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Spatial Join Operation cardinality relationships

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07-18-2023 03:00 PM
Craig_Eissler_Iceman
Occasional Contributor III

For a Spatial Join I see there is a One-to-Many (1:M) Join Operation option.

However, my data may really be a Many-to-One (M:1)

Is there a reason that Join Operation option does not exist?
...Seems to me that if I just switch the Inputs I could achieve a M:1, no?

In other words, I guess the 1:M is really bi-directional?

4 Replies
DanPatterson
MVP Esteemed Contributor

one polygon, many points

target the polygon, with keep all target features results in many polygons... all duplicates of the initial one.

manytoone.png

before you start playing around with your data, experiment with a simple case so you don't inadvertantly end up with duplicates that you don't want.

Reversing the direction?  of course you can continue with my simple example to see what happens


... sort of retired...
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Craig_Eissler_Iceman
Occasional Contributor III

Thanks, Dan, yes I realize this. I'm just wondering how to best explain it to my students. I think I'm going to 1:M Join Operation as "bi-directional" and to not take the 1:M term to literal, since it really depends on the inputs -- more specifically, the order of the inputs -- and the output is going to be different; it just depends on what you want to know. ...If there's a more correct or eloquent way of saying all this, please let me know. 

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DanPatterson
MVP Esteemed Contributor

Doing the example would be prudent rather than relying on terms since not all software has, or will, behave the same.  kind of like, there is more than one spatial join type


... sort of retired...
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Craig_Eissler_Iceman
Occasional Contributor III

Oh yes, we're going to do the exercise in the lab. 

I just taught them about all four cardinality scenarios with respect to Attribute Joins, so that's why I want it to be clear now with Spatial Joins ...but I anticipate questions from some students about this difference.

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