GPS - Ever Heard of WGS 84?

Once we have GPS co-ordinates for every place, you know where every place is, right? Wrong, writes Greg Milner in his book, Pinpoint:

“Those GPS coordinates do not mean much to us without a map on which to peg them.”

Huh? The maps of the world, regional or global, are called Geographic Information Systems (GIS). Combining GPS with GIS is not one bit easy, rather it is a “creative merger of computer science, cartography, and database management! But why should that be the case?

 

Map making has historically involved a method calling triangulation. Like the name suggests, it involves measuring triangles and laying them next to each other. An alternate method is to find the latitude and longitude of a place using astronomy. Ideally, the triangulation and astronomical methods should match. Except they don’t. Why? Because the earth isn’t a perfectly shaped ellipsoid. Rather, as it is a “complicated potato shape”. Therefore, to make the two methods align:

“The early land surveyor had to cook the books a little.”

 

Further, keep in mind each country originally cared only about its own map. And so it came to be that “different countries came up with different ellipsoids and different orientations” to align their own data. Which meant:

“The system worked on a local level, but it made the world a patchwork of measurements. In the lingo of geodesists, the world was not “tied together”.

 

This had practical consequences as long back as World War II, when the US Army realized that “confusion over the various European datums was causing its troops to miss artillery targets”! And so they began to “plunder map and surveying information” from occupied areas to “update the US military grid”. During the Cold War, this problem became even more critical: if the Russian datum was different from NATO’s datum, both sides knew their missiles would miss their target. Conflicting coordinates aside, the earth not being a perfect ellipsoid means that the direction of gravity is slightly different in different places. Over long enough distances (e.g. USSR to US), the slight differences in gravity would add up enough to cause the missile to miss its target!

 

And so the US military established the World Geodetic System (WGS), a system that would use the earth’s center as the reference point. After all, who wanted to trust any point on the surface of the earth when those pesky tectonic plates kept moving?! (The idea of any place, like Greenwich, being considered the center sounds so egoist and… naïve). In 1984, they did a massive overhaul of the same system and it came to be called WGS 84. This WGS 84 was accurate enough for missiles, to predict the orbits of satellites… and yes, it is what enables GPS to be applied onto our maps of the world.

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