Although IDL can open and read a variety of standard file formats some files may not fit this category and would need custom code to be able to access the data. Given that files can have any extension the creator of those files wants to give them i.e. you can save a text file with an *.xyz extension, have you tried opening the file a plain text editor?If it has all squares in it it probably is a binary file.... Consider the following: 1) What software created these *.ptw files? 2) Any idea how the data is arranged in the file and what the data type is? culumns, rows, float, integer, text.... 3) Do you have any documentation available from the source of those files ?