Contour Maps

Selecting Make Contour Map from the Edit menu brings up a dialog box which allows one to specify how the contour map will be created for the current image in the current graph (indicated by the green bounding box)...

Image Range: Minimum and maximum pixel values of the selected image.

Contour Range: Minimum and maximum values to contour over. The values are initially taken from a "clean" approximation of the min and max pixel values of the image, but the user can change these values to whatever is desired. (Note that the min value need not be less than the max value.)

# Contours: Number of contour levels to produce between the range points. One can either click on the < or > buttons to decrease/increase the number or type in a new number directly.

Scale: Scaling method for interpolating contour levels. The linear method will space the contours over a constant interval from the min and max range. The other two options -- sqrt and log -- do much the same thing but based on the square root or logarithm of the min and max values. These latter options bring out the finer details at the lower contour levels in images with a large dynamic range.

Contours: This is the actual list of contours to draw. The previous contour items update this list each time they are clicked, but the list may be customized prior to clicking the Make Contours button.

Resolution: This determines how detailed (and accurate) the contours will be. At high resolution, contours will be produced based on the value from each pixel of the image. Low resolution will produce contours using the average pixel value in 3x3 sections, resulting in a lot few points to be plotted and, therefore, faster performance. Medium resolution uses 2x2 sections. Using high resolution when the contour intervals are similar in size to the image noise will create an enormous number of points, significantly slowing down POW.

Separate Graph: If this box is checked, when the contours are created, the contours will be placed in a separate graph by itself. Otherwise, the contours will be placed on top of the current graph along with the original image.

Click on the Make Contours button to build and draw the contours. After a set of contours is created, its parameters can still be modified in this dialog box and the contours recreated (replacing the previous version) with the Make Contours button. Contours are created as a POW curve, so one can control its display features (color, line style, etc) from the Edit Graphs dialog box.

If the original image contains WCS information, the contours will be created in terms of celestial coordinates. (Without WCS information, contours will be drawn in pixel coordinates.) As a result, one can place the contours made from one image into a graph containing another image (with its own, possibly different, WCS information) and the contours will be positioned appropriately for the new graph. This can be used to compare observations of the same object at different wavelengths (eg, radio and optical).