I'm fairly new and entirely self-taught in using QGIS 3.4. I'm looking at turtle nest sites and trying to run a nearest neighbor analysis to analyze their distribution and establish whether or not they are clumped, dispersed or random. I'm doing this for a number of different years to determine whether nest sites are becoming more clumped as a result of environmental changes.

However, i'm not sure i understand the results that are being output. the z scores are just astronomical and the expected values are effectively 0s. How is QGIS establishing expected distances?

Maybe i'm just doing something fundamentally wrong, but i'm selecting the vector analysis -> Nearest Neighbor Analysis -> and selecting my vector layer containing the nest site points. These are some of the outputs I'm getting for different years:

Observed mean distance: 5.451281160546463 Expected mean distance: 0.00026372940677887315 Nearest neighbour index: 20669.97847197658 Number of points: 125 Z-Score: 442084.1070632401

Observed mean distance: 5.40372033661974 Expected mean distance: 0.00019992349282925835 Nearest neighbour index: 27028.941222203965 Number of points: 134 Z-Score: 598544.2290450602

Maybe i'm just doing something fundamentally wrong? Or maybe theres just a better way of achieving what i'm trying to do. I'm not sure.

  • 1
    It sounds like something is wrong (EPSG?). I don't have the explaination, but, may be, this link will help you (for ArcGis, but you have somes similitudes): pro.arcgis.com/en/pro-app/tool-reference/spatial-statistics/…. In particular, it is write: "If an Area parameter value is not specified, then the area of the minimum enclosing rectangle around the input features is used. Unlike the extent, a minimum enclosing rectangle will not necessarily align with the x- and y-axes." As i see, you can't choose the area in QGIS. So may be with extra points? – Christophe Nov 16 '18 at 10:03
  • Thanks for the help! I'll look into it! I was wondering if it had something to do with my CRS, but i don't really know enough about it to know or to attempt to fix it. I'll report back if i get this figured out. – Dan.L Nov 16 '18 at 13:11
  • This link, especially the definition of "A", may be will help you: github.com/alexgleith/Quantum-GIS/blob/master/python/plugins/… – Christophe Nov 20 '18 at 11:32

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