[GIS] Translate (convert format): Set the size of the output file

gdalgdal-translatepythonqgis

I have a raster .asc file with Pixel Size = 0.000863923,-0.000863923 (as given in Rightclick -> Properties -> Metadata -> Properties -> Pixel Size) and Dimensions = X: 5788 Y: 2315 (where X is width and Y is Height).

I want to export this raster (with a new name) after changing the properties into Pixel Size = 0.000833333,-0.000833333 and Dimensions = X: 6000 Y: 2400.

For this, I want to use Translate (convert format) in the Processing Toolbox -> GDAL/OGR -> [GDAL] Conversion.

In Translate (convert format), I am able to see an option :

Set the size of the output file (In pixels or %)

What value should I give in order to get a raster with Pixel Size = 0.000833333,-0.000833333 and Dimensions = X: 6000 Y: 2400 (from a raster with the same extents; but with Pixel Size = 0.000863923,-0.000863923 and Dimensions = X: 5788 Y: 2315) ?

Also, I am able to see another option :

Output size is a percentage of input size

Do I have to select (select by ticking or by giving 'Yes') this option or not ?

If I give Set the size of the output file (In pixels) = 100, then the python code will be :

processing.runalg("gdalogr:translate","C:/Users/Sreeraj/Desktop/1.asc",100,False,"",0,"","96.993358471,98.9933584709,14.0067404817,17.0067404815",False,5,4,75,6,1,False,0,False,"","C:/Users/Sreeraj/Desktop/2.asc")

You can see the value 100 in the above line of code (after the input file 1.asc).

If I want to give -outsize 6000 2400 ; then how can I change the given below python code (change from Set the size of the output file (In pixels) = 100 to -outsize 6000 2400) ?

processing.runalg("gdalogr:translate","C:/Users/Sreeraj/Desktop/1.asc",100,False,"",0,"","96.993358471,98.9933584709,14.0067404817,17.0067404815",False,5,4,75,6,1,False,0,False,"","C:/Users/Sreeraj/Desktop/2.asc")

Best Answer

Conversion does not change the extent of the image. It you define the number of pixels then the size of the pixel is calculated automatically, of if you define the pixel size then the number of pixels comes automatically from equations "width/pixel size" and "height/pixel size"

The user interface in QGIS gives controls for a subset of the options of gdal_translate http://www.gdal.org/gdal_translate.html and what you want to do is not supported. Fortunately you can edit the generated command directly before executing it. You have two alternatives. Either set the size

gdal_translate -outsize 6000 2400 input.tif output.tif

or set the resolution

gdal_translate -tr 0.000833333 0.000833333 input.tif output.tif

Here is the place for doing the edits. Use the pen icon for enabling edits and press OK for running the process.

Related Solutions

[GIS] Reducing File Size without losing quality

For aerial images like this, I suggest using TIFFs with JPEG compression. That way, there are no separate world files needed, all the metadata is stored in the TIFF as tags. Note, this will only work on RGB files with 8 bits per band.

It's lossy, so it's not suitable for DEMs (for those, lossless LZW will reduce file size by ~30 % to 40% in my experience) And keep your originals as it's impossible to regain lost quality if you lose those.

gdal_translate \
  -co COMPRESS=JPEG \
  -co JPEG_QUALITY=70 \
  -co PHOTOMETRIC=YCBCR \
  source.tif compressed.tif

JEPG_QUALITY works like the similar setting in GIMP and Photoshop. Higher quality (max. 100) means bigger files. The default is 70. The lower the number, the higher the compression.

You're unlikely to see any compression artifacts if the value is 90 or higher, unless you're a nit-picky digital photographer like me (and even then, it's a struggle to see the difference) :)

It's still lossy compression, so if you if you can't afford any data changes DEFLATE will be more suitable, at the expense of larger file sizes.

Artifacts tend to be more noticeable in areas with high contrast (like edges of buildings) so look for those when assessing quality, rather than low-contrast areas like tree canopy or fields.

There's a nice blog post on the subject by Paul Ramsey, which covers some additional settings which are suitable for certain scenarios (like serving images over web services)

[GIS] gdal_grid, how to set pixel size

GDAL version <3.2

To set the pixel size in gdal_grid, you have to play with the following options:

-txe xmin xmax:

Set georeferenced X extents of output file to be created.

-tye ymin ymax:

Set georeferenced Y extents of output file to be created.

-outsize xsize ysize:

Set the size of the output file in pixels and lines.

because:

xcellsize = (xmax - xmin) / xsize
ycellsize = (ymax - ymin) / ysize

New in GDAL version 3.2.

-tr xres yres

Set output file resolution (in target georeferenced units). Note that -tr just works in combination with a valid input from -txe and -tye

Best Answer

Related Solutions

[GIS] Reducing File Size without losing quality

[GIS] gdal_grid, how to set pixel size

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