# COG -- Cloud Optimized GeoTIFF generator

New in version 3.1.

Driver short name

COG

Driver built-in by default

This driver is built-in by default

This driver supports the creation of Cloud Optimized GeoTIFF (COG)

It essentially relies upon the GTiff -- GeoTIFF File Format driver with the `COPY_SRC_OVERVIEWS=YES` creation option, but automatically does the needed preprocessing stages (reprojection if asked and creation of overviews on imagery and/or mask) if not already done, and also takes care of morphing the input dataset into the expected form when using some compression types (for example a RGBA dataset will be transparently converted to a RGB+mask dataset when selecting JPEG compression)

## Driver capabilities

Supports CreateCopy()

This driver supports the `GDALDriver::CreateCopy()` operation

Supports Georeferencing

This driver supports georeferencing

Supports VirtualIO

This driver supports virtual I/O operations (/vsimem/, etc.)

## Creation Options

Creation options can be specified in command-line tools using the syntax `-co <NAME>=<VALUE>` or by providing the appropriate arguments to `GDALCreate()` (C) or `Driver.Create` (Python).

### General creation options

- BLOCKSIZE=<integer>: Defaults to `512`. Sets the tile width and height in pixels. Must be divisible by 16.

- COMPRESS=[NONE​/​LZW​/​JPEG​/​DEFLATE​/​ZSTD​/​WEBP​/​LERC​/​LERC_DEFLATE​/​LERC_ZSTD​/​LZMA]: Defaults to `LZW`. Set the compression to use. Defaults to `LZW` starting with GDAL 3.4 (default in previous version is `NONE`).

  - `JPEG` should generally only be used with Byte data (8 bit per channel). But if GDAL is built with internal libtiff and libjpeg, it is possible to read and write TIFF files with 12bit JPEG compressed TIFF files (seen as UInt16 bands with NBITS=12). For the COG driver, JPEG compression for 3 or 4-band images automatically selects the PHOTOMETRIC=YCBCR colorspace with a 4:2:2 subsampling of the Y,Cb,Cr components. For a input dataset (single-band or 3-band), plus an alpha band, the alpha band will be converted as a 1-bit DEFLATE compressed mask.

  - `LZW`, `DEFLATE` and `ZSTD` compressions can be used with the PREDICTOR creation option.

  - `ZSTD` is available when using internal libtiff and if GDAL built against libzstd >=1.0, or if built against external libtiff with zstd support.

  - `LERC` is available when using internal libtiff.

  - `LERC_ZSTD` is available when `LERC` and `ZSTD` are available.

  - `JXL` is for JPEG-XL, and is only available when using internal libtiff and building GDAL against https://github.com/libjxl/libjxl . JXL compression may only be used on datasets with 4 bands or less. Option added in GDAL 3.4

- LEVEL=<integer>: DEFLATE/ZSTD/LERC_DEFLATE/LERC_ZSTD/LZMA compression level. A lower number will result in faster compression but less efficient compression rate. 1 is the fastest.

  - For DEFLATE/LZMA, 9 is the slowest/higher compression rate (or 12 when using a libtiff with libdeflate support). The default is 6.

  - For ZSTD, 22 is the slowest/higher compression rate. The default is 9.

- MAX_Z_ERROR=<threshold>: Defaults to `0`. Set the maximum error threshold on values for LERC/LERC_DEFLATE/LERC_ZSTD compression. The default is 0 (lossless).

- MAX_Z_ERROR_OVERVIEW=<threshold>: (GDAL >= 3.8) Set the maximum error threshold on values for LERC/LERC_DEFLATE/LERC_ZSTD compression, on overviews. The default is the value of `MAX_Z_ERROR`

- QUALITY=<integer>: Defaults to `75`. JPEG/WEBP quality setting. A value of 100 is best quality (least compression), and 1 is worst quality (best compression). For WEBP, QUALITY=100 automatically turns on lossless mode.

- JXL_LOSSLESS=[YES​/​NO]: Defaults to `YES`. Set whether JPEG-XL compression should be lossless (YES) or lossy (NO). For lossy compression, the underlying data should be either gray, gray+alpha, rgb or rgb+alpha.

