Raster Format Converter

Converts a raster from any format GDAL can read into any format it can write — ECW to GeoTIFF, GeoTIFF to JPEG 2000, IMG to GeoTIFF, and back.

It exists for the orthophoto that is too big to convert in one go: the aerial survey that arrives as a 200 GB ECW and has to become a GeoTIFF before anything else can touch it.

How it survives a long run

The source is read in square blocks. Each block is written straight to disk as a temporary GeoTIFF — GeoTIFF whatever the final format is, because it is the one format that takes reliable windowed writes — and a small <output>.checkpoint.json beside the output records how many blocks are down. Cancel the run, lose QGIS, lose the machine: run it again with the same output path and it carries on from the next unfinished block. When the last block lands they are mosaicked and written once into the chosen format, which is the fast part, because it reads local uncompressed blocks instead of decoding the original again.

Small rasters skip all of that. Anything that fits inside one block is converted in a single pass.

How the pyramid is written

Overviews are built on the throwaway mosaic and copied into the output as it is created (COPY_SRC_OVERVIEWS), not built into the finished file afterwards. Re-opening a freshly written multi-gigabyte raster for update puts hours of work under a write that can still run out of room; the mosaic is disposable, so a failure there costs a rebuild and nothing else.

Disk space is checked three times — before the merge, before the pyramid, and again after it. The pyramid and the output have to fit on the drive at the same time, because nothing is cleaned up until the output is written. If they do not:

1. The pyramid is skipped and the raster is written without it.
2. Once the blocks are deleted and their space is back, it is tried again as a sidecar .ovr, with the raster opened read-only — so that attempt cannot touch the data even if it fails.
3. If there is still no room, the log says so and tells you the raster is complete and the pyramids can be built later.

Erdas IMAGINE always takes the sidecar route: COPY_SRC_OVERVIEWS is a GeoTIFF creation option and does not apply to it.

1. Open KGA Data Conversion > Raster Format Converter.
2. Pick the source raster and the output format, and name the output file.
3. Leave the defaults unless you have a reason: LZW is lossless, overviews are worth having, and 4096 px blocks suit most orthophotos.
4. Press Run and read the header the log prints first — size, band count, data type, creation options and how much temporary disk space the blocks will need. Stop there if that number does not fit on the drive.
5. Leave it running. The progress bar moves per block, then again through the merge and the overviews.
6. If you have to stop it, press Cancel. The log says which block it reached. Run it again later with the same source and output path, and it says "Resuming: N of M blocks were already converted".

• Writing ECW needs a Hexagon licence. QGIS reads ECW out of the box and the driver advertises write support, but the encoder refuses unless ECW_ENCODE_KEY and ECW_ENCODE_COMPANY are set with the key bought from Hexagon. The tool checks the licence, not just the driver, when you press Run — GDAL would otherwise refuse at the final write, hours in. JPEG 2000 goes through the open-source JP2OpenJPEG driver and needs no licence.
• Budget the temporary disk space. The blocks are LZW GeoTIFF, so roughly the uncompressed size of the source sits beside the output until the merge is done. This is the cost of being able to resume.
• A pyramid is never worth risking the raster for. Whatever happens to the overviews, the converted raster is complete and correct, and the log says which happened. Missing pyramids can be built any time from Raster > Build Overviews.
• Changing the block size or the output path starts over. The blocks no longer line up, and the tool says so rather than mixing them. It also notices when the source file itself has changed since the checkpoint was written.
• JPEG and PNG have format limits: JPEG takes 8-bit data, 1, 3 or 4 bands and no side over 65535 pixels; PNG takes 8- or 16-bit data and up to 4 bands. Both are checked before the run.
• JPEG, JPEG 2000 and a JPEG-compressed GeoTIFF are lossy. Converting an archive ortho into one of them throws away detail that no later conversion brings back. GeoTIFF with LZW or DEFLATE is the lossless choice.
• A source with no georeferencing — a plain PNG or JPEG with no world file — cannot be mosaicked back from blocks, so it is converted in one pass and that run cannot be resumed.
• Cancelling during the merge keeps every block, so the next run only redoes the merge. The half-written output is removed, so there is never a truncated file that looks finished.
• Resuming is the same idea DEM Elevation Correction uses — these two are the toolbox's resumable jobs, and they behave the same way.