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I am trying to visualize forest raster in Google Earth Engine (blue=forest, white=no_forest). However, on zoom levels lower than 12 there are pixel artifacts that look like they are following rivers. At zoom level 12 they disappear. It is the same in GEE Javascript API, as well as in Python API using geemap library. I get this also when I export visualization images with 'getThumbURL' method. When I export GeoTiff with 'Export.image.toDrive' everything is as it should be (so the pixels are just visualizing artifacts). What could be the reason for this?

zoom level 10 zoom level 10

zoom level 11 zoom level 11

zoom level 12 zoom level 12

GeoTiff exported with Export.image.toDrive GeoTiff exported with Export.image.toDrive

2 Answers 2

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The problem was, as Jobbo90 suggested, in pyramids. While uploading the original raster, the 'Pyramiding policy' was set as 'MEAN'. However, for categorical data, 'SAMPLE' or 'MODE' is recommended (Pyramiding policy). I uploaded the original image, but this time with the 'SAMPLE' PP, and pixel artifacts disappeared. Also, as the rivers are not related to this (just seemed like there was a pattern of the artifacts following the rivers) I edited the original title.

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  • next time it helps if you provide that information straight away in your question!
    – Jobbo90
    Commented Mar 22, 2022 at 14:23
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    I would have if I knew what was the problem. Thanks!
    – vladobl
    Commented Mar 22, 2022 at 15:50
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The zoom level depends the scale level of pyramids that are being used. For smooth plotting earth engine uses a fixed scale to create these pyramids.

https://developers.google.com/earth-engine/guides/scale

So most likely your data actually contains these 'artefacts' and they are a result of the analysis used to create them in the first place. You'll probably have to clean up the data somehow, for example by using a cubic interpolation, smoothing or something similar.

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  • Thanks for the tip, it pointed me in the right direction. I uploaded the original data again, but this time with the 'sample' pyramiding policy.
    – vladobl
    Commented Mar 22, 2022 at 12:08

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