- JXL_EFFORT=1-9: Defaults to `5`. Level of effort for JPEG-XL compression. The higher, the smaller file and slower compression time.

- JXL_DISTANCE=0.01-25: Defaults to `1.0`. Distance level for lossy JPEG-XL compression. It is specified in multiples of a just-noticeable difference (cf butteraugli for the definition of the distance) That is, 0 is mathematically lossless, 1 should be visually lossless, and higher distances yield denser and denser files with lower and lower fidelity. The recommended range is [0.5,3].

- JXL_ALPHA_DISTANCE=[-1​/​0​/​0.01-25]: (GDAL >= 3.7) Defaults to `-1`. (libjxl > 0.8.1) Distance level for alpha channel for lossy JPEG-XL compression. It is specified in multiples of a just-noticeable difference. (cf butteraugli for the definition of the distance) That is, 0 is mathematically lossless, 1 should be visually lossless, and higher distances yield denser and denser files with lower and lower fidelity. For lossy compression, the recommended range is [0.5,3]. The default value is the special value -1.0, which means to use the same distance value as non-alpha channel (ie `JXL_DISTANCE`).

- NUM_THREADS=[<number_of_threads>​/​ALL_CPUS]: Enable multi-threaded compression by specifying the number of worker threads. Default is compression in the main thread. This also determines the number of threads used when reprojection is done with the `TILING_SCHEME` or `TARGET_SRS` creation options. (Overview generation is also multithreaded since GDAL 3.2)

- NBITS=<integer>: (GDAL >= 3.7) Create a file with less than 8 bits per sample by passing a value from 1 to 7. The apparent pixel type should be Byte. Values of n=9...15 (UInt16 type) and n=17...31 (UInt32 type) are also accepted. From GDAL 2.2, n=16 is accepted for Float32 type to generate half-precision floating point values.

- PREDICTOR=[YES​/​NO​/​STANDARD​/​FLOATING_POINT]: Defaults to `NO`. Set the predictor for LZW, DEFLATE and ZSTD compression. If YES is specified, then standard predictor (Predictor=2) is used for integer data type, and floating-point predictor (Predictor=3) for floating point data type (in some circumstances, the standard predictor might perform better than the floating-point one on floating-point data). STANDARD or FLOATING_POINT can also be used to select the precise algorithm wished.

- BIGTIFF=[YES​/​NO​/​IF_NEEDED​/​IF_SAFER]: Control whether the created file is a BigTIFF or a classic TIFF.

  - `YES` forces BigTIFF.

  - `NO` forces classic TIFF.

  - `IF_NEEDED` will only create a BigTIFF if it is clearly needed (in the uncompressed case, and image larger than 4GB. So no effect when using a compression).

  - `IF_SAFER` will create BigTIFF if the resulting file *might* exceed 4GB. Note: this is only a heuristics that might not always work depending on compression ratios.

  BigTIFF is a TIFF variant which can contain more than 4GiB of data (size of classic TIFF is limited by that value). This option is available if GDAL is built with libtiff library version 4.0 or higher. The default is IF_NEEDED.

  When creating a new GeoTIFF with no compression, GDAL computes in advance the size of the resulting file. If that computed file size is over 4GiB, GDAL will automatically decide to create a BigTIFF file. However, when compression is used, it is not possible in advance to known the final size of the file, so classical TIFF will be chosen. In that case, the user must explicitly require the creation of a BigTIFF with BIGTIFF=YES if the final file is anticipated to be too big for classical TIFF format. If BigTIFF creation is not explicitly asked or guessed and the resulting file is too big for classical TIFF, libtiff will fail with an error message like "TIFFAppendToStrip:Maximum TIFF file size exceeded".

- RESAMPLING=[NEAREST​/​AVERAGE​/​BILINEAR​/​CUBIC​/​CUBICSPLINE​/​LANCZOS​/​MODE​/​RMS]: Resampling method used for overview generation or reprojection. For paletted images, NEAREST is used by default, otherwise it is CUBIC.

