| 摘要 |
A two-part navigational measurement device comprising a transparent circular disc having a coordinate grid thereon. An annular ring is coaxially positioned about the circular disc, the circular disc being rotatable within the annular ring. The annular ring has gradations around at least half of its circumference. A tangent ruler is fixed to the annular ring, the tangent ruler having a tangent edge which is tangent to the annular ring at the mid-point of the gradations. Means, such as a set screw may be provided for locking the disc and the annular ring against rotation relative to each other after the course to be adhered to has been determined.
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