Both responses nailed it. The amount of gravity affecting a bullets performance when shooting a steep incline or decline will make the bullet act like the yardage is closer. Its hard to explain, but easier to understand with a drawing. If you take point A (being your location) point B (the target location)
A ========= (Horiziontal line = 180 yards)
---- *
-------- *
----------- *
------------- *
-----------------B (line of sight 240 yards)
The amount of gravity affecting the bullet is the distant of the straight line horiziontally between the two rather than the distant from line of sight, so to make a long story short, aim lower (as if the target is closer). And as JNDEER mentioned, it doesn't matter if the target is up or down.
That is a great drawing.
An easy way to think about it, as well, is that gravity only works perpendicular (vertical) to the earths level surface. the bullet will only be affected in this direction by gravity, so horizantal distance only affects the time the bullet, or arrow, is in the air i.e. the time that gravity has to act on the object.
The vertical distance to angle distance will always be d=R*cos(theta), where R is the actual line of sight distance, and theta is the angle. easy things to remember are that cos(30)=sqrt(3)/2~~.87 and cos(45)~~.71
so if you are shooting 100yds at an angle of 30degrees, then this is like shooting 87yds horizontal. 100yds at an angle of 45degrees is like 71yds horizontal.
Just remember those two numbers and you should be pretty good with a fairly flat shooting rifle.
