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- 17 - the intra-linear variance, is negligible. The corresponding relative variance is cr2 -5 2 E 16.10 «„-3 -y = y = 4.10 - (6.3 X 10 ) t 4.io'": i.e. t is estimated at ± 13 %. Counter experiment Imagine now that we modify neither the number of samples, nor the size of the zone, but that we adopt a regular square grid as a sampling pattern. This means (approximately) 22 lines with 22 points per line, and a^ = = 5 mm/22 = 0.227 mm. The estimation variance -2 '2 -5 4 10 -2 a^= 5.10 3 + [(0.1667 + 0.0739)5.16x10 Z] '2 -5 -5 -5 ai = 5.10 + 0.45.10 -5.10 A better distribution of the sampling points has totally absorbed the spatial fluctuations, but has left the noise unchanged. The new relative error is given by a'p/t2 = 5,10 = 12. 5.10"4 = (3.5 x 10~2) , 4.10 Z ant t is estimated at ± 7 %. Comments a) Note the similarity between the 2-D transitive and intrinsic rules of a1 a2 correspondence. If we remark in rel. (25,5) that N = -y—, the contribution Co of the term Cq <5 becomes -y, exactly as in the intrinsic case. In the same way, the term in in rel. (25,10) becomes À, a, a~ 2 A, - - S x-jg- [-^ + M) 0.0609 a,] ^1 a2 2 i.e. - S . [0.1667 a1 + (^) 0.0609 a^