The arithmetic operations allows the user to perform operations on one or more images of the same geographical area. In addition to the operations that can be made between the bands of the images, it is also possible to apply gain and offset to the images.
The operation is pixel-based using the defined mathematical rule (sum, subtraction, division and multiplication for a constant (linear enhancement), resulting in a band representing a combination of original bands.
These operations allow data compression reducing the number of bands but in other hand there is original information loss. The operation results can be over range 0-255 and those results will be automatically normalized thus saturating values below 0 and over 255 respectively, causing spectral information loss.
These operations may require a gain (multiply) or offset to improve image contrast quality. The value definition depends on user skills, in operation definition among bands and spectral characteristics of used bands.
In general, sum operation is used to improve similarities between bands or different images while subtraction, multiplication and division is used to improve spectral differences.
Some standard operations are defined in the interface:
Gain x A + Offset
Gain x (A + B) + Offset
Gain x (A - B) + Offset
Gain x (A x B) + Offset
Gain x (A / B) + Offset
(Gain x (A - B) / (A + B)) + Offset
(2 x A + 1 - (((2 x A + 1) x (2 x A +1)) – 8 x (A – B)))/2
Customized
Select PROCESSING → RASTER PROCESSING → ARITHMETIC OPERATION in the main menu.
To use predefined
arithmetic operations click on the icon
.
Set the layer and correspondent bands and enter the gain and offset values.
Click on the OK button. The complete operation formula will be presented in the field below.
Check in Normalize box and the values of the resulting image will be normalized.
Repository -
Select one type of repository by
clicking on the
button to save the output layer as a file or on the
button to save it in the database.
Click on the OK button to start the operation.
The user can create customized arithmetic operations using layer selection and the calculator operations.
Click on the icon
an in the List of Layers select the raster layers to apply the
operation.
Choose the band to
operate and click on icon
.
Write the desired operation using the calculator keyboard.
For future use the
customized operation can be save by clicking in the icon
.
Note: For asin, acos and atan operation the values must be between -1 and 1.