ok first replacing objects is a for experienced modders . but Rockstar knows well the modding boost consequently its sells so the game is made a way that modding is not much difficult (i wouldn't have bought the game without the modding myself) . thanks to all the guys here also that create really amazing softwares like OpenIV Spark and all the others . here's what i know concerning the objects placement .
i think it'll be easier with an example . let's say you want to suppress a barge in manhattan and replace it with a sunken ship (what i'm doing myself right now)
you saw the barge on the IG map but you don't know where it is located
- install openIV tool . then make a global research with barge word. you'll get several results
- browse the results (double-click) to find the type of barge you want to remove . when you found it remember it's name and make a research but now with the placement search you'll see a map with all the barges of that type in game highlighted .
- you'll see a column at the map left . click on the results until you found the barge you want to remove .
- once you got it , double-click on the result the program will "jump" to the file that contains your barge . extract the file
- now download and install Dageron Placement tool . open the file and find the barge (same name as in openIV without extention) . if you have several same barges you'll need to make an in-game research of the object coordinates , using a script . i use DK location tool script . you'll see a line of numbers beside the file name .
- now it's important according to where the file you extracted was there are two possibilities . if the file was in an img file you can simply and purely remove the line with placement tool . if the file wasn't in an img file example nj_01.wpl then i suggest you not to erase the object or you may experience crashes or game instability , but there's an alternative solution it's to "burry" the object . with placement tool add negative values to PosZ until your object is totally burried . save
- import you modified file with OpenIV . done your object has been deleted . then for adding the new object just add a line to the file (with placement tool) you've just modified with the coordinates (the same as precedent object)
- make an in-game test to see if the collision has properly been removed , normally it shoud but in some very rare cases it doesn't , then when that happens i get "stuck" (for now)
- then when it's all done when you'll approach the barge until you come close you may still see a barge . these are lods . to remove them . you need spark program . browse the file where you extracted your wpl from for textures that have chance to be the one you see when you are far from the object , and replace (import) them with transparent textures . backup the original files