I am trying to weghted overlay in Google Earth Engine I want to overlay PDSI, SPI and SMI applying weights for each parameter.
This is how I am going to overlay them:
var overlay = spi.add(smi.add(pdsi))
How do I apply weighted overlay?
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1Can you specify your question please: How exactly do you want to apply weighted overlay? Using one fixed weight per layer? Do you need to re-organize your layers into categories, and if yes how? Is this a conceptual example of want you want to do, and how would you modify it to your specific application?– malin-fischerCommented Aug 8, 2021 at 13:28
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I do this based on AHP. Each layer has a specific weight. I want to apply weight to each parameter then overlay together to create a new image. ex: SPI * 0.25, PDSI*0.14– Chathu_TCommented Aug 9, 2021 at 5:38
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1 Answer
You can use GEE's ee.Image.expression() function for more complex mathematical operations like both addition and multiplication as in your case (more info e.g. here and here). As it works on an ee.Image, you need to combine your layers into one at first using ee.Image.addBands().
Example code (factors need to be adjsted accordingly):
var image = spi.addBands(smi).addBands(pdsi)
var overlay = image.expression(
'2 * spi + 5 * smi + 1.5 * pdsi', {
'SPI': image.select('spi'),
'SMI': image.select('smi'),
'PDSI': image.select('pdsi')
});
