Spatial Join Multiple Polygons joined to a Point

Discussion created by sunelles on Sep 17, 2013
Latest reply on Sep 18, 2013 by xander_bakker

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Hi,

I am busy spatially joining the attributes from a polygon layer (regional ecosystem information) to a point layer (scat survey sites). When I complete the spatial join (join features based on polygons which the points lie completely within) I find that the attributes from multiple polygons are joined to each of the points when the point only lies within one polygon (e.g. point 1 = vegtype1(wetland) AND vegtype2(non-remnant)). I suspect this may be due to the fact that the layers have different coordinate systems. I have attempted to project the layers in one coordinate system and reduce the XY tolerance however these changes don't seem to help. Does anyone have any suggestions.

Sunelle :D

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xander_bakker Sep 17, 2013 11:51 PM
Hi,

I am busy spatially joining the attributes from a polygon layer (regional ecosystem information) to a point layer (scat survey sites). When I complete the spatial join (join features based on polygons which the points lie completely within) I find that the attributes from multiple polygons are joined to each of the points when the point only lies within one polygon (e.g. point 1 = vegtype1(wetland) AND vegtype2(non-remnant)). I suspect this may be due to the fact that the layers have different coordinate systems. I have attempted to project the layers in one coordinate system and reduce the XY tolerance however these changes don't seem to help. Does anyone have any suggestions.

Sunelle :D

Hi Sunelle,

Assuming it isn't the result of differences in projections (and it is always wise to use the same projection when combining spatial data), there can be other reasons. Let's have a look at the help:
http://resources.arcgis.com/en/help/main/10.2/index.html#//00080000000q000000

I understand that the number of points in your output point feature class is higher than the number of points in your input featureclass. Is that correct?

If so, then I assume that during the spatial join your "join_operation" is set to "JOIN_ONE_TO_MANY". From the 10.2 Help:

If multiple join features are found that have the same spatial relationship with a single target feature, the output feature class will contain multiple copies (records) of the target feature. For example, if a single point target feature is found within two separate polygon join features, the output feature class will contain two copies of the target feature: one record with the attributes of one polygon, and another record with the attributes of the other polygon.

If you look at an input point that is found multiple times in your output and you perform an identify on your polygons, how many polygons are found at that location? If this is more than 1, your polygon feature class may contains regions (overlapping polygons).

Kind regards,

Xander
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sunelles Sep 18, 2013 3:45 PM
Hi Sunelle,

Assuming it isn't the result of differences in projections (and it is always wise to use the same projection when combining spatial data), there can be other reasons. Let's have a look at the help:
http://resources.arcgis.com/en/help/main/10.2/index.html#//00080000000q000000

I understand that the number of points in your output point feature class is higher than the number of points in your input featureclass. Is that correct?

If so, then I assume that during the spatial join your "join_operation" is set to "JOIN_ONE_TO_MANY". From the 10.2 Help:

If multiple join features are found that have the same spatial relationship with a single target feature, the output feature class will contain multiple copies (records) of the target feature. For example, if a single point target feature is found within two separate polygon join features, the output feature class will contain two copies of the target feature: one record with the attributes of one polygon, and another record with the attributes of the other polygon.

If you look at an input point that is found multiple times in your output and you perform an identify on your polygons, how many polygons are found at that location? If this is more than 1, your polygon feature class may contains regions (overlapping polygons).

Kind regards,

Xander

Hi Zander,

The number of points in my output point feature class in the same as my input point feature class (My join operation is set to JOIN_ONE_TO_ONE). As such, I believe it must be due to inconsistencies in projections. I have attempted to project all my layers in the same coordinate system but this doesn't seem to help. I have also changed the two layers I am joining to the same projection but the same problem occurs. The projections contained within my layers (prior to attempting projection in GDA 1994 MGA Zone 56) are as follows:

GDA 1994 MGA Zone 56
GCS Australian 1984 (Polygon and Point layer)
WGS 1984 ARC System Zone 10
GDA 1994 Australian Albers
GCS GDA 1994
GCS WGS 1984
GDA94 - Datum (EPSG ID 6283)

I have also tried to change the data frame properties to match the projection of the polygon and point layers however the same problem arises. I have checked the original attributes within the polygon layer as well to make sure that polygons only have one vegetation value assigned. Any ideas? The point should only have the attributes of the blue polygon joined, but somehow the pink polygon is also being joined.

[ATTACH=CONFIG]27569[/ATTACH]

Thanks,
Sunelle
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- ArcGIS Polygon Problem.png13.6 KB
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xander_bakker Sep 18, 2013 10:00 PM

The number of points in my output point feature class in the same as my input point feature class (My join operation is set to JOIN_ONE_TO_ONE).
...
I have checked the original attributes within the polygon layer as well to make sure that polygons only have one vegetation value assigned.

Hi Sunelle,

What I don't understand is that there are the same amount of points and a single point may have two vegetation types joined to it, while the polygon layer only defines one vegetation type. So there are more vegetation type columns in the resulting point feature class?
The only reason I can imagine for this to happen it that you joined the vegetation polygon layer twice to the points, once before and once after changing something related to the projection...

Could you post a small part of both layers to see if this can be reproduced?

Kind regards,

Xander
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