# Spot Calculator

One early Saturday morning I was standing in front of the posted hand written winds aloft sheet next to manifest going through the numbers in my head figuring out where I should be leaping into the great blue sky to make it home and it hit me. This is repeatable mathematics and so many of us are walking around with smart phones, why not have the world of computers figure this out for us?

Okay, okay, before the nay saying begins… This was never meant to replace human neurons and the crafty skill of spotting. The idea was to provide another resource to double-check our maybe not quite awake minds as the sun rises. There are a handful of assumptions that have to be used in any automatic spotting calculation, so at best this is a generalization of spotting.

First off the raw winds aloft data… it is computer generated forecasts, it isn’t perfect and could be totally different from real life.

Freefall drift is vastly different depending on the discipline and group size. A 4-way RW group will have a much different horizontal drift distance that a 2-way freefly group… and wingsuiters… sorry that doesn’t exactly fit into the norm for these calculations. Then there are differences depending on exit and deployment altitudes to consider. Canopy drift… vastly different depending on canopy size, wing loading, and deployment altitudes.

So how do we guesstimate to the best of the computers ability to combine all these elements into a relatively accurate jump run and spot? Poof! Magic. I wish…

The assumptions used in the calculations are 60 seconds of freefall belly to earth, exit altitude 13,500ft, opening altitude 3000ft, with a 1000ft per minute canopy descent. There are a few variables that you are able to change on the fly, the speed of the aircraft on jumprun and the desired horizontal separation used to calculate exit delays.

Things get real interesting when there are light and variable conditions in the forecast, I won’t go into the details, let’s just say the calculations try their best to come up with a workable average given the various data points for the different altitudes.

There are some data points that are not considered in the calculations currently, such as forward throw on exit, actual ground wind data since it is not generally available in a computer generated forecast, and the usage of tear drop jumpruns to accommodate light uppers and reduced pass numbers. The calculations will generate a straight line jump run.

In short, it’s a generalization tool that is a great tool not to be used as the only source of data for your skydive, but it is a pretty neat use of technology nonetheless

The list of dropzones that are using the spotter application include –

The spotter application is available under the GNU GPLv2 license. If you would like install the spot calculator for your dropzone you can download the latest version here. Open up the config.inc.php file to set it up.

I’d love to hear from you if you have installed the spotter and if you would like to be listed here.

• ### Matt

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Version 1.2.1 released to fix retrieval error from NOAA’s site changes, enjoy!