On Fri, 16 Dec 2022, Clive Swan wrote:
Greetings,
I want to shift the UL x, UL y of an image.
I am trying to find out what the *dx_dy + dy_dx* is.
They represent (part of) the rotation;
technically I think that they are they are the "skew".
For a *translation* they are both equal to zero.
Is dx_dy, dy_dx = (0,0)
The Top Left/Right value??
For your transform, yes they are zero,
but no, not the Top Left/Right.
The code is:
import gdal
### open dataset with update permission
ds = gdal.Open('/data/coastal-2020.tif', gdal.GA_Update)
### get the geotransform as a tuple of 6
gt = ds.GetGeoTransform()
### unpack geotransform into variables
x_tl = (-180.0000000)
x_res = 0.000800000000000
*dx_dy* =
y_tl = (90.0000000)
*dy_dx* =
y_res = -0.000800000000000
x_tl, x_res, *dx_dy*, y_tl, *dy_dx*, y_res = gt
# compute shift of 1 pixel RIGHT in X direction
shift_x = 1 * x_res
# compute shift of 2 pixels UP in Y direction
# y_res likely negative, because Y decreases with increasing Y index
shift_y = -2 * y_res
# make new geotransform
gt_update = (x_tl + shift_x, x_res, dx_dy, y_tl + shift_y, dy_dx, y_res)
x and y are in *degrees*.
You *can* shift your map left/right = east/west, but odd things may happen
at 180 degree longitude, since a common convention is that this
is the left-right border.
However, you cannot shift your map up/down = north/south as this will move
(in your case) the top of the map north of the north pole.
This is particularly bad with any Mercator projection,
since the north pole is already at infinity on the map.
From gdalinfo:
Size is 450000, 225000
Coordinate System is:
GEOGCRS["WGS 84",
DATUM["World Geodetic System 1984",
ELLIPSOID["WGS 84",6378137,298.257223563,
LENGTHUNIT["metre",1]]],
PRIMEM["Greenwich",0,
ANGLEUNIT["degree",0.0174532925199433]],
CS[ellipsoidal,2],
AXIS["geodetic latitude (Lat)",north,
ORDER[1],
ANGLEUNIT["degree",0.0174532925199433]],
AXIS["geodetic longitude (Lon)",east,
ORDER[2],
ANGLEUNIT["degree",0.0174532925199433]],
ID["EPSG",4326]]
Data axis to CRS axis mapping: 2,1
Origin = (-180.000000000000000,90.000000000000000)
Pixel Size = (0.000800000000000,-0.000800000000000)
Image Structure Metadata:
COMPRESSION=LZW
INTERLEAVE=BAND
PREDICTOR=3
Corner Coordinates:
Upper Left (-180.0000000, 90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"N)
Lower Left (-180.0000000, -90.0000000) (180d 0' 0.00"W, 90d 0' 0.00"S)
Upper Right ( 180.0000000, 90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"N)
Lower Right ( 180.0000000, -90.0000000) (180d 0' 0.00"E, 90d 0' 0.00"S)
Center ( 0.0000000, 0.0000000) ( 0d 0' 0.01"E, 0d 0' 0.01"N)
--
Andrew C. Aitchison Kendal, UK
[email protected]
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