I'm beginning to use ENVI+IDL (most of this relates to IDL, I think, in case you know one but not the other) at work, completely new to both. Since ENVI isn't an existing tag, I'll explain that it's a toolkit and GUI, integrated with IDL, for geospatial image processing. ENVI has some predefined image manipulation functions, in this case, one called envi_convert_file_map_projection, which accepts as argument the FID of the image you wish to reproject.
I don't totally understand the FID idea, but I gather it's an integer assigned to an open file; meaning I have to open the file first. So I use the IDL function: variable_name = READ_TIFF(filename), which I suppose makes variable_name point to the array. I'm not sure how I get an FID out of this; two unrelated concepts?
EDIT: It seems that most of the posts tagged IDL are referring to Interface Description Language. I'd never heard of this until now. This post is about the Interactive Data Language, with no relation to the other, and the tag system doesn't seem to reflect the ambiguity. If you have higher rep than I do, are you able to make a new tag, such as idl-data, idl-ittvis (after the language developer) or interactive-data-language?
EDIT EDIT: I've come across a different command, openr, which assigns the file to a lun (logical unit number) which seems to be similar to the FID. I'll try using this instead and let you know.
Use ENVI_OPEN_DATA_FILE to get a FID from the filename of a TIFF file. See the ENVI Programmer's Guide for more information about ENVI specific routines that can be used from IDL, though ENVI's IDL interface is not really documented very well.