Sunday, January 18, 2009

Simple Survey: Suunto Tandem Inclinometer

Once piece of equipment that isn't in my bag yet but is very handy is the Suunto Tandem compass & inclinometer.

This simple piece of equipment can do many things. It can help you figure out where the sun is going to be at a certain time of day. It is very handy when setting up for golden hour shoots. It can help you figure out how many degrees a camera pans. It can also help you align your directTV satellite dish!

The tip I'm going to talk about today is how to use an inclinometer to do some simple and quick outdoor set surveys. All you need is the inclinometer/compass, a laser tape measure, and some simple math.

This piece of equipment is sometimes referred to as a "sighting" compass because the compass/inclinometer is read by looking through an eyepiece on the side of the unit. There are two eye pieces and two floating dials on the device. For purposes of this tip we're just using the top dial, the inclinometer.

Hold the inclinometer vertically so that you're looking through the eyepiece with one eye. It takes some getting used to reading because you actually read the inclinometer's scale with one eye, and sight your target with the other eye. If you're using this for the first time it may be simpler to close one eye first, focus on the scale, then open your other eye and sight your target. You need to relax your gaze so that you are looking at both the scale and your target that you want to measure. This is the most difficult to master when using this device.

Hold the scale to your dominate eye and sight with the other eye. Then simply tilt your head up and down and the scale will tell you what degree of incline you are sighting.

Ok, now that we know how to use it, let's measure something. In the above image I want to measure/survey the wall height.

In this example A, D, M, N are values that you can easily measure. B, C are difficult to measure. The Wall Height = B (calculated side of our triangle) + D (distance of our eyesight from the ground).

The first measurement we will take is "M". "M" is the angle from your eye to the top of the building. A good way not to use a lot of math is to use the 45/45/90 deg triangle method. Sight the top of your target and move closer or further away until the angle of sight is 45 degrees. Using simple math principles we know that that "N" is also 45 degrees.

From this position sight down to zero degrees. This is the base of our right angle triangle. Now we calculate distance A by using our handy Hilti laser tape range finder, measure the distance from where you are standing to the "Zero Degree incline" line you just measured.

A = 25'5". Because in a 45/45/90 degree triangle A = B. We also know that B = 25'5"

Now we measure the D. D is how far your eye line is from the ground. In this case D = 5'4".

B + D = Wall Height
25'5" + 5'4" = 29'9"

There you have it. Our wall is approximately 29'9". Now of course this isn't EXACT but its close enough to help us set a scene scale or create tracking geometry for this building.

If you can't for use the 45/45/90 degree triangle method you can still derive the distance of B but you'll need to do a bit more math. I'm not a mathemagician so I use cheat sheets. A buddy of mine (Mike Romey) showed me these Quick study laminated charts. These reference charts are another great addition to the tracking kit.

I'd like to thank Les Ekker for showing me this tip. Les is a VFX Supervisor at Zoic Studios and is an encyclopedia of VFX knowledge. I'm sure we'll see many more tips from Les.

And thanks to Max for helping me out and posing for the reference pictures.

If you have any tips that you would like to share please email me at vfxtips(at)wilkoff(dot)net.