Next Previous Up Contents
Next: SkyDensity Form
Up: Plot Forms
Previous: Contour Form

A.4.5.16 Grid Form

The Grid form () plots 2-d point data aggregated into rectangular cells on the plotting plane. You can optionally use a weighting for the points, and you can configure how the values are combined to produce the output pixel values (colours). You can use this layer type in various ways, including as a 2-d histogram or weighted density map, or to plot gridded data.

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

The shading is done using the shared colour map. This colour map is used by all currently visible Grid, Aux and Weighted layers. When at least one such layer is being plotted, the Aux Axis control is visible in the control panel, which allows you to configure the colour map, range, ramp display etc.

Example Grid plot

Example Grid plot

Grid form configuration panel

Grid form configuration panel

The configuration options are:

Weight
The weight value applied to each plotted point. Fill this in with a column name or expression from the table just like for a positional coordinate. The exact way this quantity is used depends on the setting of the Combine control below. If it's left blank, the weighting is considered to be unity (all values are 1); this makes sense for some combination types (e.g. sum) but not others (e.g. mean).
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.
Combine
Determines how the Weight values for the points falling within a given data bin are combined to produce the numeric value used for that bin's colour. For unweighted values (a pure density map) it usually makes sense to use count or sum. However, if there is a non-blank Weight coordinate, one of the other values such as mean may be more revealing.

The following options (some are more useful than others) are currently available:

Transparency
Adjusts the transparency of the filled area.
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).


Next Previous Up Contents
Next: SkyDensity Form
Up: Plot Forms
Previous: Contour Form

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