Future Proof

[Gabriel Funvake]

I am interested in knowing more about the accuracy of the plus code.

I have been working with organisations who need to define locations for delivery and servicing applications.  We have been looking at "What Three Words" as well, but I prefer your system and licensing model.

I am assuming that your system is based on the WGS84 spheroid?

We are also hoping that we can add a fourth value after the "+" in your Code to bring the grid down to a 1mx1m.  Is that a possibility?

We also want to make provision for a process / system that is Geoid based - i.e. relative to specific tectonic plates.  Why is this relevant?  Our services are relative to real world places - i.e. places that are fixed to the Tectonic plates, not the WGS84 Spheroid - which as you know is fixed to observations external to earth - i.e. celestial bodies.  Given that the average tectonic movement is a negligible 2-3 cm per year, this may seem unimportant.  When the plate moves - as during or after earthquakes - the WGS84 coordinates will of course be different for the same place.  In some places, especially around the so-called "ring of fire" movements can be as much as just around 10cm per year with jumps of several meters during earthquakes.  Over a ten year period that makes a lot of difference.

So my question is this - have you considered how to future proof your system?

Regards - Gabriel

[Doug Rinckes]

Hi Gabriel,

You can add as many digits after the "+" sign as you wish - that's up to you. But you then quickly get into the difference between "accuracy" and "precision". For example, a plus code with four digits after the "+" sign has high precision, but the accuracy depends on the equipment you are using to provide the coordinates, and also the equipment you use to locate those coordinates after decoding. If you're relying on smartphones, even though they might claim to give a GPS location within +/-5 meters, their real-life accuracy may be significantly worse! In that case, you end up with a code for a 1x1 meter area, very precise, but it's 20 meters away from where you think it is.

One other aspect you will want to consider is the ease of use. For use in Google Maps, we decided that two digits after the plus sign gives a code that's short enough to remember, and precise enough to get you close, without giving a spurious accuracy. (That is, if the device that generates the code isn't accurate to a meter, there's no point generating a code that is precise to a meter.) 

Long codes can be useful in small or specialised communities - but if it's for the public, I'd suggest that using the 4+2 version of a short code is going to be accurate enough and it will become more familiar over time. If people display their code by their doorway it won't matter if you're a few meters out, just like how you still look around for a house number when using navigation now.

Secondly, yes, we are using WGS84, and I assume anyone else supporting the codes publicly will do so as well. If you are using them within a closed community, then you could use any system or projection you like, but you may get unintended results if anyone enters a code into Google Maps. Remember that the input is basically just two floating point numbers between -180 and 180, and -90 and 90. There's nothing in the plus code algorithm that requires or prevents those to being WGS84, or NZ Map Grid or the Swiss Grid or anything else.

I'm actually from New Zealand, so I'm familiar with tectonic movements (very familiar), but we decided that in terms of the problems we want to address, even high speed movements are unlikely to be a problem for many years. Even if somehow all of India moves by 10 meters, the addresses will still mostly work because you'll look for the signs or just ask. The point being, that getting to within 10 meters of someone's house or business is better than not getting there at all. (This is also another reason to avoid using extremely precise codes, since they will become inaccurate faster.)

Whether these are issues for you really comes down to what you want to use the codes for, what problems you're trying to solve, and what attributes are important. It's very important to consider those carefully. In our case, we are mostly concerned with trying to give 75% of the world a way to tell you where they are, in a way they can remember and describe easily, and that uses existing information like town and state information. If you want to be able to do drone deliveries into the third floor window of an apartment building, or to track locations of objects for a span of thousands of years, plus codes on their own probably isn't going to do it.

Happy new year!

Doug

Doug Rinckes, Technical Program Manager, Google Switzerland GmbH; 9GHJ+P88 Zürich

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Public site: http://www.openlocationcode.com/
Github project: https://github.com/google/open-location-code
Demo site: http://plus.codes/
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