A.4.5.22 VecField Form

The VecField form () plots a field of arrows on a regular grid corresponding to the aggregated (typically averaged) value of a 2-d vector quantity for the points in each grid cell.

Currently, the X and Y components of the vectors are normalised separately and scaled so that the arrows are shown with components comparable to the dimensions of the grid cells. 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.

The X and Y dimensions of the grid cells (or equivalently histogram bins) can be configured either in terms of the data coordinates or relative to the plot dimensions.

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

Example VecField plot

VecField form configuration panel

The configuration options are:

X Component

Y Component

Components of the 2-d vector values to combine per cell and plot. 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 cell grid.

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.

Combine

How the vectors in each cell are combined to determine the plotted vector in that cell. By default this is Mean, but Sum is also an option.

X Bin Size

Y Bin Size

A scale for the horizontal/vertical extent of of the rectangular bins into which the data is aggregated. There are two ways to specify this. If the left-hand radio button is selected, the adjacent slider will adjust the bin size, which is also affected by the actual width of the plotting window in pixels. Slide the slider left to get narrower bins or right to get wider ones. If the right-hand radio button is selected, you can enter a numeric value giving the actual extent in data units of the dimension. If the axis in question is logarithmic, this value is a factor.

X Bin Phase

Y Bin Phase

Controls where the horizontal/vertical zero point for bins is set. For instance if your X/Y bin size is 1, it controls whether bin boundaries on the X/Y axis are at 0, 1, 2, ... or 0.5, 1.5, 2.5, ... etc. If the slider is at either end of the scale, there will be a bin boundary at X/Y=0 (linear axis) or X/Y=1 (logarithmic axis).

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

X Offset

Y 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 per-cell combined vector values that you can see in the plot, in the form of a table. The table has one row for each plotted vector, with columns giving the central, lower and upper bounds for each the X and Y grid coordinates, as well as a columns giving the aggregated vector X and Y components. 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.