at a target 300 yards directly below you. A 300 yard horizontal shot would require around a foot of holdover, but if you are shooting down gravity will pull the bullet strait forward along the path you aimed it rather than deflecting it away, so there is no holdover required and you can aim directly at what you’re shooting at.

If you are shooing on a 60 degree slope, some holdover is required, but not nearly as much as would be needed for a shot on flat ground. The same concept applies to a shot strait up, the closer to vertical you are shooting, the less gravity deflects the bullet away from a strait flight path and the less holdover is required.

This doesn’t really come into play much till you start shooting on 25-30 degree slopes.

Art Eatman

April 1, 2009, 12:35 AM

Lucky, draw a triangle. Say, 30/60/90 degrees. 30 degree angle on your left as Point A, the 90 degree angle at the same level on your right as Point B. That puts the 60-degree angle at the upper right as Point C.

Okay: You are at the lower left at Point A. Bambi is at the upper right at Point C. The rangefinder gives you the distance up that long side, A to C. The bullet thinks it only has to go from A to B, the horizontal distance.

The ratio of the length of AB to AC is called the cosine of the angle at A.

Now for some basic trig. A 30/60 right triangle is easy to remember. The length of BC is one-half the length of AC. Now comes old man Pythagorus and his deal about AB squared plus BC squared = AC squared. Confused? Good. Generations of algebra and trig students have had the same problem.

AC squared is 4; BC squared is 1. So, AB has to be the square root of four minus one = three, or 1.73205. The cosine of 30 degrees is then the square root of three, divided by two, or 0.866. All this is easy to remember.

45 degrees is even easier, since the short sides are equal at one each. So, the long side is the square root of two, or half of what everybody is supposed to commit to memory, 1.41421. Half of that is 0.7071.

Isn't that easy? All hail that ancient Greek for making life easy! Seventh grade for the Algebra, Tenth grade or thereabouts for the Trig.

But first, draw the picture.

:D, Art

Lucky

April 1, 2009, 02:50 PM

Thank you for the explanation, it is good to refresh trig.

Why does the bullet think it has to go from A to B? If you read the distance of A to C, albeit upwards, to be X, can't you adjust sights to compensate for drop over X distance?

sam700 that makes perfect sense. These range finders calculate that? Otherwise a formula could be interesting. Would you try to aim horizontal, and calculate how far below the LOS the bullet would fall if shooting downhill, or aim directly downhill at the target and calculate how the TOF and gravity vectors affect it?

P.S. It's funny how 30 degrees sounds small until you have to walk up it.

MCgunner

April 1, 2009, 03:32 PM

Well, unless you have a way to actually measure the angle, it ain't going to be THAT accurate, anyway. I'm thinkin' Art's example, about a 30 degree angle, I can figure .866 is pretty close to .9, so if he's 400 yards, 10 percent of 400 is 40 and subtract gives you 360 yards, only 15 over actual, but heck, if it's actually 20 some (whatever a cosign of .90 is), you're dead on. Besides, I'd probably use the 350 yard hold from the chart taped to my scope, anyway, so I'd only be 5 yards off. Anyway, out to 400 yards, this'll giterdun. I don't do that 800 yard stuff anyway, but if you do, you have time to measure angle and calculate cause you ain't likely spookin' the deer. LOL

I had this thing when I was a kid, very simple, made of wood. It had a circular tube with water in it and a thing that pinched off the tube which would stop the flow of the water. I can't do a drawing or I'd try. Anyway, it was from Estes Industries and was for measuring the angle from a model rocket launch (yeah, I was a nerdy kid). What you did was you knew the distance to the pad on a vertical launch. You measured angle to apogee and multiplied the tangent of the angle to get altitude. You could build something like that pretty easily for the purpose of measuring shooting angle. Be cheaper than a new range finder if you already have one and are just worried about the shooting angle.

I have drop tables taped to my scope already. I guess I could tape a cosign/angle chart to the stock. Amongst my redneck friends, I'd probably get some razzin' about it, but most of them have never hunted where there was a hill, let alone a mountain.

MCgunner

April 1, 2009, 03:41 PM

Seems they have a different set up, but might work the same way. Says it gives angle. Hell, probably digital now days. :rolleyes: Just lookin' at it, though, looks like it's got a deal that swings on a protractor. Would have to use it backwards if shooting downhill. LOL You could EASILY make something like this, though, using a protractor to measure with.

http://www.esteseducator.com/content/search_details.php?id=EST%20302232

~z

April 2, 2009, 11:07 AM

If you are hunting in the hills a lot and shooting at extended distances and/or extreme angles get a angle cosine indicator like this: http://www.ustacticalsupply.com/anglecosineindicator-2.aspx

no they are not cheap but sure are handy.

~z

ArmedBear

April 2, 2009, 11:22 AM

BTW there's one thing that can make slope detection REALLY important.

