it depends on what you mean with isometric ;-)

do you mean to create stuff for a 2d game with isotiles?
that´s not this heavy...:
create a normal topview square tile of your wanted underground,and save it as bitmap.
in the renderprogram create a square polygon at topview.use the bitmaptile as texture map for the square polygon.
now change to the frontview,and rotate the square polygon 30 degrees to front (x -rotation) and 45 degrees to left(z-rotation).
fit it to your screen,and render it as maybe w 64 x h 32 isotile--->the background colour will be the transparent colour for your isotile.that´s your first groundisotile for an 16x16 dot grid ;-)
if you want to put a building or a tree with bigger size on it,simply put your building/tree in the middle of the square polygon,and double or triple the rendersize and texture mapping for the square polygon as much as needed:128x64 (texture mapping for square polygon double filling),192x96(texture mapping triple filling)...and so on :-)

if you mean to create something like a technical draw with a cube´s frontside will be facing to front,and the rest will be showing something like to the right top back:this can´t be done with rendersoftware without to deform the cube...

tilesets games sprites
http://www.reinerstileset.de

[edited by - tiles on May 5, 2003 10:17:58 AM]

[edited by - tiles on May 6, 2003 4:04:05 AM]