# gdal_merge.py

Mosaics a set of images.

## Synopsis

    gdal_merge.py [--help] [--help-general]
                  [-o <out_filename>] [-of <out_format>] [-co <NAME>=<VALUE>]...
                  [-ps <pixelsize_x> <pixelsize_y>] [-tap] [-separate] [-q] [-v] [-pct]
                  [-ul_lr <ulx> <uly> <lrx> <lry>] [-init "<value>[ <value>]..."]
                  [-n <nodata_value>] [-a_nodata <output_nodata_value>]
                  [-ot <datatype>] [-createonly] <intput_file> [<intput_file>]...

## Description

This utility will automatically mosaic a set of images. All the images must be in the same coordinate system and have a matching number of bands, but they may be overlapping, and at different resolutions. In areas of overlap, the last image will be copied over earlier ones. Nodata/transparency values are considered on a band by band level, i.e. a nodata/transparent pixel on one source band will not set a nodata/transparent value on all bands for the target pixel in the resulting raster nor will it overwrite a valid pixel value.

--help

  Show this help message and exit

--help-general

  Gives a brief usage message for the generic GDAL commandline options and exit.

-o <out_filename>

  The name of the output file, which will be created if it does not already exist (defaults to "out.tif").

-of <format>

  Select the output format. Starting with GDAL 2.3, if not specified, the format is guessed from the extension (previously was GTiff). Use the short format name.

-co <NAME>=<VALUE>

  Many formats have one or more optional creation options that can be used to control particulars about the file created. For instance, the GeoTIFF driver supports creation options to control compression, and whether the file should be tiled.

  The creation options available vary by format driver, and some simple formats have no creation options at all. A list of options supported for a format can be listed with the --formats command line option but the documentation for the format is the definitive source of information on driver creation options. See Raster drivers format specific documentation for legal creation options for each format.

-ot <type>

  Force the output image bands to have a specific data type supported by the driver, which may be one of the following: `Byte`, `Int8`, `UInt16`, `Int16`, `UInt32`, `Int32`, `UInt64`, `Int64`, `Float32`, `Float64`, `CInt16`, `CInt32`, `CFloat32` or `CFloat64`.

-ps <pixelsize_x> <pixelsize_y>

  Pixel size to be used for the output file. If not specified the resolution of the first input file will be used.

-tap

  (target aligned pixels) align the coordinates of the extent of the output file to the values of the -tr, such that the aligned extent includes the minimum extent. Alignment means that xmin / resx, ymin / resy, xmax / resx and ymax / resy are integer values.

-ul_lr <ulx> <uly> <lrx> <lry>

  The extents of the output file. If not specified the aggregate extents of all input files will be used.

-v

  Generate verbose output of mosaicing operations as they are done.

-separate

  Place each input file into a separate band.

-pct

  Grab a pseudo-color table from the first input image, and use it for the output. Merging pseudo-colored images this way assumes that all input files use the same color table.

-n <nodata_value>

  Ignore pixels from files being merged in with this pixel value.

-a_nodata <output_nodata_value>

  Assign a specified nodata value to output bands.

-init <"value(s)">

  Pre-initialize the output image bands with these values. However, it is not marked as the nodata value in the output file. If only one value is given, the same value is used in all the bands.

-createonly

  The output file is created (and potentially pre-initialized) but no input image data is copied into it.

Note

gdal_merge.py is a Python script, and will only work if GDAL was built with Python support.

## Examples

### Creating an image with the pixels in all bands initialized to 255

    gdal_merge.py -init 255 -o out.tif in1.tif in2.tif

### Creating an RGB image that shows blue in pixels with no data

The first two bands will be initialized to 0 and the third band will be initialized to 255.

    gdal_merge.py -init "0 0 255" -o out.tif in1.tif in2.tif

### Passing a large list of files to gdal_merge

A large list of files can be passed to gdal_merge by listing them in a text file using:

    ls -1 *.tif > tiff_list.txt

on Linux, or

    dir /b /s *.tif > tiff_list.txt

on Windows. The text file can then be passed to gdal_merge using --optfile:

    gdal_merge.py -o mosaic.tif --optfile tiff_list.txt

### Creating an RGB image by merging 3 different greyscale bands

Conduct "merging by stacking" with the `-separate` flag. Given three greyscale files that cover the same area, you can run:

    gdal_merge.py -separate 1.tif 2.tif 3.tif -o rgb.tif

This maps `1.tif` to red, `2.tif` to green and `3.tif` to blue.

### Specifying overlap precedence

The last image in the input line comes out on top of the finished image stack. You might also need to use `-n` to note which value should not be copied into the destination image if it is not already defined as nodata.

    gdal_merge.py -o merge.tif -n 0 image1.tif image2.tif image3.tif image4.tif