Wednesday, 15 May 2013

Setting up object based change detection rule set in Ecognition

I have been very busy lately, so apologies. But on the flip side I am writing something that may be useful for many of you. eCognition is pioneer software for doing object based image classification.  Let’s say, you have two images classified, time T1 and time T2 and you want to identify change and no change areas.

Time T1

Time T2

Change/ No Change map to be produced
You would proceed like this:
1)      Copy Map T1 to Map Change detection in lvl T1
2)      Copy lvl T1 above and name as lvl T2
3)      Synchronize Maps T2 with lvl T2 in Map Change Detection.
4)      At lvl T2Ł convert to sub object
5)       Copy lvl T2 in Map Change Detection.above and name as lvl change
6)      Delete classification in lvl change

By doing these steps, you have a level, lvl change where each object is of same size as in the sub objects  in lvl T1 and lvl T2. Now you want to classify objects in lvl change as change and no change. How would you proceed?

Majority of people proceed like this:
1)      Set up a class No change, and look for change in class for each objects  using ‘Existence of sub object”’ in sub objects in lvl T1 and lvl T2 (c.f. figure).  Figure shows set up of rule set for one class only.
Class description when one class only 
2)       Generally classification has many numbers of classes. When adding one more class in the rule, the class description of No change class becomes a bit complex.
Class description when two classes only 
3)      Now imagine you have 20 different classes. Of course you can perform the process by adapting the rule description for No change for another 18 class. But it’s a tedious process and takes time. Further, the same class description cannot be used for another set of classification maps which may have different class names.So what would you do in this case?

4)      Wise, things to do is to take benefit of array handlings capabilities of eCognition, concept of sub-object, super-object and Parent Process Object (PPO).I am showing you a solution that is short and does not need any set up of class description in change and No change class, easily adaptable to other projects.

6)      I leave up to you to try to understand the rule set shown above. If you don’t understand, give me a shout, I will try to explain.

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