In MongoDB, the $degreesToRadians aggregation pipeline operator converts an input value measured in degrees to radians.

$degreesToRadians accepts any valid expression that resolves to a number.

The $degreesToRadians operator was introduced in MongoDB 4.2.

Example

Suppose we have a collection called test with the following document:

{ "_id" : 1, "data" : 180 }

And let’s assume that the value of the data field is in degrees (therefore, in this case it’s 180 degrees).

We can use the $degreesToRadians operator to convert the data field to radians:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        degrees: "$data",
        radians: { $degreesToRadians: "$data" }
      }
    }
  ]
)

Result:

{ "degrees" : 180, "radians" : 3.141592653589793 }

By default, the $degreesToRadians operator returns values as a double, but it can also return values as a 128-bit decimal as long as the expression resolves to a 128-bit decimal value.

128-Bit Decimal Values

If the expression provided to $degreesToRadians is 128-bit decimal, then the result is returned in 128-bit decimal.

Suppose we add the following document to our collection:

{ "_id" : 2, "data" : NumberDecimal("0.1301023541559787031443874490659") }

Let’s run the the $degreesToRadians operator against that document:

db.test.aggregate(
  [
    { $match: { _id: 2 } },
    { $project: { 
        _id: 0,
        degrees: "$data",
        radians: { $degreesToRadians: "$data" }
      }
    }
  ]
).pretty()

Result:

{
	"degrees" : NumberDecimal("90.1301023541559787031443874490659"),
	"radians" : NumberDecimal("1.573067041239514175890278047405623")
}

The output is 128-bit decimal.

Null Values

Null values return null when using the $degreesToRadians operator.

Suppose we add the following document to our collection:

{ "_id" : 3, "data" : null }

Let’s run the the $degreesToRadians operator against that document:

db.test.aggregate(
  [
    { $match: { _id: 3 } },
    { $project: { 
        _id: 0,
        radians: { $degreesToRadians: "$data" }
      }
    }
  ]
)

Result:

{ "radians" : null }

We can see that the result is null.

NaN Values

If the argument resolves to NaN, $degreesToRadians returns NaN.

Example:

db.test.aggregate(
  [
    { $match: { _id: 3 } },
    { $project: { 
        _id: 0,
        radians: { $degreesToRadians: 0 * "$data" }
      }
    }
  ]
)

Result:

{ "radians" : NaN }

Infinity

If the argument resolves to Infinity or -Infinity, the $degreesToRadians operator returns Infinity.

Suppose we add the following document to our collection:

{ "_id" : 4, "data" : Infinity }

Let’s run $degreesToRadians against the data field:

db.test.aggregate(
  [
    { $match: { _id: 4 } },
    { $project: { 
        _id: 0,
        radians: { $degreesToRadians: "$data" }
      }
    }
  ]
)

Result:

{ "radians" : Infinity }

Non-Existent Fields

If the $degreesToRadians operator is applied against a field that doesn’t exist, null is returned.

Example:

db.test.aggregate(
  [
    { $match: { _id: 4 } },
    { $project: { 
        _id: 0,
        radians: { $degreesToRadians: "$name" }
      }
    }
  ]
)

Result:

{ "radians" : null }

Combined with Other Operators

Some aggregation pipeline operators accept their expressions in radians. If a field has its value in degrees, you can combine $degreesToRadians to convert the expression to radians.

Suppose we have a collection called test with the following document:

{ "_id" : 1, "data" : 3 }

And in this case, the data field’s value is in degrees.

Now suppose we want to use the $cosh operator to return the hyperbolic cosine of the data field. The only problem is that the $cosh operator accepts expressions in radians.

No problem! We can use $degreesToRadians to convert the input expression to radians.

Example:

db.test.aggregate(
  [
    { $match: { _id: 1 } },
    { $project: { 
        _id: 0,
        hyperbolicCosine: { $degreesToRadians: { $cosh: "$data" } }
      }
    }
  ]
)

Result:

{ "hyperbolicCosine" : 0.17571384980422547 }