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Mystery Cache

The Flash Lag Effect

A cache by Iowa Tom
Hidden : 2/14/2006
In Iowa, United States
4 out of 5
1 out of 5

Size: Size: micro (micro)

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Geocache Description:

This is a do-it-in-the-dark pictocache of sorts. You’ll have to find where I was when I took two sets of pictures and you’ll have to drive to various radio towers in the pictures. You’ll cover more than 20 miles total and will need to measure distances and back bearings.

Please read the instructions for finding the cache carefully and keep good records so that I may resolve any problems quickly.

I named this cache “The Flash Lag Effect” because I believe that that is the phenomenon that I observe while looking at various stroboscopic radio towers like the white radio tower used in this cache sequence: namely the one pointed to by the stop sign arrow shown to the left here. My son (geocacher Lycan) and I are the only two people I am aware of that purposefully look for this strange looking occurrence in the radio towers that sprout up across the countryside. It’s only visible when a person moves their eyes –or their whole heads– during the event. The movement spreads the blinking light over the field of view. Ideally moving one’s eyeballs in what are called saccadic (jerky) movements is the best way to go; that way you are less likely to drive off the road while looking at towers while you roll by and other people won’t notice what you are doing. Actually, for the maximum experience my son and I go all out and wiggle our eyes very rapidly, sometimes to the extent that we get eye strain! The rapidly blinking lights look like a series of brilliant dots –separated- from the tower. An extreme example of this weird phenomenon is shown in the image to the right. The tower in this picture is one of many like it along I-380. It has a strobe-on, strobe-off time of about 2 seconds. In this image I count more than 70 dots in the one pulse! The blinking strobe within a strobe effect can be visualized this way: […….____…….____…….] where the dots represent the brief flashes that occur during each pulse and the underscores represent the off-times. This strobe within a pulse effect occurs only at night. During the day the towers blink using individual flashes that would be enormously bright if seen that way at night. Years ago a new tower I saw must have been set up wrong because it didn’t have the strobe/pulse mode and veritably lit up the countryside every time it went off! I thought to myself that that must be a real annoyance. Since then it has been programmed to act like the others.

The white tower, above the arrow in picture 1b, does not strobe anywhere near 70 times per pulse. To see the flash lag effect for yourself, as you approach the white tower, look past it at the moment the lights come on. If necessary repeat this until you see a string of dots.

For an AWESOME website that shows the flash lag effect and a lot of other spectacular optical illusions see this.

Following is brief summary for finding the cache. You should print off the pictures that I have labeled 1a, 1b and 1c. You’ll have to find two locations from where I took pictures of distant radio towers, then you'll have to drive to those towers that are initially in the distance. You’ll need to use your GPSr to measure distances and to determine “back bearings”, both of which will help you zero in on where the cache is. Finally you’ll need to find at least one of two FireTacks. The 1st Tack is a triangular orange-brown one. The second Tack is flat and white and in a small depression directly above the cache.

The Meaning of Bearing and Back Bearing
Here is an explanation of a back bearing. In order to understand the meaning of “back bearing” one should begin with the meaning of “bearing.” [NOTE: I never use bearings when hunting caches with a GPSr (only with a compass) and only use back bearings as a part of a puzzle cache.] A bearing is the angle around a compass: N = 0° (or 360°), E = 90°, S = 180° and W = 270° that points directly to an object. Now here is an example of when you could, if you wanted to, use a bearing and a back bearing. Let’s say that you rode your mountain bike to find a cache but had to abandon your wheels because of dense undergrowth. Up to that point you have been working with a GOTO to the cache. You look and see that the bearing to the cache is 90°. That means that the cache is directly E of your position. Before you take off you want to make sure you can find your bike again so you make a quick waypoint for it. You find the cache then turn back to find your bike. Now you make a GOTO BACK to your bike’s coords. You read off the “bearing,” which at this point is actually the back bearing back to your bike. It is 270°, directly W. A back bearing is simply the bearing back to a point that you originated from.

