Tuesday, June 30, 2020

Over-Solving or Solving Problems You Don't Have

Sometimes we call them "Belt and Braces" solutions. As a former suspenders person who switched to belts, the idea of wearing both is a little like over-engineering. In the unlikely event of catastrophic failure of one system, your pants can still remain properly hoist. There's a weird, but defensible reason for that. Most over-engineering lacks a coherent reason. 

Sometimes we call them "Bells and Whistles." The solution has both bells and whistles for signaling. This is usually used in a derogatory sense of useless noisemakers, there for show only. Again, there's a really low-value and dumb, but defensible reason for this. 

While colorful, none of this is helpful for describing over-engineered software. Over-engineered software is often over-engineered for incoherent and indefensible reasons.

Over-engineering generally means trying to solve a problem that no user actually has. This leads to throwing around irrelevant features.

Concrete Example

I lived on a boat. I spent a fair amount of time fretting over navigation. 

There are two big questions: 
  1. How far apart are two points, really. 
  2. What's the real bearing from one point to another.
These are -- in some cases -- easy to answer.

If you have a printed, paper chart at the right scale, you can use dividers to compute a distance. It's actually a very easy task. Similarly, you can read the bearing off the chart directly. There's a trick to comparing a course to a nearby compass rose, but it's easy to learn and very accurate.

Of course, we don't want to painstakingly copy our notes from a paper chart to a spreadsheet to add them up to get total distance. And then fold in speed to get time and fuel consumption. These summary computations are a pain.

What you want is to do all of this with a computer.
  1. Plot the points using a piece of software like OpenCPN (https://opencpn.org).
  2. Extract the GPX file.
  3. Compute distances, bearings, and durations to create a route.
"So?" you ask.

So. When I did this, I researched the math and got a grip on the haversine formula for doing the spherical geometry computation of distances between points on a sphere.

It's not too bad. The formula are big-ish. But manageable. See http://www.edwilliams.org/avform.htm#Dist for the great circle distance formula.

For airplanes and powered freighters crossing oceans, this is perfect.

For a small sailboat going from Annapolis, Maryland, to the Bahamas, this level of complexity is craziness. While accurate, it doesn't really solve the problem I have. 

I don't actually need that much accuracy. 

I need this much accuracy.

And no more. This is the essential hypotenuse distance using an R-factor to convert the difference between latitudes and the distance between longitudes into pretty-close distances. For nautical miles, R is 60×180÷π. 

This is simpler and it solves the problem I actually have. Up to about 232 miles, the answer is within 1 mile of correct. The error grows quickly. Double the distance and the error seems to jump to 8 miles. A 464 mile sailing journey (at 6 knots) takes 3 days. Wind, weather, tides and currents will introduce more error than the simplifying assumptions.

What's important is this can be put into a spreadsheet without pain. I don't need to write sophisticated Python apps to apply haversine to sequences of way-points. I can do a simpler hypotenuse computation on waypoints converted to radians.

Is there a lesson learned?

I think there is.

There's the haversine a super-general solution. It handles great-circle routes elegantly. 

But it doesn't solve my actual problem. And that makes it over-engineering.

My problem is what we call rhumb-line sailing. Over short-enough distances the world may as well be flat. Over slightly longer distances, errors in the ship's compass and speedometer make a hyper-accurate great circle route moot. 

(Even with several fancy GPS-based navigation computers, a prudent mariner has paper backups. The list of waypoints, estimated times and directions are essential when the boat's GPS reciever fails.)

I don't really need the sophistication (and the potential for bugs) with haversine. It doesn't solve a problem I actually have.

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