09-08-2025, 12:43 PM
I have two images of the same thing. The extent and alignment are the same. One is low resolution (ppi) and has been geo referenced. The other is the same image where the ppi has been increased and the image size reduced with the scaling tool.
I don't want to georeference all those points on this new image. It seems like there should be a relationship between the x, y coordinates of the pixels of the two images so I should be able to calculate where a pixel from the old image will line up on the new image.
I thought it would be a simple ratio, but so far it hasn't worked. The points move but are off by a lot.
I have read that the scaling tool has a number of interpolation methods and that may influence this relationship.
Does anyone know anything about this?
I'm actually trying to load the points from the old image onto the new higher ppi image in QGIS. I'm not sure if it has any settings for interpolation method when doing this operation or not. From there is will be warped. The warping included interpolation methods, but I'm not there yet.
Here is some info on the two images:
My old low ppi raster is:
3772 x 1936 pixels
0.4517 x 0.2319 inches print size (11.47318 x 5.89026 mm)
8350 x 8350 PPI
The new high ppi raster is:
10190 x 5225 pixels
16.883 x 8.708 in print size (428.8282 x 221.1832 mm)
600 x 600 PPI
I don't want to georeference all those points on this new image. It seems like there should be a relationship between the x, y coordinates of the pixels of the two images so I should be able to calculate where a pixel from the old image will line up on the new image.
I thought it would be a simple ratio, but so far it hasn't worked. The points move but are off by a lot.
I have read that the scaling tool has a number of interpolation methods and that may influence this relationship.
Does anyone know anything about this?
I'm actually trying to load the points from the old image onto the new higher ppi image in QGIS. I'm not sure if it has any settings for interpolation method when doing this operation or not. From there is will be warped. The warping included interpolation methods, but I'm not there yet.
Here is some info on the two images:
My old low ppi raster is:
3772 x 1936 pixels
0.4517 x 0.2319 inches print size (11.47318 x 5.89026 mm)
8350 x 8350 PPI
The new high ppi raster is:
10190 x 5225 pixels
16.883 x 8.708 in print size (428.8282 x 221.1832 mm)
600 x 600 PPI