# principal meridian

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## principal meridian

One of the imaginary lines running north and south,used as references for the public land survey system. There are 37 principal meridians in the United States. Each one has a name, and the names are used to distinguish the various surveys that eventually made up the public land survey system.

References in periodicals archive ?
+2.00D will be the principal meridian with the higher absolute BVP, so as that is less than 3.00D, the tolerance is [+ or -] 0.12D in each principal meridian (see Table 1).
Therefore, the user must be aware of the importance of considering both principal meridian powers.
Dorsher concluded that, The marked degree of correspondence noted in this qualitative study between the distributions of the anatomically derived myofascial meridians to those of acupuncture Principal Meridians is unlikely to be coincidental'.
In existing work [3], bending along principal meridians as an eigenvalue modification was not considered; only fresh angles were adopted by the meridians of a compensation lens.
Principal power differences and principal meridians are seen to vary along perpendicular paths in symmetric power space.
Corneal factors include the difference in the radii of curvature of the principal meridians. The non-corneal factors include the lenticular errors, including the difference in the curvature, refractive index or position of the lens.1
Astigmatism is also classified on the relationship between the two principal meridians as regular (when two principal meridians are perpendicular to each other) and irregular (when the two principal meridians are not perpendicular to each other).
The base lines running north and south are known as "Principal Meridians" while the east and west base lines are called simply "Base Lines." The township numbers east or west of the Principal Meridians are designated as ranges, whereas the numbers north and south of the Base Line are tiers.
Suppose that a lens surface has a principal meridian at an angle whose tangent is 12/5 in Figure 1(a) and the other principal meridian is at an angle whose tangent is 8/-15 in Figure 1(b) with powers 4D and -7D, respectively.
The principal meridian u, where the principal power is 44 D, is any multiple of [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] and is plotted in Figure 1(a).
* Patients who have difference in principal meridians from keratometer readings greater than 0.6mm
Pre-regs should, if using a keratometer, ensure they find the principal meridians, but also ensure that they know the theory on both one and two-step design keratometers.

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