Now if you want a shot at my “Iowa Tom’s Hall of Fame” [see the brown table below] in the category of “Overachiever,” you’ll not only need to find this cache but you will also need to take –and post– a picture of the white flashing tower showing only ONE series of pulses. [Paragraph one above explains this]. To get a picture you’ll need a digital camera that can take pictures more than 1 second long. I suspect most digital cameras will work. Still interested? If so, read on….

The specific instructions follow.

(1) To begin with, print out the tower pictures so that they fit on a page. To do this I copied them to a Word document and brightened them up a bit to make them easier to study. Pictures 1a and 1b, and 1c were taken from site-1 (S1). All the telephoto images should be compared to the on site scene by holding them farther from your eyes. That will allow you to see the towers as I saw them in the distance. IT WILL HELP TO KEEP THINGS IN PERSPECTIVE IF YOU USE A CLIPBOARD AND PAPER TO DRAW A MAP OF THE TOWERS AS YOU PERCEIVE THEM. ON THE MAP YOU'LL EVENTUALLY LABEL THE TOWERS (T) AND SITES (S) WHERE YOU DETERMINE DISTANCES AND BACKBEARINGS: T1, S1, S2, T2, S3, AND T3. IT WOULD BE GREAT TO DRAW LINES AND LABEL BACKBEARING ANGLES AS WELL.

When you find S1 you'll make your first waypoint that you'll use to determine distances and backbearings. As mentioned above, pictures 1a, 1b, and 1c are necessary to complete your mission. Picture 1c is a blown up and cropped version of 1a. S1 is pretty high above the Cedar Valley. To locate S1 you should start at the webpage coords that put you on Ansborough in Waterloo. An annotated picture of what you will see from the tower there is shown here. At the webpage coord you’ll be about 60 feet W of the red light radio tower, (tower-1 or T1) looming large in the image I took from there. From the point where you are parked (in the tower driveway preferably) look through the guy-wires attached to T1. Look for the white tower (tower-2 or T2). It's near site-2 (S2). I labeled it in the picture linked to above. T2 is a few miles away from T1. You can just see what I am calling tower-3 (T-3) in the picture I took from the first tower.

Study carefully the pictures 1a and 1b and 1c (linked above) to figure out what direction you need to drive to get to S1. Pictures 1a and 1b were taken from S1. Use the sign with the upward pointing arrow to get an accurate alignment of the white tower; that will guarantee you're at S1. In picture 1c it’s especially easy to see that T1 (linked above) is just to the left of the road. You must decide which direction you must drive from T1 to keep T1 to the left of T2, as you look back toward them.

Once you arrive at S1, the place from which I took pictures 1a (cropped = 1c) and 1b (all linked above) get a waypoint. You will later use that waypoint to measure two distances and two angles (back bearings in this case). If you’re within 100 feet of the right spot you should be OK.

(2) After you get the waypoint at S1, you will need to find FireTack-1 (FT1) by S2. [NOTE! S2 is actually the location of the dashboard of your car when you are parked in the farm field entryway near FT1.] The tack is about 6-feet up on the west side of a sign post just south of a road you’ll drive on. To find that tack (use a flashlight to add to the fun), first locate the driveway that leads to T2. Now zero your odometer. Drive clockwise, using the paved roads closest to the tower. If you want help with locating S2 use this map. The odometer should read just under 0.5 miles when you pass the sign with the tack (it’ll be on your right). Park on the field entryway at S2. 15 feet east of the Tack. Determine the distance (D1) and the back bearing (BB1) from S2back to S1. Record your data in the appropriate cells in tables 2 and 3 provided below.

(3) Get close enough to the sign to read and record the second word down from the top.

(4) Convert word two (WT) to a number using the phone-pad letter-to-number sequence shown in table-1 below. The value of all those numbers added together is represented by WT. Simply add up the list of letter numbers, like 2 + 8 + 5 and so on. Record your data under WT in table-2.

(5) From S2 drive another 0.3 miles to the next field entryway. Again, use this map if you like. From the second field drive S2 look for then drive to the tallest red tower (T3) at site-3 (S3), shown in picture-2. That tower is the tallest of all that you will see in its area and is a little more than 5 miles away from S2. It’s on the other side of town.

