I'm pretty new to prof. GIS, but have OSM experience. So if I just miss something obvious, please don't punish me :)

I have a .CSV file with measurements of wifis along a track. I like to guess the wifi accesspoint positions by using the signal levels during the tracking. Adding the locations works fine (coloured circle markers), but applying the mean coordinates feature to the point layer results only in the usual centeroid (blue star) of the convex shape polygon(soft yellow) and seems to take not the signal level into account:

no mean coordinates

But I expect that the new position should be usual more close to the dark blue circles in the south.

So my CSV structure looks like

trackpoint_id, lat, lon, ..., signal
1, 54... , 12... ,..., -90.0
2  54... , 12... ,..., -74.0

The signal col is recognized as double. As signal is in dbm so negative (smallest negative number is best) and logarithmic scale, I might need to create a new column containing a cleared signal_strength? Sadly the log doesn't tell me if the mean coordinates tool has problems with computation :(

1 Answer 1


Did you try Vector -> Analysis Tools -> Mean coordinate(s)? You can specify a weight field for this tool.

OK, I guess you did use that tool. If so, it seems you really need to transform those logarithmic values. Mean coordinate(s) tool seems to work with linear values (the bigger it gets, the more it influences the mean position of the whole dataset).

  • 3
    Ok so I had to do it like that. I used the editmode and the field calculator to get a new field "signal_mw" that was generated by this formular: 10 ^ ("signal_dbm" / 10). Even if specified as double, you need to adjust length:20 and precision:10 as QGis doesn't warn you and otherwise zeros your results. Works now as expected!
    – Mapper
    Commented Nov 16, 2013 at 20:37

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