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What does "Precision" do within "Join Attributes by Location"?

Also, what's the difference between the "Geometric Predicates"?

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    Precision is a broken option - it's been removed in version 3.0. I'd just ignore it! – ndawson Jan 23 '18 at 21:19
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It's easier to explain them the other way around.

Geometric predicate: this is the topologic rules you wish to use to join features. If you only want to join features that intersect with one another, you check only the "Intersects" box. If you want to join features that either crosses or touches another, you check both these boxes. You can read more about how topologic relations work here.

Precision: this is how precise you want your topologic rules to evaluate. Let's say you have a Linestring that ends at the point (230989.435 8900443.366), and a Linestring close to it starts at the point (230989.436 8900443.362). These two geometries are very, very close to one another (millimeters close), but do not exactly touch or intersect. If you want these cases to be treated as if they were in fact touching, then you set your precision for a value above this gap (say, 1cm). It's basically how close you want the software to consider that it's basically the same position. This is a unitless value (or, rather, it's in the units you are working with in your project CRS).

  • See my note above - the precision option is actually broken, and is dangerous to use since it makes ANY results unreliable. It's been dropped from QGIS 3.0 as a result. – ndawson Jan 23 '18 at 23:12

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