Etymology of the words 'vedi' in ritual context

[Sudalaimuthu Palaniappan]

In a message dated 6/26/2000 11:20:50 AM Central Daylight Time,
s.hodge at PADMACHOLING.FREESERVE.CO.UK writes:

> the vedi is a narrow flat raised
>  ridge constructed around the hearth upon which ku`sa grass is
>  carefully laid to encircle the fire pit.

I am just curious. Was the vedic sacrificial altar created first by digging a
pit and then building a fire inside so that the fire could be below ground
level and thought of as fire pit? Was it ever built  from ground level up
with enclosing materials (bricks?) to contain the fire?  Among these two,
which was more prevalent? Thanks in advance.

Regards
S. Palaniappan