Convert kilometers to radians

Question

I need to convert kilometers to radians. Is this correct formula? I need radians for nearsphere in MongoDB.

If I need to convert 5 kilometers to radians I do this:

5/6371

And I get this result (does it seem correct):

0.000784806153

UPDATE

This is not a math issue, I really need to know if I am doing the correct calculations from kilometers to radians to be able to do geospatial queries with MongoDB.

@duffymo I would guess, the earth. Nautical miles were arrived at by converting degrees to a form of distance, after all. — Jon Hanna, Aug 29 '12 at 14:33
So division is a problem? Is the answer to pat the OP on the head? — duffymo, Aug 29 '12 at 14:47
The reason I ask if the above is correct formula/result is that I need my iOS app to use correct radians when performing calculations against my MongoDB backend. — Jonathan Clark, Aug 29 '12 at 14:50
Meridional radius of earch is about 6,367.445 km. Radian measure of angle is given at http://www.themathpage.com/atrig/arc-length.htm. Looks like you're fine. — duffymo, Aug 29 '12 at 15:36
Dupe of http://stackoverflow.com/questions/25138862/converting-earth-kilometers-to-radians-in-php. Your equation matches that answer. — Steve Tarver, Sep 14 '15 at 03:17

score 18 · Answer 1 · edited Aug 17 '19 at 13:23

Kilometers to radians or distance and radian conversion

I arrived here, was confused, then I watched some Khan academy videos and it made more sense at that point and then I was able to actually look at equations from other sources to further educate myself.

Here's my train of thought.

I see a diagram about radians and I first think radius from a geolocation point which is wrong.
Instead, imagine the earth cut perfectly in half and just focus on one of the halves.

Now face that half toward you and look at the math diagram.

Think of the math diagram as showing from the center of the earth measuring the edge of the earth based on the arc length, after all the earth is curved so any measurement will be curved on the surface of the earth.
Radians are like degrees in a circle and the arc length is literally the distance between A and B in the math diagram.
To you, its a straight line on a bird's eye view, but really it's just a curve in 3d space along the edge of the earth.
Eureka! A lightbulb went on in my head.

distance = earth radius * radians

Thus with some very easy algebra...

radians = distance / earth radius

km radians = distance in km / 6371

mi radians = distance in mi / 3959

Sometimes thinking it through is fun.

Double-check this... https://www.translatorscafe.com/unit-converter/en/length/7-89/kilometer-Earth%E2%80%99s%20equatorial%20radius/

Now in regards to Mongo v3.2 specifically using mongoose in node.js

Despite my best efforts, mongo would not behave correctly as documented for a $geoNear query on a 2d index. never worked

  let aggregate = [
    {
      $geoNear: {
        near: { type: 'Point', coordinates: lonLatArray },
        spherical: false,
        distanceField: 'dist.calculated',
        includeLocs: 'dist.location',
        maxDistance: distanceInMeters / (6371 * 1000),
        query: {
          mode: 'nearme',
          fcmToken: { $exists: true }
        }
      }
    },
    { $skip: skip },
    { $limit: LIMIT }
  ];

However, when I changed to a 2dsphere index, it worked perfectly.

  let aggregate = [
    {
      $geoNear: {
        near: { type: 'Point', coordinates: lonLatArray },
        spherical: true,
        distanceField: 'dist.calculated',
        includeLocs: 'dist.location',
        maxDistance: distanceInMeters,
        query: {
          mode: 'nearme',
          fcmToken: { $exists: true }
        }
      }
    },
    { $skip: skip },
    { $limit: LIMIT }
  ];

But education never seems like a waste of time.

score 3 · Answer 2 · answered Nov 25 '15 at 11:59

You are correct. In spherical geometry you divide the distance with the radius of the sphere. Mind that you should keep the units. So if you calculate the sphere radius in kilometers then you should use distance in kilometer. If you use miles then you should use Earth radius in miles (approx: 3,963.2).

score 3 · Answer 3 · answered Mar 08 '19 at 09:23

The equatorial radius of the Earth is approximately 3,963.2 miles or 6,378.1 kilometers.

Note : 1 KM = 0.621371 Mile

Here is some simple formulas for calculation :

100 KM to Miles : (100 * 0.621371)

100 KM to radiant : 100 / 6378.1

100 Miles to radiant : 100 / 3963.2

So if you have data in Kilometers then you must use (100 / 6378.1) and for Miles data you can use (100 / 3963.2)

score 1 · Answer 4 · answered Aug 03 '18 at 18:14

To convert:

distance to radians: divide the distance by the radius of the sphere (e.g. the Earth) in the same units as the distance measurement.
radians to distance: multiply the radian measure by the radius of the sphere (e.g. the Earth) in the units system that you want to convert the distance to.

for Example: if you want to convert 5 miles in radians, then we need to divide the distance(5 miles) by the radius of the sphere(also in miles), which is 3959. then 5/3959 is 0.0012629451...

Thanks

Convert kilometers to radians

4 Answers4

Kilometers to radians or distance and radian conversion

Now in regards to Mongo v3.2 specifically using mongoose in node.js