- OVERVIEW_RESAMPLING=[NEAREST​/​AVERAGE​/​BILINEAR​/​CUBIC​/​CUBICSPLINE​/​LANCZOS​/​MODE​/​RMS]: (GDAL >= 3.2) Resampling method used for overview generation. For paletted images, NEAREST is used by default, otherwise it is CUBIC. This overrides, for overview generation, the value of `RESAMPLING` if it specified.

- WARP_RESAMPLING=[NEAREST​/​AVERAGE​/​BILINEAR​/​CUBIC​/​CUBICSPLINE​/​LANCZOS​/​MODE​/​RMS​/​MIN​/​MAX​/​MED​/​Q1​/​Q3]: (GDAL >= 3.2) Resampling method used for reprojection. For paletted images, NEAREST is used by default, otherwise it is CUBIC. This overrides, for reprojection, the value of `RESAMPLING` if it specified.

- OVERVIEWS=[AUTO​/​IGNORE_EXISTING​/​FORCE_USE_EXISTING​/​NONE]: Defaults to `AUTO`. Describe the behavior regarding overview generation and use of source overviews.

  - `AUTO` (default): source overviews will be used if present. If not present, overviews will be automatically generated in the output file.

  - `IGNORE_EXISTING`: potential existing overviews on the source dataset will be ignored and new overviews will be automatically generated.

  - `FORCE_USE_EXISTING`: potential existing overviews on the source will be used. If there is no source overview, this is equivalent to specifying `NONE`.

  - `NONE`: potential source overviews will be ignored, and no overview will be generated.

    Note

    When using the gdal_translate utility, source overviews will not be available if general options (i.e. options which are not creation options, like subsetting, etc.) are used.

- OVERVIEW_COUNT=<integer>: (GDAL >= 3.6) Number of overview levels to generate. This can be used to increase or decrease the number of levels in the COG file (when GDAL computes overviews from the full resolution dataset, that is when there are no source overviews or the user specifies `OVERVIEWS=IGNORE_EXISTING`), or decrease the number of levels copied from the source dataset (in `OVERVIEWS=AUTO` or `FORCE_USE_EXISTING` modes when there are such overviews in the source dataset).

  If not specified, the driver will use all the overviews available in the source raster, in `OVERVIEWS=AUTO` or `FORCE_USE_EXISTING` modes. In situations where GDAL generates overviews, the default number of overview levels is such that the dimensions of the smallest overview are smaller or equal to the `BLOCKSIZE` value.

- OVERVIEW_COMPRESS=[AUTO​/​NONE​/​LZW​/​JPEG​/​DEFLATE​/​ZSTD​/​WEBP​/​LERC​/​LERC_DEFLATE​/​LERC_ZSTD​/​LZMA]: Defaults to `AUTO`. Set the compression method (see `COMPRESS`) to use when storing the overviews in the COG.

  By default (`AUTO`) the overviews will be created with the same compression method as the COG.

- OVERVIEW_QUALITY=<integer>: JPEG/WEBP quality setting. A value of 100 is best quality (least compression), and 1 is worst quality (best compression). By default the overviews will be created with the same quality as the COG, unless the compression type is different then the default is 75.

- OVERVIEW_PREDICTOR=[YES​/​NO​/​STANDARD​/​FLOATING_POINT]: Set the predictor for LZW, DEFLATE and ZSTD overview compression. By default the overviews will be created with the same predictor as the COG, unless the compression type of the overview is different, then the default is NO.

- GEOTIFF_VERSION=[AUTO​/​1.0​/​1.1]: Defaults to `AUTO`. Select the version of the GeoTIFF standard used to encode georeferencing information. `1.0` corresponds to the original 1995, GeoTIFF Revision 1.0, by Ritter & Ruth. `1.1` corresponds to the OGC standard 19-008, which is an evolution of 1.0, which clear ambiguities and fix inconsistencies mostly in the processing of the vertical part of a CRS. `AUTO` mode (default value) will generally select 1.0, unless the CRS to encode has a vertical component or is a 3D CRS, in which case 1.1 is used.

