Distance and Angle

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.

• To activate the Distance and Angle measurement, click on the Image Toolbar or use Measure > Distance & Angle on the pull-down menu.

Procedure

First, activate the Distance & Angle command. [more] Marking mode is automatically enabled when the toolbar is opened. If marking mode has been disabled by prior commands, activate it by clicking the button). Then:

1. Be sure the drawing and centroiding preferences are set as desired. [more] To set these preferences, click to set open the Marker Preferences dialog,

2. Move the mouse pointer to the starting position. Press the primary mouse button and hold it down.

3. Drag the mouse pointer to the ending position and release the mouse button. The result is listed in the Distance Measurements Report window.

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.

If the image has a World Coordinate System ("WCS") calibration, then X1, X2, Y1, and Y2 give the WCS values corresponding to listed column,row coordinates.

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

Marking 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 Marker 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