Faded Twilight
The Great Zero
The Great Zero, GZ for short and apparently Rezeero in the native tongue, as strange as it sounds, was the center of the polar coordinate system that was established by the D'ni in all of the Ages that they visited. Not surprisingly, it was also established in their home age, on an island in the middle of the great underground sea. Ri'neref always stressed the importance of constructing all significant buildings along the Line of the Great Zero; the Line being a blue beam that was projected from the massive device.
Quite appropriately, the tunnel to the surface was also most likely built along the Line. The angular measurements that you see taken along the map are certainly taken with respect to the Line of the Great Zero. The purpose of these measurements, though, aren't entirely clear at first glance.
Before we can talk about that, however, we need to set up some conventions first. Here's an apt segment of the map, at Ae'gura:
Look at those rays on the map that originate from the Zero. There are twenty-six of them, going in a radial direction till they intersect the borders of the map, where Aitrus drew the border to correspond with how each ray touched it. Looking closer at the island, you can see that the twenty-six rays (from here on out, referred to as "minor rays") do not originate at a single point. Five rays, drawn in bolder lines, meet at the Zero, and these we'll call "major rays." Any circle that originates at the Great Zero will be a "major circle"; these circles are very faint, but can be seen with clarity when they cross the center of a node.
Now, let's do some work with the circle that these rays form, since we apparently have a polar coordinate system now. Divide 360 degrees by 5, and you'll get 72 degrees of arc. There are five of these divisions, and we'll call them "major sectors." The 25 "minor sectors," as divided up by the minor rays, contain 14.4 degrees of arc apiece. We aren't going to use radians for our measurements because they don't play as nicely with torahns.
When converted into torahns, we have 12,500 torahns per major sector, and 2,500 torahns per minor sector. Nice round numbers. Have a look at the picture at the bottom of the Great Zero sometime; each ray is numbered, and wraps around at zero.
Now that we have some terminology, again look at the Zero. Notice the thick blue line that extends from the center along a minor ray, intersecting the edge of a major circle and the centers of both node one and node two. Each blue line that extends from a node will not necessarily begin at a minor ray, which you can see by examining the other nodes, but it will meet the edge of a major circle and the center of a node. (You'll probably need a larger image of the map to see this - it's rather tricky to see.) This, of course, is the Line of the Great Zero.
Right now, the Line means nothing to us, so let's return to the Zero again. Look at the blue numbers that have been written between two arcs: 1)[[ ([1][10][20][20]). This is actually an angular measurement in torahns, measured from the major ray pointing straight upward to the line of the Zero. It is 22,395 torahns of arc or 129 degrees. From just eyeballing it, you can see that it is indeed greater than 90 degrees, but less than 180 degrees (a straight line). This is also the bearing to the next node.
Refer to the following figure for the next part of the discussion; some things are marked that are not discussed:
Continuing along the nodes, noting where the Line meets a node each time, you can see where a faint vertical line has been drawn that meets the edge of a major circle, and then the blue torahn measurement was taken (labeled "node bisector" in the figure). Following that, there are also red measurements, such as 3{5 (2,430 torahns or 14 degrees) at node three. Now, what do those mean? Simply put, it's a relative angular measurement from where the route they had to take deviates from where the Line extended before. If one were to do this with a compass, you'd place the point where the lines meet, place the pencil on the new line, then swing back to the old one. Initially, you might think this to be the bearing back to the previous node from the current node, but this is not the case. Instead, it's the number of torahns needed in order to correct yourself to the new bearing.
Interestingly enough, the Line traces out the shortest path from D'ni to the surface, though we can of course see that they had to take some liberties with the route along the way.
The green pathway through the tunnels is the true (walking) path to the volcano. Aitrus must have drawn this in for quite obvious reasons. Then, there are the orange dash-dot lines with the red triangles. This line represents the fault mentioned in the faultline text.
Finally, we have the green crystal under the map title. I've posited in the past that this crystal is the compass point for the map; i.e. it either points to the northern cardinal direction, or to whatever direction that the D'ni considered to be their primary direction. However, this, in reality, may depend entirely on how the Zero is oriented. If this is true, then zero torahns is their north (remah), as the Zero is usually oriented with respect to magnetic north.
Those of you familiar with the KI (a D'ni PDA of sorts) know that you can calibrate it to the Great Zero, and then be able to accurately find your position in the cavern using the coordinates displayed on its face (distance, angle, and elevation). Guess what? You probably have both distance and angle right here on the map! Angle you can already see, distance is most likely measured by the seventeen seals. However, the units of distance seem not to be the span, as that is a smaller unit of measurement than what the numbers on the seals seem to describe.
What about elevation, then? Unfortunately, this is not measured in any numeric quantity on the map; you have to use your imagination a bit to find that one. Look at the cutaway view of the tunnel near the bottom of the map. You'll see arrows pointing at specific points; these merely indicate relative elevation and the water level. The rest of the lines that have been drawn through the diagram most likely indicate the elevation coordinate needed for the set. See the figure below.
