Tuesday, November 24, 2015

Navigation: Latitude, Longitude, Haversine, and all that

For a few years, I was a tech nomad. See Team Red Cruising for some stories of life on a sailboat. Warning: it's pretty dull.

As a tech nomad, I lived and died (literally) by my ability to navigate. Modern GPS devices make the dying part relatively unlikely. So, let's not oversell the danger aspect of this.

The prudent mariner plans a long voyage with a great deal of respect for the many things which can go wrong. One aspect of this is to create a "Float Plan". Read more about it here: http://floatplancentral.cgaux.org.

The idea is to create a summary of the voyage, provide that summary to trusted shore crew, and then check in periodically so that the shore crew can confirm that you're making progress safely. Failure to check in is an indicator of a problem, and action needs to be taken. We use a SPOT Messenger to check in at noon (and sometimes at waypoints.)

Creating a float plan involved an extract of the waypoints from our navigation software (GPS NavX). I would enrich the list of waypoints with estimated travel time between the points.  Folding in a departure time would lead to a schedule that could be tracked. I also include some navigation hints in the form of a bearing between waypoints so we know which way to steer to find the next point.

The travel time is the distance (in  nautical miles) coupled with an assumption about speed (5 knots.) It's a really simple thing. But the core haversine calculation is not a first-class part of any spreadsheet app. Because of the degrees-to-radians conversions required, and the common practice of annotating degrees with a lot of internal punctuation (38°54ʹ57″ 077°13ʹ36″), it becomes right awkward to simply implement this as a spreadsheet.

Some clever software has a good planning mode. The chartplotter on the boat can do a respectable job of estimating time between waypoints. But. It's not connected to a computer or the internet. So we can't upload that information in the form of a float plan. The idea of copying the data from the chart plotter to a spreadsheet is fraught with errors.


Enter navtools. This is a library that I use to transform a route into a .csv with distances and bearings that I can use to create a useful float plan. I can add an estimated arrival time calculation so that a change to departure time creates the entire check-in schedule.

This isn't a sophisticated GUI app. It's just enough software to transform a GPS NavX extract file into a more useful form. The GUI was a spreadsheet (i.e., Numbers.) From this we created a PDF with the details.

Practically, we don't have good connectivity on the boat.  So we would create a number of alternative plans ("leave tomorrow", "leave the day after", "leave next Monday", etc.) we would go ashore, find a coffee shop, and email the various plans to ourselves. They could sit in our inbox, waiting for weather and tide to be favorable.

Then, when the weather and tides were finally aligned, we could forward the relevant details to our trusted shore crew. This was a quick spurt of cell phone connectivity to forward an email. It worked out well. When the scheduled departure time arrived, we'd coax Mr. Lehman to life, raise the anchor and away.

Literate Programming

This is an exercise in literate programming. The code that's executed and the HTML documentation are both derived from source ReStructured Text (RST) documents. The documentation for the navigation module includes the math along with the code that implements the math.

I have to say that I'm enthralled with the intimate connection between requirements, design, and implementation that literate programming embodies.

I'm excited to (finally) publish the thing to GitHub. See https://github.com/slott56/navtools.  I'm looking at some other projects that require the navtools module. What I wind up doing is copying and pasting the navigation calculation module into other projects. I had something like three separate copies on my laptop. It was time to fold all of the features together, delete the clones, and focus on one authoritative copy going forward.

I still have to remove some crufty old code. One step at a time. First, get all the tests to pass. Then expunge the old code. Then make progress on the other projects that leverage the navtools.navigation module.

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