Let’s pick this up right where the geoid story gets practical — Page 8, Lesson 12

Let’s pick this up right where the geoid story gets practical. I want to walk you through why the choice of geoid actually matters to you as a pilot, and how two modern systems forced the world to standardise. Remember, a geoid is the mathematical model of the Earth’s shape that a mapping agency uses as its reference surface. Now, here’s the key point: each agency has tended to optimise its geoid to give the best fit to the actual shape of the Earth over the area in which it is interested in mapping. That means the geoid fits beautifully over its own home territory, but it may not fit the actual Earth in another part of the world. So you get a patchwork of different models, each perfect locally and imperfect globally. Let me give you the concrete examples from the text. The UK Ordnance Survey uses a geoid based on a survey of 1936 — that’s called OS36. France has tended to use the Nouvelle Triangulation de France 1970 model — that’s NTF70. Some other European countries use the European Datum 1950 model — ED50. And the USA uses the World Geodetic System 1984 — WGS 84. So four different agencies, four different reference surfaces. Now, why does this matter? Because using different geoids can result in arriving at different values for defining latitude and longitude. The same physical point on the ground gets different coordinates depending on which geoid you measure it against. And the differences are not trivial. There can be differences of up to the order of 200 metres for positions on the extremities of the European ED50 and the UK OS36. So if you’re near the edge of the overlap between those two datums, your position could be off by up to 200 metres just because of the datum choice. Now, the text makes a lovely point here. It says this may not sound much to an airline pilot — though it would to the programmer of a cruise missile which was guided to latitude and longitude coordinates. So 200 metres is a big deal if you’re trying to hit a specific point, but for a while, in aviation, these differences were not considered significant. Until recently, that is. Two recent developments changed this. The first is the arrival of the Global Positioning System — GPS. The second is the widespread use of Flight Management Systems — FMS. Let’s take GPS first. GPS is an electronic navigation system in which aircraft receivers compare signals from several of the 24 transmitters in the satellites which make up the GPS constellation. So there are 24 satellites, and your receiver listens to several of them at once and compares the signals to work out your position. It can be received over the whole globe, and its accuracy is of the order of tens of metres. So GPS is accurate to tens of metres, worldwide. Now, here’s the crucial point: the system accuracy is such that the differences in geoids become significant. If your geoid is off by 200 metres, and your GPS is accurate to tens of metres, then the geoid error swamps the system accuracy. So the US government adopted WGS 84 for GPS. That was the decision — GPS uses WGS 84 as its datum, worldwide. Now the second development, FMS. The Flight Management System compares the output of Inertial Reference Systems — IRS — with positions derived from range information received from Distance Measuring Equipment — DME. So you have two sources: the IRS gives you an inertial position, and the DME gives you range information from ground stations. The FMS compares the two. Now, the positions of the DME stations are stored in latitude and longitude held in the data base of the FMS computer. So the FMS has a database of where every DME station is, in lat and long. Here’s the problem. DME is a very accurate system, and any inaccuracy in the datum positions would degrade the position calculation of FMS. So if the DME station’s stored coordinates are wrong because they’re on the wrong geoid, the whole FMS position calculation suffers. The text gives a vivid example. If the data base held the positions of all the UK DMEs in OS36 and the French DMEs in NTF70, it could cause large discontinuities in the calculation of FMS position as the aircraft crossed the English Channel. So you’re flying from the UK to France, and suddenly the datum changes from OS36 to NTF70, and your FMS position jumps. That’s a real operational problem. So the takeaway is this: the geoid choice is not academic. GPS forced a worldwide standard — WGS 84. And FMS, with its database of DME positions, forced the same standardisation, because mixing datums across a route creates discontinuities. That’s why, in modern navigation, WGS 84 is the common reference.