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Comment by Animats | Hacker News Reader

Well, there's Earth Centered Earth Fixed, which is what you actually get from GPS systems. Latitude and longitude are just for human-readable I/O. ECEF has the Z axis through the poles, the XY plane through the equator, and the X axis in a plane that goes through Greenwich.

This is converted to latitude and longitude from GPS using the WGS-84 standard, at least for the western world. Russia uses the PZ-90 reference frame for GLONASS. China uses the BeiDou coordinate system. There's also an obfuscated latitude and longitude used by China for public consumption, GCJ-02. This introduces an error of 100 to 700 meters. Not that anyone is fooled; the obfuscation algorithm is known.

But everybody uses the same ECEF 3D coordinates, and combining satellite data is done in that frame. So if you need precise locations, there's something to be said for working in ECEF.

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I am with you, https://en.m.wikipedia.org/wiki/Earth-centered,_Earth-fixed_... would be my preference, too. Especially if we’re talking internal representation. My background is having to track objects that fly or go below the surface. Though as others have mentioned, asking “is this object on the ground currently” is a tiny bit complicated.

I've spent many hours looking for open-source geo data sets, but don't recall finding any that preserve the raw ECEF. Anyone know of any?

Yeah but usually people want to do something silly like place a point on the surface of the Earth :).

Yeah, it is actually silly if you don't have a decent DEM, though you can get away with a geoid approximation if you only want to render a pretty picture for someone navigating to the nearest ATM.

fortunately, between SRTMv3, ASTER, ArcticDEM and REMA, a free DEM is available for practically the entire world (maybe missing some subantarctic islands?).

But we want to do more than that, and dealing with elevation can be surprisingly complex and fragile.

Imagine you are building an offshore wind turbine. You want 20m of clearance between the sea and turbine blade. That means 20m above highest possible tide. Tidal data is only available for a port 200 miles away and will be different for you based on local gravity. So you use modelling from satellite based gravity data to make an assumption. And then give instructions to a ship out at sea who has a contractual obligation to place the turbine within 30cm of the required position. Which coordinate system do you use? And when you are modelling a microwave link from that turbine to an onshore location on the horizon that is surveyed using a local datum what coordinate system do you use?

In reality geocentric coordinates are unlikely to be used. But maybe it would be easier if they could be. The world it not flat and it is not easily modelled as a sphere. A set of coordinates always carry with them a set of assumptions that may not work with your particular task. Geocentric coords can remove some of those assumptions.

> In reality geocentric coordinates are unlikely to be used. But maybe it would be easier if they could be

I agree. When objects interact with each other, having everything as (x,y,z) makes many things a lot easier.

So in your example, ideally, you send the ship the (x,y,z) coordinates. Internally, we would have to worry about tides and figuring out exactly what the coordinates should be, but crew installing it would know exactly where to place it.

Not even counting the tides, sea level is not flat across the globe. We live on a spheroid with varying densities and other things which goes around bigger sphere and has big sphere next to it. In the end real world is quite messy.

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