  Note

  Write support for GeoTIFF 1.1 requires libgeotiff 1.6.0 or later.

- SPARSE_OK=[TRUE​/​FALSE]: (GDAL >= 3.2) Defaults to `FALSE`. Should empty blocks be omitted on disk? When this option is set, any attempt of writing a block whose all pixels are 0 or the nodata value will cause it not to be written at all (unless there is a corresponding block already allocated in the file). Sparse files have 0 tile/strip offsets for blocks never written and save space; however, most non-GDAL packages cannot read such files. On the reading side, the presence of a omitted tile after a non-empty one may cause optimized readers to have to issue an extra GET request to the TileByteCounts array.

- STATISTICS=[AUTO​/​YES​/​NO]: (GDAL >= 3.8) Defaults to `AUTO`. Whether band statistics should be included in the output file. In `AUTO` mode, they will be included only if available in the source dataset. If setting to `YES`, they will always be included. If setting to `NO`, they will be never included.

### Reprojection related creation options

- TILING_SCHEME=[CUSTOM​/​GoogleMapsCompatible​/​...]: Defaults to `CUSTOM`. If set to a value different than CUSTOM, the definition of the specified tiling scheme will be used to reproject the dataset to its CRS, select the resolution corresponding to the closest zoom level and align on tile boundaries at this resolution (the actual resolution can be controlled with the `ZOOM_LEVEL` or `ZOOM_LEVEL_STRATEGY` options).

  The tile size indicated in the tiling scheme definition (generally 256 pixels) will be used, unless the user has specified a value with the `BLOCKSIZE` creation option, in which case the user specified one will be taken into account (that is if setting a higher value than 256, the original tiling scheme is modified to take into account the size of the HiDPi tiles).

  In non-CUSTOM mode, TARGET_SRS, RES and EXTENT options are ignored. Starting with GDAL 3.2, the value of `TILING_SCHEME` can also be the filename of a JSON file according to the OGC Two Dimensional Tile Matrix Set standard, a URL to such file, the radical of a definition file in the GDAL data directory (e.g. `FOO` for a file named `tms_FOO.json`) or the inline JSON definition. The list of available tiling schemes can be obtained by looking at values of the TILING_SCHEME option reported by `gdalinfo --format COG`.

- ZOOM_LEVEL=<integer>: (GDAL >= 3.5) Zoom level number (starting at 0 for coarsest zoom level). Only used for `TILING_SCHEME` different from CUSTOM. If this option is specified, `ZOOM_LEVEL_STRATEGY` is ignored.

- ZOOM_LEVEL_STRATEGY=[AUTO​/​LOWER​/​UPPER]: (GDAL >= 3.2) Defaults to `AUTO`. Strategy to determine zoom level. Only used for `TILING_SCHEME` different from CUSTOM. LOWER will select the zoom level immediately below the theoretical computed non-integral zoom level, leading to subsampling. On the contrary, UPPER will select the immediately above zoom level, leading to oversampling. Defaults to AUTO which selects the closest zoom level.

- TARGET_SRS=value: to force reprojection of the input dataset to another SRS. The string can be a WKT string, a EPSG:XXXX code or a PROJ string.

- RES=value: Set the resolution of the target raster, in the units of `TARGET_SRS`. Only taken into account if `TARGET_SRS` is specified.

- EXTENT=<minx,miny,maxx,maxy>: Set the extent of the target raster, in the units of `TARGET_SRS`. Only taken into account if `TARGET_SRS` is specified.

- ALIGNED_LEVELS=<integer>: Number of resolution levels for which GeoTIFF tile and tiles defined in the tiling scheme match. When specifying this option, padding tiles will be added to the left and top sides of the target raster, when needed, so that a GeoTIFF tile matches with a tile of the tiling scheme. Only taken into account if `TILING_SCHEME` is different from CUSTOM. Effect of this option is only visible when setting it at 2 or more, since the full resolution level is by default aligned with the tiling scheme. For a tiling scheme whose consecutive zoom level resolutions differ by a factor of 2, care must be taken in setting this value to a high number of levels, as up to 2^(ALIGNED_LEVELS-1) tiles can be added in each dimension. The driver enforces a hard limit of 10.