.45-70

Art Eatman

April 2, 2009, 11:30 AM

Lucky, good old gravity is the deal with horizontal distance. And that's about as far as I can go with a simple but unsatisfactory answer.

From a practical standpoint: Few shots are taken beyound 300 or so yards. If you zero the typical deer rifle at 200 yards, the drop at 300 is only about six inches. You have to get a really steep angle for the difference in drop to change the point of impact from a kill shot to a wound shot. At only 100 or 200 yards? It doesn't really matter what the angle is: Just point it and pull.

A hunter who seriously expects 400-yard shots and 45- or 60-degree angles really needs to go out and practice under those conditions before hunting. You're getting into two feet of drop, and an error in trajectory of six to twelve inches is a bummer.

MCgunner

April 2, 2009, 03:17 PM

Ah, if I was that much of a distance shooter, I'd get the Burris laser scope with the slope compensation built in and be done with it. But, I'm with Art. I zero at 250 and there normally isn't much error to my max extremes. I only have a max of 300 I can practice at and, well, I'd have to rent a helicopter down here to do much practice with slope. Good knowledge here, though, and I appreciate the discussion.

sscoyote

April 4, 2009, 02:47 AM

I use the old Sniper tools ACI as well myself when i'm hunting the mountains here--simple enuf to use.

Mr. T

April 4, 2009, 05:00 AM

I bought the Bushnell Scout 1000 ARC and it works well out to a little over 600 yards. On reflective objects it is supposed to go out to 1000 yards, but for practical purposes it will only go out to 600+ yards for game. The furthest range it's recorded is 622 yards and it gives you the angle and related compensation in the site window. It has a bow mode and a rifle mode. It was on sale last summer for $345 at Gander Mountain. I'm not sure where the prices are now for them.:)

JWF III

April 4, 2009, 11:50 AM

Art Hit the nail on the head with his answer. IE-it really isn't neededin most practical hunting situations. If it is needed, you need a lot of practice at the shots,then those rangefinders might be worth the money.

Basically, the closer to vertical you are shooting, the less gravity affects the trajectory of the bullet. An extreme example would be shooting strait down at a target 300 yards directly below you. A 300 yard horizontal shot would require around a foot of holdover, but if you are shooting down gravity will pull the bullet strait forward along the path you aimed it rather than deflecting it away, so there is no holdover required and you can aim directly at what you’re shooting at.

Basically correct, but if you're shooting at 90 deg. downhill, the line of sight is not the trajectory of the bullet. If you aim dead on, you'll miss by as much as a couple of feet at 300yards. This could possibly be the hardest shot possible to make. We're just lucky that it doesn't come around very often (if ever in our own hunting life).

Correction to previous paragraph- If your vertical wire of your scope is along the length of an animal, you may hit it holding dead on, just not where you're aiming. If your horizontal wire is along the length of the animal, holding dead on you'll definently miss it.

Wyman

Dr. Tad Hussein Winslow

April 4, 2009, 12:06 PM

And interestingly, it doesn't matter whether you're shooting uphill or downhill. The point of impact is always HIGHER with an uphill/dowhill shot, than the lasered distance would make you think you should hit, compared to a horizontal shot of the same lasered distance. So you have to estimate holdUNDER instead of holdover with an uphill/downhill shot. Actually you have to estimated holdunder, the take the difference between the holdunder and the appropriate holdover for the straight lasered range. Might be a net holdunder, or might be a net holdover, depending upon circumstances.

Some people think you aim lower on a downhill shot, but higher on an uphill shot, but that is not the case. Always aim lower than otherwise.

This can become very important with shots over 20 yards with a bow/arrow, from a treestand, particularly if you're way up there, 15 feet or higher.

sam700

April 4, 2009, 10:46 PM

Thanks for the help. I ended up going with the Bushnell 800 since it seems like it's unlikely I'll have shots that are both long and steep. Also I figured that with the 800 it should range animals pretty easy at 400 and the other model might have a bit of trouble with it. The fact that they had a great deal on the 800 at Sportsmans warehouse helps too.

I did a bit of testing yesterday and found that it works pretty reliably dirt hillsides to 600, trees to 650 and reflective objects like white buildings have worked to just a bit over 900!

MCgunner

April 5, 2009, 10:46 AM

I have a 400 yard bushnell rangefinder. It'll range that far on a deer if there's a nice white rock near him. Otherwise, it has problems beyond 300 on some dark objects. I got it when they first came out. I may get a better one if I go back west, but 400 yards is my absolute shooting limit, so I really don't think I need much more rangefinder.

~z

April 6, 2009, 10:30 AM

Doc, it is all the same, gravity acts on the horizontal length of the bullets path regardless of the angle. That is the easy part, now what the wind is doing across there...

~z

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