Up to this point, we've spent a lot of time looking at how we mathematically measure and model the Earth. We learned that:
- the topographic surface is the actual Earth surface upon which we walk;
- a geoid is a model of gravity, representing local mean sea level to establish a proper zero point for elevation measurements;
- reference ellipsoids comes in two varieties - local and global - and upon those reference ellipsoids a geographic grid is drawn;
- geographic grids require an Earth-centered, Earth-fixed coordinate system and an affine transformation;
- Earth-centered, Earth-fixed coordinate systems require an angular unit of measure and a principle meridian;
- local reference ellipsoids fit inside the geoid really great somewhere and really poorly everywhere else;
- global reference ellipsoids fit everywhere inside a geoid, however, the fit pretty okay everywhere but not really great anywhere;
- latitude and longitude, which happen to use an Earth-centered, Earth-fixed coordinate system utilizing the Prime Meridian as the principle meridian and degrees as the angular unit of measure, is just one example (albeit the most common) of a geographic grid;
- geographic grids are not, by themselves, a datum, nor a geographic coordinate system;
- benchmarks are real world objects, found on the topographic surface, which are the base of datums. These benchmarks allow for a series of points to connect the XY coordinates of a reference ellipsoid with the XY coordiantes and Z values of the geoid. Once several benchmarks connect the geoid to the reference ellipsoid, all of the remaining control points can be found utilizing a Cartesian Coordinate System;
- benchmarks are a real world objects that are converted into control points within the datum. Some, but not all, control points within a datums started out as benchmarks. The rest were mathematically derived;
- datums, horizontal or vertical, are the product of connecting a geoid to a chosen geograhic grid via control points;
- horizontal datums (the focus of Introduction to GIS) contain only X and Y coordinates, assuming the entire world exists at an equal elevation;
- and vertical datums contain just Z values, while three-dimensional datums (used inside GPS receivers) are made up of XY and Z coordinates/values .
Geographic coordinate systems are the whole shabang - a datum connected paired with a geographic grid - and their purpose in cartography, geodesy, GIS, and navigation is to represent all of the land and ocean masses graphically and give an "address" to every point on the Earth's surface. Much like grocery store down the street which has an address of 123 Bob Street, Boston Mass., it also has an address for it's location on the Earth's surface. This GCS address is measured not in an odd-even value/street name/city like it's Post Office address, but in - most commonly - latitude and longitude, degrees, minutes, seconds. Long after the grocery store goes out of business and the street is demolished and replaced by a speed train track, the latitude and longitude - the GCS address - will continue to be true. Since the point found on the GCS is tied to the topographic surface, even is if a huge hole or man-made mountain is put in place, the X and Y coordinates will never change (if we make a huge hole or mountain, the Z value will change - not to be confused with the gravitational pull, but the distance above local sea level).
Often, in Intro to GIS classes, datums and geographic coordinate systems are mixed up and the words used interchangeably, which is incorrect. Geographic coordinate systems require a complete and labeled geographic grid paired with a datum. The datum is, indeed, a reference ellipsoid connected to a geoid via control points, which are transferred from the topographic surface to the geoid and mathematically derived via a temporary grid drawn on the reference ellipsoid, but that without a complete geographic grid, it's still just a datum and not a Geographic Coordinate System. It's not until a datum is selected and a geographic grid is "stretched" and "conformed" to match a reference ellipsoid, using the control points of the datum as the "anchors" that a Geographic Coordinate System is created.
Every geographic coordinate system is based upon a datum and when you look at the properties of that GCS, you can find the name of the datum (as well as the geoid, for that fact). Knowing which datum a geographic coordinate system is based on (and how to look it up) is pretty important when it comes to accuracy (for now, we will leave it at that, as the full explanation will make more sense later). It is important to note that geographic coordinate systems are a stand alone thing - there is no need to move any further forward if your goal is to simply navigate around using GPS unit (as the job of a GPS unit is to find places in the real world on the topographic surface). In the next section, we will learn what it takes to get the round earth to be a flat map utilizing a method where we project the data from a 3D object to a 2D object. This is necessary if we need to measure things such as area (since we think better in units like feet and meters squared instead of degrees squared) or wish to accurately navigate without drawing on a globe.
|FIGURE 2.14: A Graphical Breakdown of the Parts of a Geographic Coordinate System|
|Geographic Coordinate Systems are a complete package: A datum with a specific geographic grid laid over the reference ellipsoid. While it is important to understand how each part is made, this graphic is a simple breakdown of the parts and how they connect.|
|The Main Point...|
Creating a Geographic Coordinate System:
Select a specific geodetic datum (attaching a reference ellipsoid to a geoid via control points) and match it to a geographic grid (a labeled grid combining an Earth-Centered, Earth-Fixed coordinate system (a principle meridian and an angular unit of measure) and an affine transformation). The output is an a means to locate and label specific locations on the Earth’s surface. Once these places are labeled, tools such as GPS units can lead you right to them.
The term Geographic Coordinate System is often incorrectly used interchangeably with datum. While they seem similar, a geographic coordinate system is the whole package: a geographic grid and a datum, while a datum is just the part which links a reference ellipsoid to a geoid.