NGA’s LAS File Format – NITF LAS & the VLR
Please note that NGA “wraps” LAS files in a NITF wrapper and adds a variable length record (VLR) to the LAS header. These two processes are independent of each other, but are frequently performed together. Here’s what it means to you:
NITF Wrapping: Wrapping a LAS point file in NITF does not alter the content of the LAS file itself. It simply “wraps” the file with a NITF-compatible wrapper so it can be stored in systems designed exclusively for classified data. It does, however, change the extension of the file to “.ntf”. It may also cause the file to be unreadable in many applications. QT Modeler and QT Reader both seamlessly unwrap NITF LAS files. Furthermore, NITF wrapped files will appear in search results from QT Modeler’s “Model Search” tool (File Menu…Model Search) as well as in indexing operations to catalog your data holdings in SHP/KMZ format (Export Menu…Create File Index).
Variable Length Record (VLR): The VLR is simply additional, informative text that is written in the LAS header. NGA writes information such as collection data, collection platform, classification level, etc. This process is technically independent of the NITF wrapping noted above, but frequently happens together. QT Modeler’s data preview capability (i.e., looking at the header information when you open/import a file) will enable you to read the NGA VLR prior to opening or importing a LAS file.