# A Heuristic Approach to Calculating Geometric Medians

I was recently given an interesting problem:

Given a set of points in on a common plane , find a point on that minimizes the net cost of traveling from to each other point.

Ignoring the problem of calculating cost The differences between traveling by car, bike, and foot include important time and cost differences. Here, we’ll employ Euclidean distance as an uncomplicated, normalizing cost function. , this problem may seem fairly straightforward. Let’s look at some approaches.

## #0: as the Median

As it happens, the median of a set of points is guaranteed to be the
cost-minimum center of those points **only in
space**. We can show that the median does not hold in
via a counter-example: create a triangle with vertices
, the median of which is at
:

The net cost of traveling from the vertices to the center point can be improved by if we make that center point :

For most, isn’t all that much, and the median is a
pretty good approximation for the latter point - the **geometric median**
More info.
,
or **center** as I will refer to it from here. But what if the percent
improvement grows higher, say to or
or ? How do we derive this geometric median?

## #1: Function Minimization

What if we attempt to minimize the net cost function? Say that for some point , the net cost of traveling from a set of points with coordinate form to is

Note that this function does *not* have the same minimum as
, the minimum of which is the *median*.

Hey, pretty good! Let’s do , , throw in a second derivative test, and call it a day.

Oh man. Maybe not. That looks like a pain in the ass to solve for
, let alone simultaneously with
I have
no idea how to do this efficiently, but if you do, please
let me know.
Looks like it could be worth a **lot** of money .
.
Don’t forget this process would have to be automated, too - *wow* that looks fun!

## #2: Heuristic Approximation

You know the old saying:

If you can’t derive ‘em, approximate ‘em!

…and so heuristics come to the rescue! How? Well, a good place to start
is ~~WolframAlpha~~ a contoured version of the function whose extrema are
in question.

As expected, the contour slopes in around the previously-shown minimum
. Interestingly, there seems to only be one
minimum for this function - and in general, ** there is** only one
center
See
this paper, which
shows that the geometric center is unique and convergent for any set of
non-co-linear points.
for any set of points! This means we can employ
some descent-type optimization method.

Okay, but where do we descend from?

A convenient place would be from the median. It’s cheap to compute and fairly close to the center.

How do we descend?

This is a bit trickier. Of course, we can only travel in
, so let’s just say we can go up, down,
left, or right
Formally, `+y`

, `-y`

, `-x`

, or
`+x`

. A more thorough traversal could also employ diagonal directions.
.
To find the best direction of descent, we travel to a candidate point in each
direction and determine the one that most improves the cost.

What if no direction lowers the cost?

Then we’ve found our center! Except we could probably do better, by decreasing our search radius and repeating the descent mechanism again. And again, and again, until the approximation of the geometric center is within some acceptable margin of error .

Alright, let’s code it!
These examples are in `Rust`

,
but it should be easy to implement the logic in any language of your choice. The
full code can be found here, with a `C`

version here.
First, the boilerplate - a euclidean cost function.

And now, we can use the previously-defined specification to write a complete algorithm.

Let’s plug in our three points from before, and…

`[0.21126302083333331, 0.21126302083333331]`

Awesome!

## #n: Some Details

This function runs in time A method for exact computation of the geometric center in near-linear is proposed here. , and its precision can be improved by reducing (epsilon) to an arbitrary minimum.