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I'm planning to put up some holiday lights around the eaves of my home. The house is basically Tudoresque with multiple pitches. In order to make the measurements I would like to avoid climbing on a ladder with a tape measure.

Is there a way to make reasonably accurate visual measurements while standing on the ground? Or perhaps a method for measuring using photos?

Reasonably accurate would mean that a small overestimation would be ok, but an underestimation not so much.

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3  
How's your trigonometry? –  The Evil Greebo Nov 9 '11 at 19:08
    
Considering that I last used trig ~25 years ago, I'd have to brush up a bit... –  wdypdx22 Nov 9 '11 at 19:20
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Well, if you want measurements from the ground, then you are pretty much talking surveyors rig and a lot of angles, which means trig... –  The Evil Greebo Nov 9 '11 at 19:29
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That sounds like more work than climbing the ladder with a tape measure. –  wdypdx22 Nov 9 '11 at 20:07
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More brainwork, yes. What's the stereotypical saying? "Work smarter, not harder". –  KeithS Nov 9 '11 at 20:12

2 Answers 2

up vote 2 down vote accepted

For a quick and dirty estimate, I'd try the following:

  1. Take a camera and tripod as far away from the front of the house as you can get.
  2. Level the tripod and camera.
  3. Take a picture of the full face of the house, zoomed in as much as possible.
  4. Verify your scale:
    1. Measure a few points on the house (windows are good, you can measure the picture and the inside of the window) on either side and different floors.
    2. Make sure the scale of the picture at those points is consistent.
    3. Note: they're not all going to be the same. Use the least favorable scale for your estimates.
  5. Assuming the scale is reasonably consistent, measure things on the picture and multiply by the scale.
  6. Add 10% (standard practice).

This works because you specifically wanted to overestimate rather than underestimate. It will be more accurate the further you can get from the house for the picture. I would never recommend this for construction work; but it should be good for holiday lights.

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Thank you. This is the type of thing I was thinking of, but wasn't familiar with the procedure. –  wdypdx22 Nov 10 '11 at 21:08
    
Oh yeah. I definitely wouldn't do this for a construction project. The end section is only 6 feet up, so I can always run over to Home Despot for an additional strand if need be... –  wdypdx22 Nov 10 '11 at 21:30

Basically, if you want the answer without climbing up a ladder, you'll need some math and a few tools.

Let's take the basic example of the height of your home from the ground to the eaves (which is good to know because you'll need an extension cord at least that long to get to the ground, plus however much more you'll need to get to the outlet). First, measure out the distance along the ground to some point of observation. A click wheel is good, or you can pace it out if you know your average stride length. Then, you'll need some tool to measure a vertical angle. You can improvise a sextant (which is the true tool for the job) using a protractor and weighted string. Measure the angle from your point of observation up to the eaves.

Now comes the trig. The tangent of an angle theta is defined in terms of the right triangle that includes that angle; the tangent is the ratio of the side of that triangle opposite the angle, to the side of the triangle adjacent to the angle.

In other words, from your point of observation away from the house, the height of your house can be determined using the distance you are away from it, and the angle at which you have to look up to see the eave of your roof. You have the angle and the length of the adjacent side: tan(theta)= O / A, so O = A * tan(theta). Now, because you were probably standing when you took the angle measurement, add your height to the answer.

You can use similar methods to find the length of a horizontal line on the roof, the length along a gable, etc etc.

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