Angle Measurement Definition

Different image formats are displayed with their origin in different relative locations in the image window. The convention for FITS format is to display the image with the origin in the lower left corner but other formats display the origin in the upper left corner. This means that angles are measured differently in FITS format and other formats. The convention for graphics software is to measure positive angle going clockwise from the positive x axis but it may be different for a FITS image that does have or does not have a World Coordinate System Calibration. Therefore, to accommodate all conventions, angle measurements follow a different sense when measured in different format images:

Definition

If the image has a World Coordinate System calibration, angle as a Position Angle (PA) beginning at North=0 and increasing counterclockwise through East.
If the image has no WCS calibration or it is from a graphics format such as TIFF, BMP, or JPG, the angle is measured in the trigonometric sense starting at 0 degrees pointing to the right and 90 degrees pointing in the direction of increasing row number. A FITS format image is required by the FITS standard to be displayed in the traditional Cartesian sense with x to the right and y upward. Therefore, angle is measured with 0 degrees pointing right and 90 degrees pointing upward.