Distance and Angle Measurements
The Distance & Angle tool measures the distance
between two points and the angle of the line connecting them. The
results are reported in the Distance Measurements report
window, which has all the built-in functionality of a Report Window. The
Distance & Angle command operates from a Command Toolbar, which
opens with Marking Mode active.
Coordinate Systems
-
If the image has a World Coordinate
System calibration, then distance is measured in arcseconds and
the angle in degrees is reported as a Position Angle (PA). Position
angle is measured with 0 aligned North, with increasing angle
measured through east (with increasing right ascension). See
Angle
Measurement Definition.
-
If the image has no WCS calibration, the
distance is measured in pixel units and the angle in degrees is
measured in the trigonometric sense: An angle of 0 degrees points
in the direction of increasing x (parallel to the rows, pointing
toward increasing column number). Positive angle increase through
increasing y (parallel to the columns, pointing toward increasing
row number). Different image formats are displayed differently,
which makes the angle as you see it, measure differently. 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.
Measurement Results
Measurements are tabulated in a Report Window designed
specifically for this command. Using this window, the measurements
may be sorted, rearranged, saved to the clipboard or a file,
etc.
Measurements for all objects in all images and all
Image Windows go into a single Report window. If you want to clear
the window of prior measurements, make sure the Report window has
focus, then use the View > Clear menu command.
The items in the Report window are described in the
table below.
Distance & Angle Quantities
|
#
|
The sequence number of the measurement.
|
|
Image
|
The name of the image that was measured
|
|
Distance
|
The distance in pixel units, or arcseconds if the
image has a WCS calibration.
|
|
Angle
|
The angle in degrees, or a Position Angle if the
image has a WCS calibration.
|
|
X1
|
The X coordinate of the starting point.
|
|
Y1
|
The Y coordinate of the starting point.
|
|
X2
|
The X coordinate of the ending point.
|
|
Y2
|
The Y coordinate of the ending point.
|
|
Col 1
|
The column coordinate of the starting point.
|
|
Row 1
|
The row coordinate of the starting point.
|
|
Col 2
|
The column coordinate of the ending point.
|
|
Row 2
|
The row coordinate of the ending point.
|
Marking the Endpoints
As with other interactive marking commands, the
Distance & Angle command can use the exact marked endpoints or
it can compute either or both of the endpoints as a precise
centroid position. All 4 combinations of centroid or no centroid
and starting point or ending point are possible. To set this
option, click 
on the toolbar to open the
Marking
Preferences dialog for this command, then select the
Centroid page.
Examples of Distance & Angle Measurement
clarifies the effect of centroiding the starting and ending points
of the measurement. In the examples, the same starting and ending
points were located with the mouse but some of them were
automatically centroided by Mira.
Related Topics
Measuring Images, Report Windows, Image Windows, Image Display
Orientation, Pixel Coordinate Definition, Sub Pixel
Coordinate Definition, World Coordinate System
on the toolbar or
click Distance & Angle in the Measure pull-down
menu.