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When I use one DEM in my study area to delineate watersheds, the results are different from when I mosaic 4 DEM layers adjacent to each other. After the mosaic process, 3 of the basins were merged in one large basin, and 2 other basins got larger extent.

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    $\begingroup$ This is a better fit for GIS Stack Exchange. $\endgroup$ – Spencer May 27 '18 at 23:25
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    $\begingroup$ @Spencer I agree that it would probably get better answers there, but I think it's still on topic here as well. Spirit Garden might want to request a transfer to GIS.SE if they don't get answers here in a few days. $\endgroup$ – Semidiurnal Simon May 28 '18 at 13:24
  • $\begingroup$ What meters do you have in these 4 DEMS ? $\endgroup$ – PROBERT May 28 '18 at 14:19
  • $\begingroup$ the maximum elevation reaches around 235 meters, I noticed that the presence of the sea water body alters the results, but when clipping the mosaic and removing the water body, the basins are back to what they were. $\endgroup$ – Spirit Garden Jun 2 '18 at 14:52
  • $\begingroup$ I'm not clear why this is surprising. You will get edge effects if the basins are truncated by splitting the landscape into separate layers. $\endgroup$ – haresfur Jun 29 '18 at 1:03
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I would recommend looking at automatic stream network delineation and/or known stream locations features, and overlaying those as a check on the final watersheds created through whatever procedure you have. Watersheds can be merged or lumped together, which is fine, you just want to make sure it is consistent with the other information you have and what your goal is. There will always be some variation in the exact extents of the watersheds depending on what algorithm you use, but you should definitely make sure the general areas and drainage patterns are consistent with what you expect. Beyond that, I am not sure there is a correct answer.

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