QGIS – How to Convert South and West Coordinates to Align with X/East and Y/North

Question

So I have an image I am trying to georeference using QGIS. The coordinates I want the image to be placed at are:

top left = 50N, 140E
top right = 50N, 160W
bottom left = 15N, 140E
bottom right = 15N, 160W

But when I attempt to load those into QGIS – Georeference tool, it only appears to want:

X / East
Y / North

How would I convert my coordinates from W to E as the tool appears to want?

Answer 1

You can do this without using the georeference tool as you have the information to write your own world file. From wikipedia the world file structure is as follows

Line 1: A: pixel size in the x-direction in map units/pixel
Line 2: D: rotation about y-axis
Line 3: B: rotation about x-axis
Line 4: E: pixel size in the y-direction in map units, almost always negative[3]
Line 5: C: x-coordinate of the center of the upper left pixel
Line 6: F: y-coordinate of the center of the upper left pixel

You should be able to find the the size in pixels and therefore calculate how big each pixel is. From your coordinates it looks like there is no rotation so 0. The last two lines require a bit more maths as you have to calculate the centre of a pixel.

World files should be saved with the same name as the image but a different extension such as .wld.

Here is an example

0.50000000
0.00000000
0.00000000
-0.50000000
438007.16150000
193255.10440000