to the second line and a flashing icon will appear at the left of five dashes
(representing the distance value to be subtracted) on the third line. Then press the
button while aiming the laser at the point from which the first
measurement was made. The LDM will measure the distance to that point, replace
the dashes on the third line with that value, and display the difference of the two
measurements on the bottom line.
Indirect Measurements of Height or Length using Triangulation
The LDM can use triangulation (one type of indirect measurement based on
Pythagorean geometry) to calculate the height or length of an object from a
distance. The instrument can perform three kinds of Pythagorean calculations:
•
Triangulation with two inputs.
This kind of distance measurement can be
made only for distances that present you with a right angle. A good example is
measuring the height of a building from across the street at ground level (Fig. 8).
Because the LDM and the bottom of the building are both at ground level, the side
of the building (whose height “A” is unknown) forms one leg of a right triangle
whose other leg is the distance across the street (“B” in the figure). In other
words, you can use triangulation to determine the height “A” using only two
inputs because “A” is perpendicular to “B”—one of the distances you can
measure. The LDM can measure “B” as well as the distance to the top of the
building (“C” in the figure), which is the hypotenuse of the right triangle. Once
the LDM has determined the values of “B” and “C”, it calculates the value of “A”
according to Pythagoras’ famous equation: A
2
+ B
2
= C
2
.
Fig. 8. Triangulating a height using two inputs
•
Triangulation with three inputs.
This kind of distance measurement can be
made for distances that do
not
present you with a right angle. A good example is
measuring the height of a building from another building across the street through
an open fourth-floor window (Fig. 9). Because the LDM and the bottom of the
target building are not both at ground level, you must measure one common leg
“B1/B2” (which is perpendicular to the wall of the building) and the hypotenuses of
13
C
B
A
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