Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.1.26

3.5.1.26

atha śamyāṃ ca sphyaṃ cādatte | tadya eṣa pūrvārdhyaḥ uttarārdhyaḥ śaṅkurbhavati tasmātpratyaṅ prakrāmati trīnvikramāṃstaccātvālam parilikhati sā cātvālasya mātrā nātra mātrāsti yatraiva svayam manasā manyetāgreṇotkaraṃ taccātvālam parilikhet

He now takes the yoke-pin and the wooden sword; and from where the northern peg of the front side is, he strides three steps backwards and there marks off the pit (kâtvâla). The measure for the pit is the same (as for the high altar); there is no (other) measure in regard to it: wherever he himself may think fit in his mind (to fix it), in front of the heap of rubbish (utkara), there let him mark off the pit.