tutorial

Okay, here we have a XML map:         (HOW TO DO INVISIBLE GROUNDS)
<C><P /><Z><S>
<S L="77" o="324650" H="77" X="133" Y="243" T="12" P="0,0,0.3,0.2,0,0,0,0" />
<S L="200" H="29" X="400" Y="391" T="0" P="0,0,0.3,0.2,0,0,0,0" />
<S P="0,0,0.3,0.2,0,0,0,0" L="77" o="324650" X="657" Y="243" T="12" H="77" /></S><D /><O /></Z></C>

and there are two grounds color,
first u'll copy and paste the xml of ur map on notepad
second u'll in editeur and click in ground that u want invisible and copy the color of it, or be: 324650
third u'll search/copy the color in notepad locate and find it
fourth when u find the color, just delete the numbers of color
example:     <S L="77" o="324650" H="77" X="133" Y="243" T="12" P="0,0,0.3,0.2,0,0,0,0" />   <- the ground with the color in o="324650"
             <S L="77" o="" H="77" X="133" Y="243" T="12" P="0,0,0.3,0.2,0,0,0,0" />       <-  let just the o="" and ur ground will be invisible