Compute the Ratio of Two Grids

Use the Compute the Ratio of Two Grids dialog (GRIDRAT GX) to compute the ratio of two grids.

Compute the Ratio of Two Grids dialog options

Numerator grid	Numerator grid (.grd file) Script Parameter: GRIDRAT.NUM
Denominator grid	Denominator grid (.grd file) Script Parameter: GRIDRAT.DEN
Output (Numerator/Denominator) grid	Output (Numerator/Denominator) grid (.grd file) Script Parameter: GRIDRAT.RAT
Constant to add to denominator grid	Constant to add to denominator Script Parameter: GRIDRAT.ADD
Max. value to allow in output grid	Maximum value to allow in output grid Script Parameter: GRIDRAT.MAX
Min. value to allow in output grid	Minimum value to allow in output grid Script Parameter: GRIDRAT.MIN

Application Notes

Division of the grids occurs grid point by grid point.

A typical application of grid division is to calculate the ratio of the concentrations of two elements as measured by a radiometric or geochemical survey.

Noise is likely to be a problem in ratio grids. In areas where the values in the denominator grid are low, then the signal-to-noise ratio is also likely to be low, and so the ratio value will be unreliable. (This is particularly true of radiometric data since it is a statistical measurement of random radioactive decays.) Areas of weak signal can produce large noise spikes in the ratio grid. One way to prevent this problem is to add a small constant value to the denominator grid before computing the ratio. This leaves the ratio unchanged in high-signal areas but reduces the ratio values in low-signal areas.

Another way of suppressing noise spikes due to low values in the denominator is to limit the output values at a given maximum level.

If one of the input values is less than zero then the ratio will also be negative. To prevent negative noise spikes you can set a minimum limit value. To reject all negative ratio values, set the minimum to zero.

If the denominator grid does not match the numerator grid in coordinate system, grid size or cell size, it will be sampled to match the numerator grid and the result grid will be spatially equivalent to numerator grid.