Why is gdalwarp flipping pixel size sign?

Question

I am using the following command:

gdalwarp -co COMPRESS=LZW \
    -ts 1024 1024 \
    -cutline cutline.json \
    -crop_to_cutline \
    input.tif \
    output.tif

the output raster has the same number of pixels as the input one, but is just cropped to a cutline.

The input raster has pixel size (from gdalinfo):

Pixel Size = (159.847049179404479,153.365855104948253)

but the output raster has:

Pixel Size = (159.820436280028900,-153.866089899794588)

Why is gdalwarp flipping the pixel size?

What can I do to have positive size everywhere?

This is with GDAL 2.1.2

Answer 1

GDAL, like almost all image libraries, places the origin coordinate for an image in the top-left corner. GDAL just has an extra property of working in a geographic space, and keeps that convention. The first row (line) is at the top and successive rows (lines) are below it. In a North-up image (or in a coordinate system with X-right and Y-up and the image aligned to the grid), this means that successive rows are positioned at successively-less Y-values.

Pixel size (x: 5 ; y: -5)
Row 0: Y 20
Row 1: Y 15
Row 2: Y 10
...

It is very common to have a negative Y pixel-size, because it allows for a simple calculation of the geographic position of any pixel (i, j) in the image.

pix_x = origin_x + j * res_x // for example
pix_y = origin_y + i * res_y // for example

As to why the image flipped after passing it through gdalwarp: The original image you supplied must have placed the geographic origin in the bottom-left corner, effectively mirroring the image across a horizontal axis. The georeferencing used, however, had the data positioned correctly.

Read more about the spatial model, particularly the geotransform in the GDAL docs