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While using the Vector->Data Management Tools->Join Attributes by Location process, and following this great tutorial on spatial joins. (http://www.qgistutorials.com/de/docs/performing_spatial_joins.html) I tried to create the sum of pumping rates from all the well (points) located within a feature (grid polygons). See image:

Each point has a monthly pumping rate

After using the Join attributes by Location feature, I can generate the grid cells with the "sum". The cells are in the correct locations

However, this sum is completely wrong. for most of the cells created, the sum is zero, and in those cells where it is not (6 out of 170) the number generated seems completely random. I repeated with with other functions like mean, max, etc. and still get nonsensical values. Are there requirements for the "receiving" layer?

What could be the source of this error? And how can I solve it within QGIS?

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    Make sure that both the grid and the point layers have the same projection. – ahmadhanb Oct 27 '16 at 8:47
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Just to extend my comment, you have to make sure that both the grid and point layers have the same projection. Spatial reference or projection in GIS in general and QGIS in particular is very important. Seeing both layers overlaying each other correctly does not mean they are in the same location.

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