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:
Definitions
-
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.
Related Topics
Image Display Orientation
Pixel Coordinate Definition
Subpixel Coordinate Definition
World Coordinate System
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