Suppose you want to know the exact coordinates of a particular point in your property, to the centimeter. In many countries there are services (oftentimes governmental services) that you can commission to do this measurement and give you all the exact data of one or more such points in your land or property.
However, pretty much always what they give you are coordinates using a coordinate system that's local to your own country. Very rarely if ever do they give you the data in a global coordinate system, like the standard GCS system, or the ECEF system.
But why is that? Is it just tradition? Stubbornness? Hesitation to move to a global standard from a long-established local one?
No. The reason is a lot more practical than that. And that reason is: The continents move.
That might sound a bit of a surprising answer at first, but it indeed is literally the reason. Giving the coordinates using a global coordinate system like the GCS or ECEF is not practical because continents move. And they move surprisingly fast. This would cause the coordinates of that particular point to deviate more and more as time passes.
Indeed, if you measure the exact coordinates of a particular point on the ground today, using a global coordinate system, and you do the same measurement a year later, you'll find out that the coordinates will have drifted by 10-20 centimeters (sometimes even more, depending on where you are on Earth). When you need the coordinates to be accurate to the centimeter (or sometimes even to the millimeter), them drifting by this much is just completely impractical.
A local coordinate system for each country, however, is not fixed to the global latitudinal and longitudinal coordinates of the Earth, but to the land itself. They are defined so that certain points on the ground do not move (or move as little as possible). Or to put it other words, for all intents and purposes the local coordinate system moves with the continent. This makes sure that exact measurements of particular points on the ground will remain relatively accurate for decades to come (with any drifting being in the range of less than a millimeter per decade, or so.)
These exact measurements are often necessary for all kinds of land surveys, city planning, construction work, road planning, and so on. They give you a quick way of knowing the exact distance, to the centimeter accuracy, between two points within the country. And, more importantly, it gives you way to know the exact distance from a newly-measured point to another existing point. All kinds of construction projects use these measurements and this data all the time.
There is also another, perhaps more secondary reason to use local coordinates rather than eg. GCS:
The GCS coordinate system in particular models "sea level" as an exact mathematical oblate spheroid, with particular standardized dimensions. This is a close approximation to Earth's actual sea level, but it's not exact, because the actual sea level is not a mathematically exact oblate spheroid.
This can cause surprising results particularly in altitude measurements. There are many parts of the world where, if you were to use GCS coordinates to measure altitudes, it would seem like certain rivers are flowing uphill. Indeed, according to the GCS coordinates the end of a river may be at a "higher altitude" than the beginning.
Of course this is just a quirk of the GCS system itself, caused by its oblate spheroid to just be an approximation. A very close approximation, but still just an approximation. (Also how gravity works on an oblate spheroid plays a role in this. It's complicated.)
Many local coordinate systems also fix this problem, using more accurate "sea level" geometry for the country, and in them all rivers flow downhill. (Although there are still some local coordinate systems that have not fixed this, and still have this same problem. Several countries have tried to move to a more accurate local coordinate system in recent decades to fix this problem, among others.)
No comments:
Post a Comment