The Seed Coordinate System

In the Flight Simulator scenery system, the earth is divided into seed coordinates that the seed system uses to determine position.

In the seed coordinate system, the equator is divided into 2 to the eighth power (256) divisions, level 8 seeds. Each division is 156.543 km wide (the earth’s circumference / 256). In the seed coordinate system, moving north from the equator in exactly 156.543 km increments in a line parallel to the equator all the way around the earth creates 256 squares, or seeds. The details of level 8 seeds are as follows:

• This first band of seeds is latitude band 0 or latband 0.
  
  
• The first seed to the right, or to the east (left border is at 0° longitude), is at coord longitude 0.
  
  
• The second seed to the right is at coord longitude 1.
  
  
• The second seed to the right is at coord lon, lat=1, 0.
  
  
• The seeds go all the way around the earth until they are at the seed to the west of 0°.
  
  
• The last level 8 seed in the band is coord lon, lat=255, 0.
  
  
• Level 8 seeds are not exactly square because the northern end is slightly shorter than the southern end; it's at a higher latitude on the sphere. This error is small and for all practical purposes a square is an accurate approximation.

The top of each seed in latband 0 is 156.495 km wide. In the seed coordinate system, moving north from the top of latband 0 in exactly 156.495 km increments in a line parallel to the equator all the way around the earth creates 256, 156.495km squares, or seeds; this is latband 1. The seeds in latband 1 are slightly smaller than those in latband 0. The details of level 8 seed coordinates are as follows:

• The first latband north of the equator is latband 0.
  
  
• Latband numbers increase as you go north.
  
  
• The first latband south of the equator is latband -1.
  
  
• Latband numbers decrease as you go south.
  
  
• As you get closer to the north or south poles, seeds get smaller.
  
  
• Seeds are always approximately square, regardless of latitude position.
  
  
• The first seed to the east of 0° longitude is coord lon=0.
  
  
• Coordinate longitude numbers increase as you move to the east.
  
  
• The last coordinate longitude number as the globe is circled is coord lon=255.

Level 8 seeds are the largest seeds available in Flight Simulator. In the seed coordinate system, seeds get smaller moving north; as a result, the latbands get narrower. Because of this latband "shrinkage," it is impossible to surround the earth completely with latbands all the way to the north pole. But, the Flight Simulator seed coordinate system gets to within approximately 19 degrees of the north pole, with 256 north latbands and 256 south latbands.

Flight Simulator's graphics system keeps a fixed definition of the latitudes of the level 8 seed border in a table called the seed_grid_table, as follows.

Note: Notice that the size of level 8 seeds in latband 0 is 156.542 km square, while the size of seeds in latband 255 is only .539 km.

Level 8 seeds establish the basis for all higher (smaller) seed levels. Level 8 seeds are assumed to be square. Level 9 seeds are formed by dividing a level 8 seed into four squares, each square being one-fourth the size of a level 8 seed. Level 10 seeds are created by dividing a level 9 seed into four squares. This process of dividing seeds continues to seed level 15.

It takes 256 level 8 seeds to wrap around the world; the coordinate longitude range is from 0–255. In keeping with the process of seed division, it takes 512 level 9 seeds to wrap around the world; the coordinate longitude range for level 9 seeds is 0–511. The following table shows the coordinate latitude range for seed levels 8–15. In the table, notice how the size of the seeds at the equator decreases to approximately 1.2 km. This is the finest resolution that the seed system boundaries can define; although, at higher latitudes, shrinkage makes finer resolutions possible.

Caution: Because of dramatic increases in processing time, level 14 and 15 seeds are no longer used in Flight Simulator scenery design. Seed levels 14 and 15 are no longer interpreted or displayed; use objects, which are much more efficient at this level, instead

Notes:

• Seed size at any given latitude can be computed as:
  seed size=size at equator * cos (latitude)
  
  
• At any given location and seed level, the coordinate latitude and longitude of the next highest level seed are always double that of the current seed level. For example, the space at the southwest corner of level 8 seed at coord lon, lat=117, 63 is overlapped by the level 9 seed at coord lon, lat=234, 126. Coordinate longitude and latitude measurements are wholly dependent on seed level.