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#### A.4.5.20 SkyVecField Form

The SkyVecField form () plots a field of arrows on a HEALPix grid corresponding to the aggregated (typically averaged) value of a 2-d vector quantity such as proper motion for the points in each HEALPix cell.

Currently, the visible length of the vectors is dependent on the order of the grid; they are normalised to appear with a length comparable to the size of the HEALPix pixels. This length can be adjusted by use of the Scale control.

By default, the absolute values of the vectors are plotted. However, if the Relative option is set, the mean vector value averaged over the visible plot area will be calculated and subtracted from all the plotted vectors. The effect of this is to show local variations rather than absolute values of the vectors over the visible area. The value of such mean offsets is displayed in the Report panel at the bottom of the layer control panel.

This form may be used in the Sky plot window. The VecField form does the same thing for a Plane plot.

Example SkyVecField plot

SkyVecField form configuration panel

The configuration options are:

Lon(*) Component
Lat Component
  Longitude and Latitude Components of the vector values to combine per cell and plot. The longitude value is considered to be pre-multiplied by cos(lat). Since these values are normalised before plotting, units are not significant.
Weight
  An optional weighting to apply to each vector.
Color
  Colour of the arrows.
Arrow
  Arrow shape selected from a range of options.
Thickness
  Controls the line thickness used when drawing arrows. Zero, the default value, means a 1-pixel-wide line is used, and larger values make drawn lines thicker.
Scale
  Changes the factor by which all vector lengths are scaled. If the arrows are too small, slide it right, if they are too big, slide it left. The slider scale is logarithmic. Alternatively, enter a fixed value in the text field. The default value of unity corresponds to vectors that are a sensible size for the current HEALPix grid.
Healpix Level
  This allows you to control the resolution of the HEALPix grid onto which the weighted values are resampled. According to the radio buttons, you can configure this using either a Absolute or Relative value. In Absolute mode you specify the HEALPix level (k) directly; the number of pixels on the sky is 12*22k. In Relative mode the level is set so that a HEALPix tile has approximately the given number of screen pixels along a side, and hence the absolute level will change if you zoom in and out. In either case, you can see the absolute level at the right hand side of the control.
Combine
  How the vectors in each HEALPix cell are combined to determine the plotted vector in that cell. By default this is Mean, but Sum is also an option.
Relative
  If true, the mean value for all the vectors in the field of view is calculated and subtracted from each displayed vector. The effect of this is to show local variations rather than absolute values of the vectors over the visible area. The value of such mean offsets is displayed in the Report panel below the configuration controls.

The Report gives you access to the calculations made during plotting:

Lon(*) Offset
Lat Offset
  If the Relative option is selected, these quantities report the field of view averaged values that have been subtracted from per-cell values before plotting.
Vector Map
  This allows you to export the vectors that you can see in the plot in the form of a table. The current implementation writes the table for the whole sky, not just the cells that are visible in the current plot. The table has a row for each healpix pixel; depending on how sparse the data is the HEALPix index might be implicit (corresponding to row index) or explicitly given as a column value. There are also columns giving the aggregated longitude and latitude values for each cell. The Import () option loads the data as a new table in the TOPCAT application, and the Save () option lets you save it directly to disk in one of the available table formats. These options are also available from the Export menu.

  Some notes apply for this export:

  1. If you want to export the table following the HEALPix-FITS convention in a form suitable for use in other applications such as Aladin, you should save it using the output format fits-healpix rather than one of the other FITS variants.
  2. When exported as FITS, the sky coordinate system (FITS `COORDSYS` header) is determined by the current value of the View Sky System, as reported in the Sky Axes Control.
  3. The HEALPix level at which the data is exported is that currently plotted as reported by the HEALPix Level report item above, not necessarily the one selected in the style configuration controls.

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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@jiscmail.ac.uk