# Coordinate system in aircraft. Question about step between lines

Good day! Can you help me? I doing a restoration of the plane's geometry. I want to do restoration use a theory. But I have not information about step (size) between B.L. (W.L.) lines. As example - this picture with bulkhead on which are B.L.4.,8.,12...

• Given the general size of what you're restoring, is it more reasonable that the dimensions are shown in inches or in centimeters? The overall height seems to be about 68 units (i.e. 27.47 of them above the waterline, and 40.53 of them below the waterline), and the max width about 53 units (i.e. 26.66 of them to either side of the centerline). The lines themselves seem to be 4 units apart, both laterally and vertically. So if you can tell us if this is all in inches or centimeters (or something else, possibly) we can discuss everything else with specifics.
– Ralph J
Jan 8 at 1:33
• @RalphJ. In this blueprint are inches Jan 8 at 14:00

The Vertical Centerline, BL4, BL8, BL12, and BL16 lines are each 4 inches apart; the WL20, WL16, WL12, and so on down to the Horizontal Centerline, aka the Waterline, are also all 4 inches apart.

So if you picture an X-Y axis with the origin of 0,0 at the point where the Vertical Centerline and the Horizontal Centerline meet...

• The top center point would be at 0,27.47" (0 because it's on centerline, and 27.47 from the dimension showing the max height -- to the left of the drawing)
• The next point on the outline, working clockwise, would be at 4, 27.159 (4 because the BL4 is 4 inches from the vertical centerline, and 27.159 because that's what's shown under BL4)
• The next points would be 8,26.204 then 12, 24.530 then 16,21.973

After that, the points are no longer every 4 inches horizontally, but every 4 inches vertically:

• 18.275, 20 (using the distance from the Vertical CL on the WL20 line)
• then 21.671, 16 then 23.982,12

and so on until reaching the Horizontal CL

• 26.660, 0 is the point on the horizontal CL

After that, the X values start to decrease, and the Y values become increasingly negative:

• 26.530,-4
• 26.136, -8

And so on down to the last WL line at -32 inches:

• 16.362, -32

Now the lines go back to every 4 inches horizontally:

• 16, -32.42 (very close to the prior point)
• 12, -36.192
• 8, -38.662
• 4, -40.071

And finally, the point at the bottom on centerline:

• 0, -40.530

And those are the points that the diagram provides for you.