GNU Astronomy Utilities

6.5.2.3 Checking grid values

Programs that use the mesh grid to calculate some value, also have checking options (that start with --check). When such options are called, the program will create a specific output file depending on what you want to check. When the operation is done on the meshes, this file shows the values assigned to each mesh grid element. It has three extensions:

1. The value that was initially calculated for each grid element. If a mesh was not successful in providing a value, a NaN value is stored for that mesh.
2. The interpolated values, see Grid interpolation and smoothing.
3. The smoothed values, see Grid interpolation and smoothing.

If more than one value is calculated for each mesh (for example the mean and its standard deviation), then each step has that many extensions. By default, these check image outputs have the same size (pixels) as the input image. So all the pixels within a mesh are given the same value. This is useful if you want to later apply these values to the image through another program for example. There is a one to one correspondence between the input image pixels and the grid showing you the mesh values for that pixel.

In other situations, the input pixel resolution might not be important for you and you just want to see the relative mesh values over the image. In such cases, you can call the --meshbasedcheck option so the check image only has one pixel for each mesh. This image will only be as big as the full mesh grid and there will be no world coordinate system. When the input images are really large, this can make a difference in both the processing of the programs and in viewing the images.

Another case when only one pixel for each mesh will be useful is when you might want to display the mesh values in a document and you don’t want the volume of the document (in bytes) to get too high. For example if the mesh sizes are 30 pixels by 30 pixels, then the mesh grid created through this option will take \(30\times30=900\) times less space! You can resize the standard output, but the borders between the meshs will be blurred. The best case is to call that program with this option.

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GNU Astronomy Utilities 0.2.28-34fb manual, October 2016.