There are two types of slope lasers. The first is single slope
that will give you a slope in one axis or direction at a time.
The second is a dual slope laser that will give you two slopes
on separate axis at the same time. Slope lasers come with different
ways to adjust the grade. The first would be a manual grade slope
laser that you would figure out what the grade is to be and set
your detector and rod to that grade, then manually moving the beam
up or down until your detector comes on to center. This type will
not have a display on the laser to read grade.
The second is a dual slope this type will have a display to see
the different grades on the laser and will be more expensive than
the single slope laser. Dual
slope lasers give you the option to induce a grade in both axes giving you
a compound grade.
Calculating
a Slope using a Slope Laser with a Topographic Map
Determining
the average slope of a hill using a topographic map
is fairly simple. Slope can be given in two different ways,
a percent gradient and an angle of the slope. The initial steps
to calculating slope either way are the same.
1) Decide
on an area for which you want to calculate the slope (note, it
should be an area where the slope direction does not
change; do not cross the top of a hill or the bottom of a valley).
2)
Once you have decided on an area of interest, draw a straight
line perpendicular to the contours on the slope. For the
most accuracy, start and end your line on, rather than
between,
contours on the
map.
3) Measure
the length of the line you drew and, using the scale of the map,
convert that distance to feet. (insert
image with
the line drawn on it, conversion calculation)
4) Determine
the total elevation change along the line you drew (subtract
the elevation of the lowest contour
used from
the elevation
of the highest contour used). You do not need to do any
conversions on this measurement, as it is a real-world
elevation change.
To calculate
a percent slope, simply divide the elevation change in feet by
the distance of the line you drew (after
converting
it to feet). Multiply the resulting number by 100 to
get a percentage value equal to the percent slope of
the hill.
If
the value you
calculate is, for example, 20, then what this means
is that for every 100 feet you cover in a horizontal direction,
you
will
gain (or lose) 20 feet in elevation.
To calculate
the angle of the slope, divide the elevation change in feet by
the distance of the line you drew
(after converting
it to feet). This is the tangent value for the angle
of the slope. Apply an arctangent function to this
value to
obtain
the angle
of the slope (hit the ‘inv’ button and then the ‘tan’ button
on most scientific calculators to get the slope angle).
The angle you calculated is the angle between a horizontal
plane and the
surface of the hill.
Rise divided
by run multiplied 100 = % slope
