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### A.4.6 Spherical Plot

Spherical plot window

The spherical plot window draws 3-dimensional scatter plots of datasets from one or more tables on spherical polar axes, so it's suitable for displaying the position of coordinates on the sky or some other spherical coordinate system, such as the surface of a planet or the sun. You can display it using the Sphere () button in the Control Window's toolbar.

In most respects this window works like the 3D Plot window, but it uses spherical polar axes rather than Cartesian ones, You have to fill in the dataset selector at the bottom with longitude- and latitude-type coordinates from the table. Selectors are included to indicate the units of those coordinates. If TOPCAT can locate columns in the table which appear to represent Right Ascension and Declination, these will be filled in automatically. If only these two are filled in, then the points will be plotted on the surface of the unit sphere - this is suitable if you just want to inspect the positions of a set of objects in the sky.

If the Radial Coordinates () button is activated, you can optionally fill in a value in the Radial Axis selector as well. In this case points will be plotted in the interior of the sphere, at a distance from the centre given by the value of the radial coordinate.

The 3D space can be rotated by dragging the mouse around on the surface - it will rotate around the centre of the sphere. By default the points are rendered as though the 3D space is filled with a 'fog', so that more distant points appear more washed out - this provides a visual cue which can help to distinguish the depth of plotted points. However, you can turn this off if you want. If there are many points, then you may find that they're not all plotted while a drag-to-rotate gesture is in progress. This is done to cut down on rendering time so that GUI response stays fast. When the drag is finished (i.e. when you release the mouse button) all the points will come back again.

Zooming is also possible. You can zoom in around the centre of the plot so that the viewing window only covers the middle. The easiest way to do this is to use the mouse wheel if you have one - wheel forward to zoom in and backward to zoom out. Alternatively you can do it by dragging on the region to the left of the plot, like the Axis Zoom in some of the 2-d plots. Drag the mouse down to zoom in or up to zoom out on this part of the window. Currently it is only possible to zoom in/out around the centre of the plot. When zoomed you can use the Subset From Visible () toolbar button to define a new Row Subset consisting only of the points which are currently visible. See Appendix A.4.1.6 for more explanation.

Clicking on any of the plotted points will activate it - see Section 8.

The following buttons are available on the toolbar:

Split Window
  Allows the dataset selector to be resized by dragging a separator between it and the plot area. Good for small screens.
Replot
  Redraws the current plot. It is usually not necessary to use this button, since if you change any of the plot characteristics with the controls in this window the plot will be redrawn automatically. However if you have changed the data, e.g. by editing cells in the Data Window, the plot is not automatically redrawn (since this is potentially an expensive operation and you may not require it). Clicking this button redraws the plot taking account of any changes to the table data.
Configure Axes and Title
  Pops up a dialogue to allow manual configuration of axis ranges, axis labels and plot title - see Appendix A.4.1.2. The only configurable axis range is the upper limit of the radial axis.
Export as PDF
  Pops up a dialogue which will write the current plot as a PDF file. In general this is a faithful and high quality rendering of what is displayed in the plot window. However, if plotting is being done using the transparent markers, the markers will be rendered as if they were opaque. Plots with very many points can result in rather large output PDFs.
Export as GIF
  Pops up a dialogue which will output the current plot to a GIF file. The output file is just the same as the plotted image that you see. Resize the plotting window before the export to control the size of the output GIF.
Rescale
  Rescales the axes of the current plot so that it contains all the data points in the currently selected subsets. By default the plot will be scaled like this, but it it may have changed because of changes in the subset selection.
Reorient
  Reorients the axes of the current plot to their default position. This can be useful if you've lost track of where you've rotated the plot to with the mouse. This also resets the zoom level to normal if you've changed it.
Stay Upright
  Toggle button which when selected ensures that the north pole (latitude = +90 degrees) is oriented vertically on the screen at all times. By default this is on.
Grid
  Toggles whether the spherical wire frame bounding the plot is drawn or not.
Show Legend
  Toggles whether a legend showing how each data set is represented is visible to the right of the plot. Initially the legend is shown only if more than one data set is being shown at once.
Fog
  Toggles whether rendering is done as if the space is filled with fog. If this option is selected, distant points will appear more washed out than near ones.
Draw Subset Region
  Allows you to draw a region on the screen defining a new Row Subset. When you have finished drawing it, click this button again to indicate you're done. The subset will include points at all depths in the viewing direction which fall in the region you have drawn. See Appendix A.4.1.6 for more details.
Subset From Visible
  Defines a new Row Subset consisting of only the points which are currently visible on the plotting surface. See Appendix A.4.1.6 for more explanation.

The following additional item is available as a menu item only:

Antialias
  Toggles whether the axes and their annotations are drawn antialiased. Antialiased lines are smoother and generally look more pleasing, especially for text at a sharp angle, but it can slow the rendering down a bit.

The Dataset Toolbar contains the following options:

/ Add/Remove dataset
  Adds/removes tabs in which the data for extra datasets can be entered. See Appendix A.4.1.1.
/ Add/Remove auxiliary axis
  Adds/removes a selector for entering an auxiliary axis to modify point colours etc. See Appendix A.4.1.5.
Toggle point labelling
  Allows text labels to be drawn near plotted points. See Appendix A.4.1.4.
Toggle Radial Coordinates
  When activated, a column selector labelled Radial Axis will be visible below the Longitude and Latitude Axis selectors. This enables you to enter a value for the radial coordinate of each point. If this is disabled, or if it has a blank value, then all the points will have an assumed radial coordinate of unity and will be plotted on the surface of the sphere.
Toggle tangential error bars
  When activated, an additional column selector appears in the dataset panel to the right of the Longitude and Latitude selectors, along with its own unit selector. You can fill this in with an isotropic error value representing the radius of a small circle associated with the selected points, in units of arcsec, arcmin, degrees or radians. This will cause 2-d error bars to be plotted. You can configure their appearance (e.g. crosshairs, ellipses, rectangles, ...) using the style editor in the usual way. See Appendix A.4.1.3 for more information.
Toggle radial error bars
  Switches radial error bars on and off. See Appendix A.4.1.3. This button will only be enabled if the Radial Coordinates are in use.

You have considerable freedom to configure how points are plotted including the shape, colour and transparency of symbols and the representation of errors if used. This works exactly as for the Cartesian 3D plot as described in Appendix A.4.5.1.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@bristol.ac.uk