Looks like each country has their own coordinate system
Why is that? Can’t they simply use lat and long in degrees like in Google Earth?
I think most countries do use latitude and longitude.
When dealing with a relatively small area of the earth’s surface (say, a country the size of Ireland), there are advantages to using a rectangular co-ordinate system.
For example, in the Irish grid the 5-digit eastings and northings in each grid square are in units of metres.
The earth is a sphere. Maps are flat.
Latititude and longitude are spherical measurements. Coordinate systems are flat measurements
Mathematics gets really really complicated on spherical surfaces, but is pretty easy on a flat surface. (for example, a triangle has 3 straight sides,and 3 angles right? No, not a spherical triangle; it has curved sides,not straight lines, and 9 angles.)
The various coordinate systems are designed to simplify the math,by changing reality. In reality, the earth is curved,so no line is straight.
But when engineers build a road or a pipeline 20 miles in a “straight” line, they don’t want to mess with the complications of curvature.So you place a flat coordinate system on top of the curved earth, and pretend that your road is straight. It makes the math easier.
To calculate the distance between two points, you imagine a triangle. Then you just use the Pythagorean theorem you learned in junior high school :
a^2 +b^2 = c^2
—or in plain English:
the distance between the two points is the square root of the total 2 distances that make up the triangle’s shorter sides.
It’s true mathematically, but not true in reality, because of the curvature of the earth.
The coordinate systems are designed by each country to cover their little section of our round planet with a flat grid . The grids vary so that they introduce as little mathematical inacurracy as possible, so that the mathematical calculations of simple algebra are almost identical to what you would calculate if you used the more complicated spherical geometry.
Mostly deleted, already covered.
I just want to add that Google uses a particular coordinate system, probably WGS84. There’s no such thing as a “generic lat/lon” system.
Most of them do use Lat/Long. But that’s not a coordinate system, really. There’s also the particular spheroid that you project your map on. There’s the projection that you use. There’s various rules for distributing the lat/long…google maps uses a version of Mercator, so the latitude lines are farther apart (go to infinity, actually), as you approach the poles. If you’re, say, Iceland, your map is all screwed up. So you use a local coordinate system which will give you less distortion. And since you have time, you make the PERFECT projection/coordinate system for your little country. You don’t care what australia looks like on that map, because you’re using it for zoning in Reykjavik.
I fail to see how you get 9 angles from a triangle on a sphere. Whether you use spherical geometry, euclidean geometry, or hyperbolic geometry, a triangle has three angles and three sides by definition (it’s a triangle after all). Those sides are also straight in the sense of being the shortest distance between two points in that geometry. The big difference is that the angles of a triangle add up to exactly 180 degrees in euclidean (flat surface) geometry, more than 180 degrees in spherical geometry, and less than 180 degrees in hyperbolic geometry.
True. For a group project, I had occasion to try to put a marker at the South Pole on a Googtle Maps based page. Can’t be done; Antarctica is this enormous mass several times the size of Asia that is cut off abruptly. Best thing I could do is locate the marker south of the Palmer Peninsula, with a note saying it was supposed to be at the Pole.
There are many coordinate systems that are slightly different versions of lat and lon, which are called “datums” (the singular being “datum”, clearly different from the word “datum/data”). These are all attempts to estimate the same one and true lat and lon, but there is a problem - your little island of Maponesia can be much more accurately mapped using optical instruments and aerial photographs than it can be placed in the larger global context. This is like the problem that your house could have construction drawings that are accurate to better than an inch, and so could your neighbor’s, but you don’t know how your house is positioned relative to the neighbor’s house to any better than perhaps a foot. So you have multiple local datums.
Now that GPS can apply one system - well, sort of - to surveying receivers the world over, these will probably go away by attrition.
The GPS system itself is accurate enough to require a new set of separate datums (IIRC there is a new name for this, though) to accomodate the movement of sections of the earth’s crust. These sections are considered to have irregular boundaries and to have motion that is pretty predictable over the scale of a few years.
Although country-specific coordinate systems are still used a lot, almost all modern topographical maps are standardizing on UTM. UTM is also a rectangular coordinate system, so it is convenient for use on small-region maps but it gets inaccurate over long distances.
UTM is not optimized for a specific country. It divides the Earth’s surface into many narrow zones, with the center of each zone being used as the reference point for all maps in that zone. Measuring distances across zone boundaries can still get rather complex, unfortunately, but if you just want to use a map for navigation in a small area it’s great, because you only need to understand a single global system and you still get the benefits of having a simple rectangular coordinate space.
Thanks for the replies. 
Now, once Google conquers the Earth, things may change a bit …
Just to jump in with another ginormous simplification… A projection is often chosen for how well it represents what you might be measuring. A flat map, digital or paper, will always distort area, distance or direction. You can choose a projection based on which you need to be the most accurate.
There’s a great book called How to Lie With Maps that does a better job of explaining all this than I can.
I’ll add that US States have thier own coordinate system(s). Many of these are variations of UTM, but there are also conic projections and a weird diagonal projection for Michigan and the Alaska panhandle
Brian
Although my Garmin hiking GPS does let you choose the datum, because you still need to match it up to any maps you are using.
I just designate every location by how long it takes to drive there from my house.
If I can’t be bothered, it doesn’t exist.
One thing they may not say is that WGS84 (the default one in typical consumer GPS) is the same or practically the same as NAD83 (the newer version for USGS topographical quadrangles, as opposed to NAD29.
You can also get some consumer items that work with ITRF00 and handle postprocessing, and get accuracies much better than a foot, but it’s a much messier world.