[thanks to Mas Yazid]
I finally know that there is an Alumny of Geodetic Engineering in DISHIDROS. His name is Mamad Yazid who entered UGM in 1988. He personally supported me to conduct fieldwork in DISHIDROS and promise me to give any data I need.
Because of our personal relationship, he even did not request me to provide a formal letter. However, I think it is agood idea to have one and Clive already agreed to write a letter for me.