- ADD_ALPHA=[YES​/​NO]: Defaults to `YES`. Whether an alpha band is added in case of reprojection.

## Update

Updating a COG file generally breaks part of the optimizations, but still produces a valid GeoTIFF file. Starting with GDAL 3.8, to avoid undesired loss of the COG characteristics, opening such a file in update mode will be rejected, unless the IGNORE_COG_LAYOUT_BREAK open option is also explicitly set to YES.

Note that a subset of operations are possible when opening a COG file in read-only mode, like metadata edition (including statistics storage), that will be stored in a auxiliary .aux.xml side-car file.

## File format details

### High level

A Cloud optimized GeoTIFF has the following characteristics:

- TIFF or BigTIFF file

- Tiled (512 pixels by default) for imagery, mask and overviews

- Overviews until the maximum dimension of the smallest overview level is lower than 512 pixels.

- Compressed or not

- Pixel interleaving for multi-band dataset

- Optimized layout of TIFF sections to minimize the number of GET requests needed by a reader doing random read access.

### Low level

A COG file is organized as the following (if using libtiff >= 4.0.11 or GDAL internal libtiff. For other versions, the layout will be different and some of the optimizations will not be available).

- TIFF/BigTIFF header/signature and pointer to first IFD (Image File Directory)

- "ghost area" with COG optimizations (see Header ghost area)

- IFD of the full resolution image, followed by TIFF tags values, excluding the TileOffsets and TileByteCounts arrays.

- IFD of the mask of the full resolution image, if present, followed by TIFF tags values, excluding the TileOffsets and TileByteCounts arrays.

- IFD of the first (largest in dimensions) overview level, if present

- ...

- IFD of the last (smallest) overview level, if present

- IFD of the first (largest in dimensions) overview level of the mask, if present

- ...

- IFD of the last (smallest) overview level of the mask, if present

- TileOffsets and TileByteCounts arrays of the above IFDs

- tile data of the smallest overview, if present (with each tile followed by the corresponding tile of mask data, if present), with leader and trailer bytes

- ...

- tile data of the largest overview, if present (interleaved with mask data if present)

- tile data of the full resolution image, if present (interleaved with corresponding mask data if present)

### Header ghost area

To describe the specific layout of COG files, a description of the features used is located at the beginning of the file, so that optimized readers (like GDAL) can use them and take shortcuts. Those features are described as ASCII strings "hidden" just after the 8 first bytes of a ClassicTIFF (or after the 16 first ones for a BigTIFF). That is the first IFD starts just after those strings. It is completely valid to have ghost areas like this in a TIFF file, and readers will normally skip over them. So for a COG file with a transparency mask, those strings will be:

    GDAL_STRUCTURAL_METADATA_SIZE=000174 bytes
    LAYOUT=IFDS_BEFORE_DATA
    BLOCK_ORDER=ROW_MAJOR
    BLOCK_LEADER=SIZE_AS_UINT4
    BLOCK_TRAILER=LAST_4_BYTES_REPEATED
    KNOWN_INCOMPATIBLE_EDITION=NO
    MASK_INTERLEAVED_WITH_IMAGERY=YES

Note

- A newline character \n is used to separate those strings.

- A space character is inserted after the newline following KNOWN_INCOMPATIBLE_EDITION=NO

- For a COG without mask, the MASK_INTERLEAVED_WITH_IMAGERY item will not be present of course.

The ghost area starts with `GDAL_STRUCTURAL_METADATA_SIZE=XXXXXX bytes\n` (of a fixed size of 43 bytes) where XXXXXX is a 6-digit number indicating the remaining size of the section (that is starting after the linefeed character of this starting line).

