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I have a few thousand points measured with totalstation from an archaeological site. The points are measured quite unevenly which makes TIN triangulation and subsequent DEM generation a mess. Some areas has a lot of tightly measured points, others almost none.

I have created a 1m2 grid overlaying the points containing XYZ.

What I would like to achieve: Display one point for each 1m2 and that this one is the one with the lowest Z elevation value.

PS: Yes, the Z-values are negative, because the excavation was conducted on old seabed.

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    Not exactly what you are trying to do, but you may be interested in "Points Filter" function, which is in QGIS Processing Toolbox | SAGA | Vector point tools | Points filter. This tool takes Point shapefile and its attribute (your Z) as input, and returns thinner point set. In this case you can filter it by keep minima option. Problem is, it takes points within a circle of defined radius, not the grid (square) as you have indicated. Just thought it may worth a try. – Kazuhito Jan 23 '17 at 12:58
  • If what you are really trying to go is create an interpolated grid of elevation over the whole site then this isn't the right way to go about it... – Spacedman Jan 23 '17 at 14:27
  • Thanks @Kazuhito it definitely thinned out the points in a reasonable way. – eyesore Jan 23 '17 at 14:29
  • @Spacedman Do you have any suggestions? – eyesore Jan 23 '17 at 14:30
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    anything from inverse distance weighting to kriging. These techniques draw strength from having lots of nearby points as that gives a better estimate of the elevation there, and also kriging lets you get a measure of the uncertainty on the elevation estimate of your predictions. Try not to use a particular tool (like generating a TIN) just because that's what your software has... – Spacedman Jan 23 '17 at 15:05

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