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:
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?