Let me give you the real-world examples from the book — Page 8, Lesson 12

Let’s pick up where we left off. I want to walk you through the practical problem that arises from having different geoids around the world. Each mapping agency has tended to optimize its own geoid model to give the best fit to the actual shape of the Earth over the specific area it is interested in mapping. That sounds sensible, but it means that a geoid that fits perfectly over one region may not fit the actual Earth in another part of the world at all. Let me give you the real-world examples from the book. The UK Ordnance Survey uses a geoid based on a survey from 1936 — they call it OS36. France has tended to use the Nouvelle Triangulation de France 1970 model, abbreviated NTF 1970. Some other European countries use the European Datum 1950 model, which is ED50. And the USA uses the World Geodetic System 1984 — that is WGS 84. So you have at least four different geoids in use just among these examples. Now, what happens when you use different geoids? You can arrive at different values for defining latitude and longitude for the same physical point on the ground. The book tells us that 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. Two hundred metres — that may not sound much to an airline pilot, though it would certainly matter to the programmer of a cruise missile that is guided to latitude and longitude coordinates. Until recently, these differences were not considered significant for aviation. But two recent developments have changed that. The first is the arrival of the Global Positioning System — GPS. The second is the widespread use of Flight Management Systems — FMS. Let me explain GPS first. GPS is an electronic navigation system in which aircraft receivers compare signals from several of the 24 transmitters in the satellites that make up the GPS constellation. It can be received over the whole globe, and its accuracy is of the order of tens of metres. That system accuracy is such that the differences between geoids become significant — a 200-metre offset is no longer negligible when your system can fix position to within tens of metres. And because GPS has worldwide application, the US government adopted WGS 84 as the geoid for GPS. So GPS positions are referenced to WGS 84. Now the second development: Flight Management Systems. An FMS compares the output of Inertial Reference Systems — IRS — with positions derived from range information received from Distance Measuring Equipment — DME. The positions of the DME stations are stored as latitude and longitude in the database of the FMS computer. DME is a very accurate system, so any inaccuracy in the datum positions — that is, the geoid used to define those coordinates — would degrade the position calculation of the FMS. Here is the concrete problem: if the database 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. You would have a jump in the computed position because the two geoids don't agree on where those stations actually are in the same coordinate system. So the key takeaway is that the choice of geoid directly affects the numerical values of latitude and longitude stored in navigation databases, and when you mix geoids across a flight path, you introduce position errors that modern systems like GPS and FMS can no longer ignore.