(6) At S3 park just off the road in the driveway leading up to the red light tower. There’s no need to approach the tower. Determine the distance (D2) between S3 and S2 and the back bearing (BB2) between S3 and S2 and determine the distance (D3) between S3 and S1 and determine the back bearing (BB3) between S3 and S1. Record your data in the appropriate cells in tables 2 and 3.

(7) Now use equation-1 in table 4 to crunch your data. The result, BB4, is the back bearing from the cache location back to S2! Finally use equation-2 to get the distance, D4, from the cache back to S2.

(8) When looking for the cache, scan for fire tack-2 on a post next to a country road. I made it so the tack is only visible from a position that is at a right angle to the direction you will be traveling. When you find it, pull completely off the road. I obtained permission from the land owner, a friend of mine. The nearest house is owned by another friend of mine, the land owner’s cousin.

THE CACHE IS 7.12 MILES FROM S3! There are quite a few roads that intersect a circle with that radius. Good luck in finding it without doing the footwork, or should I say, wheel work.

Table-1 for the Second Word to Number Conversion
2 3 4 5 6 7 8 9

Table-2 for Distances
D1, miles S2 to S1 D2, miles S3 to S2 D3, miles S3 to S1 D4, miles cache to S2 WT, number for word
D1 = [________] D2 = [________] D3 = [________] D4 = [________miles ] WT = [________]

Table-3 for the Back Bearings
BB1, S2 to S1 BB2, S3 to S2 BB3, S3 to S1 BB4, cache to S2
BB1 = [________] BB2 = [________] BB3 = [___________] BB4 = [________°]

Table-4 for the Equations
Equations 1 and 2 calculate the back bearing (BB4) and the distance (D4) from the cache to S2
BB4 = (D1 + D2 + D3 + WT + ((BB1 + BB2 + BB3) ÷ 10) ÷ 1.307
D4 = BB4 ÷ 37.97

Iowa Tom’s Geocacher Hall of Fame
FTF (obsessive) STF (compulsive) TTF (simply bored) Overachieved by successfully photographing tower-2
ricann! TheLeopolds! Squeamish! ricann, TheLeopolds and TeamTrapper!

Here is a cool link that tells us how far away one's horizon is. I just discovered on 9/23/09.

Additional Hints (Decrypt)

cubgb tnyyrel + ebpx orybj SG2 + uggc://jjj.pvgl-qngn.pbz/gbjref/pryy-Jngreybb-Vbjn.ugzy

Decryption Key


(letter above equals below, and vice versa)



  • A panned shot of the white light towerI used a tripod and panned while zoomed in from over a mile away.
  • Aeronautical chart with keyThis image shows info about the chart I am using to try to locate the various towers in picture 1b.
  • Backbearing Triangle and Route
  • Compare this map with picture 1bI marked the height of quite a few towers that might be in picture 1b with a yellow line roughly proportional to the height of the top of each tower above sea level. It is proving to be very difficult determining which towers are pictured.
  • LED tail lightsI tried to photograph the flickering LED tail lights of the car in front of me while I was driving down the road. I deliberately moved the camera during a time exposure. Notice the flickering street lights as well.
  • Map for FireTack-1A map to help find the FireTack-1 and to help locate tower-3 from near tower-2.
  • Orange FireTack This is the triangular shaped tack near the white flashing tower.
  • Picture 1a at site-1. You will need to find this location.
  • Picture 1b Site-1You will need to mark this location then drive to the white tower.
  • Picture 1b with annotations Picture 1b with annotations that note how far and tall all but one of the distant red-light towers is.
  • Picture-2 Site-2 You will need to mark this location then drive to the tallest red tower.
  • The Flashing Tower Near I-380This is a short time exposure made during one of the 2 second flashes of a radio tower at the I-380 exit to Laprte City. Notice how many times is flashes per pulse!!

44 Logged Visits

Found it 20     Didn't find it 3     Write note 9     Temporarily Disable Listing 2     Enable Listing 1     Publish Listing 1     Owner Maintenance 7     Post Reviewer Note 1     

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**Warning! Spoilers may be included in the descriptions or links.

Current Time:
Last Updated: on 4/22/2015 4:58:41 PM Pacific Daylight Time (11:58 PM GMT)
Rendered From:Unknown
Coordinates are in the WGS84 datum