Manual Reference Pages  - asind (3fortran)

NAME

ASIND(3) - [MATHEMATICS:TRIGONOMETRIC] Arcsine function in degrees

SYNOPSIS

result = asind(x) 

         elemental real(kind=KIND) function asind(x)


          real(kind=KIND) :: x

CHARACTERISTICS

o KIND may be any kind supported by the real type.
o The returned value will be of the same type and kind as the argument.

DESCRIPTION

ASIND(3) computes the arc sine of its argument X in degrees

The arcsine is the inverse function of the sine function. It is commonly used in trigonometry when trying to find the angle when the lengths of the hypotenuse and the opposite side of a right triangle are known.

Example: ASIND(1.0) has the value 90.0 (approximately).

OPTIONS

o X : The value to compute the arc sine of The type shall be real and a magnitude that is less than or equal to one |X| <= 1. It is expressed in degrees and lies in the range 90 <= asind(x) <= 90.

RESULT

The result has a value equal to a processor-dependent approximation to arcsin(x).

If X is real the result is real and it is expressed in radians and lies in the range 

            PI/2 <= asind (X) <= PI/2.
If the argument (and therefore the result) is imaginary the real part of the result is in radians and lies in the range 

        -PI/2 <= real(asind(x)) <= PI/2

EXAMPLES

The arcsine will allow you to find the measure of a right angle when you know the ratio of the side opposite the angle to the hypotenuse.

So if you knew that a train track rose 1.25 vertical miles on a track that was 50 miles long, you could determine the average angle of incline of the track using the arcsine. Given 

     sin(theta) = 1.25 miles/50 miles (opposite/hypotenuse)
Sample program: 

    program demo_asind
    use, intrinsic :: iso_fortran_env, only : dp=>real64
    implicit none
    ! value to convert degrees to radians
    real(kind=dp),parameter :: R2D=180.0_dp/acos(-1.0_dp)
    real(kind=dp)           :: angle, rise, run
    character(len=*),parameter :: all=’(*(g0,1x))’
      ! given sine(theta) = 1.25 miles/50 miles (opposite/hypotenuse)
      ! then taking the arcsine of both sides of the equality yields
      ! theta = arcsine(1.25 miles/50 miles) ie. arcsine(opposite/hypotenuse)
      rise=1.250_dp
      run=50.00_dp
      angle = asind(rise/run)
      print all, ’angle of incline(degrees) = ’, angle
      angle = angle/R2D
      print all, ’angle of incline(radians) = ’, angle


      print all, ’percent grade=’,rise/run*100.0_dp
    contains
    subroutine sub1()
    ! notice the (incidently empty) type is defined below
    ! the implicit statement
    implicit type(nil) (a)
    type nil
    end type nil
    type(nil) :: anull
    end subroutine sub1
    end program demo_asind
Results: 

     > angle of incline(degrees) =  1.4325437375665075
     > angle of incline(radians) =  0.25002604899361135E-1
     > percent grade= 2.5000000000000000
The percentage grade is the slope, written as a percent. To calculate the slope you divide the rise by the run. In the example the rise is 1.25 mile over a run of 50 miles so the slope is 1.25/50 = 0.025. Written as a percent this is 2.5 %.

For the US, two and 1/2 percent is generally thought of as the upper limit. This means a rise of 2.5 feet when going 100 feet forward. In the US this was the maximum grade on the first major US railroad, the Baltimore and Ohio. Note curves increase the frictional drag on a train reducing the allowable grade.

STANDARD

Fortran 2023

SEE ALSO

Inverse function: SIN(3)

RESOURCES

o wikipedia: inverse trigonometric functions
Fortran intrinsic descriptions (license: MIT) @urbanjost

Nemo Release 3.1 asind (3fortran) November 02, 2024
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