PURPOSE:To measure the diagonal sizes of the foundation materials of a steel tower easily and accurately by providing a base line in correspondence to the inside vertex and outside vertex of a V-shaped part to the parallel parts (legs) of an M- shaped member having width, and disposing permanent magnets to the inner edge part and outer edge part of the V-shaped part. CONSTITUTION:The inside vertex P1 and outside vertex P2 of the V-shaped part 12 of an M-shaped member having uniform width W (50mm.) are both formed of sides including a right angle; the P2 forms an arc shape of 20R curvature. The parallel parts 13, 14 of the member 11 forms a 45 deg. angle to the V-shaped part, and base lines L1, L2 are provided in correspondence to the inside vertex P1 and the outside vertex P2. Plural pieces of permanent magnets are buried in the inside and outside edge parts of the part 12. A rule 11 is placed on a foundation material 31 in such a way as to abut on the inside surface of a main columnar material 32 of a steel tower, and the size between the base lines L2 (TL) is measured, whereby the diagonal size (TL) of the material 31 is easily and accurately determined.
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