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I'm beginner with QGIS and I have task currently to make some GIS map/data.

I downloaded 3 satellite images from Landsat by USGS (multispectral, georeferenced) and I did overlapping of images using merge tool. I checked with x - no data value and load into canvas when finished. When I used merging tool I've got situation as you can see by the picture.

enter image description here

https://i.sstatic.net/FTUae.jpg

Images has been overlapped perfectly but with strange black lines on images edges (where overlapping was made). Does anybody can help me regarding this issue? Have in mind that i'm a beginner :)

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  • What does it look like if you just open the three satellite images in QGIS without merging the images to one?
    – Jakob
    Dec 11, 2014 at 8:10
  • Did you also try to build a virtual raster (Raster->Miscellaneous->Build Virtual Raster)?
    – MartinMap
    Dec 11, 2014 at 8:15
  • @Jakob They are overlapped but not joined(one image is in a front a two in the back). Dec 11, 2014 at 14:08
  • @MAP I tryed but again I have the same problem. Dec 11, 2014 at 14:09
  • Per Jakob's comment, does it look like it should if they're just stacked and not merged? It appears there may be some overlap that isn't just no-data between the images and an incorrect/inappropriate method is handling that overlap. Can you post a screenshot of the tool dialog you are using? I'm not sure if there is anything like ArcGIS's Mosaic to New Raster does, which would allow you say to blend the two or take the top value for example.
    – Chris W
    Dec 11, 2014 at 22:44

1 Answer 1

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Looks like you downloaded the JPEG "LandsatLook images with Geographic Reference" not the "Level 1 GeoTIFF Data Product". JPEGs use lossy compression which will change the values slightly so the zero values at the edge of the scene aren't exactly 0 anymore.

You can use GDAL nearblack (QGIS Raster - Analysis - Near black) to convert them back to 0 (output to a uncompressed/lossless compressed geotiff/img), then use gdalbuildvrt (using QGIS Raster->Misc.->Build Virtual Raster Catalog) to build a VRT mosaic instead of merge.

Near black dialog

enter image description here

VRT without using near black

enter image description here

VRT after using near black

enter image description here

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  • Yes Luke, I downloaded exactly what you said. I tried to use nearblack on many ways, checking and unchecking of all boxes but without luck (Raster - Analysis - Near black). I see that you have a solution for my problem but as I'm noob with QGIS please give me more detail info how can I do it (which boxes i have to check with merge and near black tool) because maybe I'm doing something wrong. Tnx! Dec 12, 2014 at 8:20
  • @Dragan see my edit
    – user2856
    Dec 13, 2014 at 7:39
  • Your precise description solved my problem with black edges! Thank you Luke Cheers! Dec 15, 2014 at 9:15

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