Is syaku an ancient Chinese unit of measurement (Suanfa tong zong)?
I believe this is the original text:

今有葭二莖生池中，並根杪齊，出水三尺，即葭一莖斜去至岸九尺，與水適平，問水深若干？

It also comes with the following, hard to read, illustration:




This is the measure word you're asking about:
尺
MDBG:

chǐ
a Chinese foot / one-third of a meter / a ruler / a tape-measure / one of the three acupoints for measuring pulse in Chinese medicine / CL: 支, 把

Wiktionary

1 chi; Chinese foot (a traditional Chinese unit of distance based on the human forearm and equal to 10 cun (寸) or 1/10 zhang (丈))
2 (Mainland China) mainland chi; Chinese foot, standardized in 1984 as 1/3 meter
3 (Taiwan) Taiwanese foot, standardized as 10/33 meter and identical to the Japanese shaku
4 (Hong Kong) chek; Hong Kong foot, standardized as 0.371475 meters
5 (informal) imperial foot
6 ruler (straightedge) (Classifier: 把)
7 tape measure

Japanese Shaku is also:

10/33 meter