The word āspada is formed by suṭ when meaning 'a place or position'.,
The word प्रतिष्ठा means 'firm place, established position, rank, dignity, authority'. Thus आस्पदम् अनेन लब्धम् ॥ Why do we say when 'meaning a place'? Observe आ पदात् = आपदम् ॥,
आस्पदं प्रतिष्ठायाम्
Adhyāya 6 · Pāda 1 · Rule 146