Basejumper.com - archive

Rockdrops are not always possible when measuring the height of cliffs due to various reasons (stealth, houses or climbers below, chance of starting a bigger rockfall, etc.) In this case, we just use our good friend laser rangefinder, right?

Well, as I recently found, laser can give you wrong numbers in some cases. Some 3000ft wall that I've jumped in WS before was lasered to have at least 1000ft+ to the first big ledge below, turned out to have only 500ft to impact (6.0s = 6.6-6.8s measured several times by video and timer, minus time for sound travel and reaction time) when I revisited it a few years later. No wonder the ledge looked so close on the jump and on video! With laser, the wall seemed to be terminal even for street clothes tracking; with rockdrop measurement, it's not even terminal without wingsuit.

My theory is that in some situations when you aim at the impact point, part of the laser beam (which has finate spread) bounces off the wall, then hits some lower ledges, and reflects back into the rangefinder, giving you much higher figure than real.

Be careful out there. Rockdrop accurately measured with video (especially if you can zoom in and actually record the impact, so you don't have to subtract sound travel and unknown reaction time) is the best. Better to overanalyze and overmasturbate than to become a rockdrop yourself!

Yuri

Agreed, always double check!

I use my laser a lot on A's. Makes for quick scouting work on the ground if you're just stopping by to check it out. How ever I will still count the rungs on the first climb up, laser it again from the top and if need be toss something off of it. Better to be on the safe side, especially when there is not much over 300ft around.

large cliffs offer up several optical illusions. the difference between 500 & 1000 foot is rather significant, but boulders do not come in standard sizes.

that said, I prefer simply using calibrated eyeballs, and counting rungs. I can generally tell if an object is "jumpable" just by a quick look. I enjoy comparing my results with my laser carrying jump buddy. (and yes, I needed those measurements to convince me a few lower objects would be ok.)

think in other terms... what do jumpers use to discern altitude at 200 ft, under canopy? how does one estimate how far they will fly under canopy?

I remember being told never blindly trust calculations. always ask if a result is "reasonable." I've seen significant errors caught by people asking, "does that seem right?"

Thanks for the info yuri! I use a rangefinder pretty often and usually cross reference and double check but now I always will...at least enough to make sure its reasonable

Keep fresh batteries in the range finder as well