- `LAYOUT=IFDS_BEFORE_DATA`: the IFDs are located at the beginning of the file. GDAL will also makes sure that the tile index arrays are written just after the IFDs and before the imagery, so that a first range request of 16 KB will always get all the IFDs

- `BLOCK_ORDER=ROW_MAJOR`: (strile is a contraction of 'strip or tile') the data for tiles is written in increasing tile id order. Future enhancements could possibly implement other layouts.

- `BLOCK_LEADER=SIZE_AS_UINT4`: each tile data is preceded by 4 bytes, in a ghost area as well, indicating the real tile size (in little endian order). See Tile data leader and trailer for more details.

- `BLOCK_TRAILER=LAST_4_BYTES_REPEATED`: just after the tile data, the last 4 bytes of the tile data are repeated. See Tile data leader and trailer for more details.

- `KNOWN_INCOMPATIBLE_EDITION=NO`: when a COG is generated this is always written. If GDAL is then used to modify the COG file, as most of the changes done on an existing COG file, will break the optimized structure, GDAL will change this metadata item to KNOWN_INCOMPATIBLE_EDITION=YES, and issue a warning on writing, and when reopening such file, so that users know they have broken their COG file

- `MASK_INTERLEAVED_WITH_IMAGERY=YES`: indicates that mask data immediately follows imagery data. So when reading data at offset=TileOffset[i] - 4 and size=TileOffset[i+1]-TileOffset[i]+4, you'll get a buffer with:

  > - leader with imagery tile size (4 bytes)

  > - imagery data (starting at TileOffsets[i] and of size TileByteCounts[i])

  > - trailer of imagery (4 bytes)

  > - leader with mask tilesize (4 bytes)

  > - mask data (starting at mask.TileOffsets[i] and of size mask.TileByteCounts[i], but none of them actually need to be read)

  > - trailer of mask data (4 bytes)

Note

The content of the header ghost area can be retrieved by getting the `GDAL_STRUCTURAL_METADATA` metadata item of the `TIFF` metadata domain on the datasett object (with GetMetadataItem())

### Tile data leader and trailer

Each tile data is immediately preceded by a leader, consisting of a unsigned 4-byte integer, in little endian order, giving the number of bytes of payload of the tile data that follows it. This leader is ghost in the sense that the TileOffsets[] array does not point to it, but points to the real payload. Hence the offset of the leader is TileOffsets[i]-4.

An optimized reader seeing the `BLOCK_LEADER=SIZE_AS_UINT4` metadata item will thus look for TileOffset[i] and TileOffset[i+1] to deduce it must fetch the data starting at offset=TileOffset[i] - 4 and of size=TileOffset[i+1]-TileOffset[i]+4. It then checks the 4 first bytes to see if the size in this leader marker is consistent with TileOffset[i+1]-TileOffset[i]. When there is no mask, they should normally be equal (modulo the size taken by BLOCK_LEADER and BLOCK_TRAILER). In the case where there is a mask and MASK_INTERLEAVED_WITH_IMAGERY=YES, then the tile size indicated in the leader will be < TileOffset[i+1]-TileOffset[i] since the data for the mask will follow the imagery data (see MASK_INTERLEAVED_WITH_IMAGERY=YES)

Each tile data is immediately followed by a trailer, consisting of the repetition of the last 4 bytes of the payload of the tile data. The size of this trailer is not included in the TileByteCounts[] array. The purpose of this trailer is forces readers to be able to check if TIFF writers, not aware of those optimizations, have modified the TIFF file in a way that breaks the optimizations. If an optimized reader detects an inconsistency, it can then fallbacks to the regular/slower method of using TileOffsets[i] + TileByteCounts[i].

## Examples

    gdalwarp src1.tif src2.tif out.tif -of COG

    gdal_translate world.tif world_webmerc_cog.tif -of COG -co TILING_SCHEME=GoogleMapsCompatible -co COMPRESS=JPEG

## See Also

- GTiff -- GeoTIFF File Format driver

- If your source dataset is an internally tiled geotiff with the desired georeferencing and compression, using cogger (possibly along with gdaladdo to create overviews) will be much faster than the COG driver.