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I have the following problem: I am currently working with Landsat 5 imagery from the tropics including quiet high cloud cover. I have classified raster images with the different land use categories I need and "Clouds" as well as "Cloud shadow" as categories where there appear on the image. I now extracted the "Clouds" and "Cloud shadow" parts of my classified image (Extraction toolset --> Extract by Attributes) and converted these to polygons. Now I have a layer with polygons for every cloud and every shadow of my classified image. I was hoping that I can now take a different classified image from the same year with less clouds in the areas where my original image has a lot of clouds and extract the data from this new classified image that falls within the polygons that I created. Then I wanted to take this new data and "overlay" my original classified image with the extracted data that falls within the area that is covered by clouds. The result would be that I replaced the areas covered by clouds with the data from the other classified image of the exact same area. However I can not find any tools to do this overlay-like operation. If someone knows how to extract data from a classified Raster image using polygon files (similar to a mask) and how to replace the data from the original classified raster image with the newly obtained data I would highly appreciate your help.

Kind regards,

Markus

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What software are you using? I believe that every raster-based GIS has a tool that directly does what you describe; it is usually called a conditional operator. –  whuber May 28 '13 at 15:21
    
I use Arcgis 10 –  Markus May 29 '13 at 13:24
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marked as duplicate by whuber May 29 '13 at 13:30

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1 Answer

One approach you can take is to use the the polygons you have (cloud and cloud shadow) and use it to cut through your original image.

So your original image

enter image description here

minus the cloudy area

enter image description here

Should leave you with the original image but with holes where the clouds are supposed to be.

enter image description here

In QGIS, it's called the Difference Tool. There should be something similar in ArcGIS.

Using the same polygons, you find an image that has no clouds in those areas and then use the polygons to clip that image.

Assuming this is your second cloud free image from a different time

enter image description here

You use the cloud polygon to clip it which should leave you with cloud free areas that occupy the polygon regions.

enter image description here

Most GIS packages have a Clip Tool that should let you accomplish this.

One you have those two areas, you can now combine them. You should be able to add them together using a raster calculator. QGIS and ArcGIS have this. Other packages would most probably have this too.

You should end up with a cloud free image that is a combination of your two layers.

enter image description here

This is rather simplistic but hopefully it should point you in the right direction. Good luck!

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Thanks a lot, I have not thought about this Approach, using the raster calculator should be a good idea. However I can not find a tool that allows me to cut my Polygons into the Image. I am using Arcgis 10 and it seems like there are a lot of Tools, like mask for example to obtain data from the inside of my Polygons but not to Keep the outside. If anybody has a suggestion it would be highly appreciated. –  Markus May 29 '13 at 11:35
    
I think it's called Erase in ArcGIS. Good luck! help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//… –  R.K. May 29 '13 at 12:16
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