How to properly rotate a raster

[Eloi Ribeiro]

Hi Regina and all,

Elaborating a bit more. I have a vector layer consisting of a rectangle (farm plot), and this rectangle is subdivided into several (polygons) stripes parallel to the outer edge of the rectangle. I want to produce a raster where the rectangle and stripes long edges are parallel to the latitude line.

Using ST_Rotate(geom, -0.035, x, y) in all vector layers together with ST_AsRaster(), I managed to get what I want. Good.
Nevertheless, if I first produce the raster and then try to rotated it using ST_SetRotation(rast, -0.035), I barely see any rotation. In this step, I must be doing something wrong.

I would like to first rasterize vector and then rotate the raster. Instead of rasterize non rotated vector and then, again rasterize rotated vector.

The change in size (width and height), I mentioned in my first message, was while testing with 0.5 rad, not with -0.035 rad.

-- input raster
SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3;
-- st_width | st_height | st_skewx
-- ----------+-----------+----------
-- 1000 | 1000 | 0

-- output raster of ST_SetRotation(rast, -0.035162353342492736)
SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3_rotated_from_raster;
-- st_width | st_height | st_skewx
-- ----------+-----------+----------------------
-- 1000 | 1000 | 0.006952538312005269

-- output raster of ST_SetRotation(rast, -0.035162353342492736)
SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3_rotated_from_vect;
-- st_width | st_height | st_skewx
-- ----------+-----------+----------
-- 977 | 964 | 0

-- output raster of ST_SetRotation(rast, -0.5)
SELECT ST_Width(rast), ST_Height(rast), ST_SkewX(rast) FROM plot_3_rotated_from_raster_05;
-- st_width | st_height | st_skewx
-- ----------+-----------+---------------------
-- 1000 | 1000 | 0.09481479675190897

Cheers,
Eloi

On Monday, March 18th, 2024 at 14:01, Regina Obe <lr at pcorp.us> wrote:

> By size, what do you mean exactly? It would change the width and height.
>
> What are you expecting rotation to do. Perhaps you can give example out of the below and some sample
>
> ST_Width, ST_Height, ST_SkewX before and after and what you were expecting.
>
> https://postgis.net/docs/en/RT_ST_SetRotation.html
>
> From: Eloi Ribeiro <mail at eloiribeiro.eu>
> Sent: Monday, March 18, 2024 4:57 AM
> To: PostGIS Users Discussion <postgis-users at lists.osgeo.org>
> Subject: How to properly rotate a raster
>
> Hi all,
>
> I need to rotate a raster, for that I'm using the function ST_SetRotation, like so:
>
> CREATE TABLE plot_3_rotated AS
> SELECT ST_SetRotation(rast, 0.03516235334249185) rast
> FROM plot_3;
>
> But I see that is not producing the expected results and is changing size of the raster. What am I doing wrong?
>
> Cheers,
>
> Eloi
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