Difference Operation

On this page, you will find general help for the ProRaster product family including links to documentation, instructional videos, and training videos.
Previous Topic: Raster Time Sequence Operation
Next Topic: Change Matrix Operation
Back to: ProRaster Scientific Help
Back to: ProRaster Help
Back to: ProRaster
Go to: ProRaster Essential help page, ProRaster Premium help page, ProRaster Scientific help page.
The ProRaster User Guide is available for download as a PDF.

 The Time Differential Operation is elsewhere referred to as the Difference Operation. Prior to version 4.0 it was only accessible via the Multispectral Satellite Product Editor as this was the only mechanism in ProRaster that generated rasters with a temporal dimension. From version 4.0 onwards you can use the Raster Time Sequence Operation to create a virtual raster with a temporal dimension and so this processing operation can now be accessed via the Processing Menu button. 

Given a source raster that contains a temporal dimension and two or more temporal events, the operation generates a virtual raster that computes the difference between data in different events. 

You can compute the difference for all fields and bands in the input raster, or for a selected field and all bands in that field. The operation will process fields of any type, but treats the input data as numeric, regardless of the data type. 

You can compute a moving or fixed difference. A moving difference is the difference between each event and the preceding event. The first event generated in the output raster will correspond to the second event in the input raster. A fixed difference is the difference between each event and a key event that you select. Usually, the key event will be the first event in the sequence. 

The difference operation is executed by calculator operations defined in the output virtual raster. You can edit the calculator expression if you wish. The standard expression is – “Event – KeyEvent”. In this expression, “Event” and “KeyEvent” are variables that refer to the current event and the key event (or preceding event). These variable names cannot be changed, but you can change the expression. For example, you can reverse the sign of the difference by using – “KeyEvent – Event”. You can compute the absolute value using – “abs(Event – KeyEvent)”. 

You can introduce a conditional expression to output only differential values of interest – for example this expression only outputs difference values that are greater than 0.1 – 

“cond(Event–KeyEvent>0.1,Event–KeyEvent,null)” 

For example, you can use this technique to target certain classes in a classified raster. You can even target imagery by using calculator functions to decompose the RGBA color values into color components and operating on those separately.