mpatte05

Spatial join of lines to closest points: discrepency in points total and count

Discussion created by mpatte05 on Jan 24, 2012
I am a graduate student at Virginia Tech Department of Forest Resources and Environmental Conservation.  I am using ArcGIS to model sample street tree inventory procedures.  The input data in my project consists of two layers: a points layer containing GPS locations and attribute data for all of the street trees in a city of interest and a TIGER/Line shapefile of all of the streets. 

In order to simulate street tree sampling, I have to generate a count of the number of tree points along each street lines.  To accomplish this I have been right clicking on my street lines layer and selecting join.  I select to join the data so that "Each line will be given a summary of the numeric attributes of the points that are closest to it.  

I have run into a bit of a snag when spatially joining a lines layer to a points layer.  In some instances, particularly when dealing with larger street tree population (>5,000) trees, the total for the tree count column in the joined attribute table is slightly greater (never less) than the number of trees in the points shapefile.  For instance, when I spatially joined 40,321 street trees to a population of street lines and total the tree count in the resulting attribute was 40,345 trees. 

I suspect that when a large population of points are joined to the closest line, a handful of points are equidistand from more than one closest streets and are subsequently joined to more than one line.  However, this is very difficult to determine manually because the only information about the points along each street line is a count.  I don't know if any type of rounding is used in determining how far a point is from nearby lines, but suspect that if rounding is used the possiblity of points being equidistand from more than one line would be increasingly likely.  Any insight into this aspect of my difficulties would be helpful.

Has anyone out there run into this type of problem before?  If so, any explanation you can offer up will be greatly appreciated and go a long way in reducing the stress of reconciling this discrepency.

Thanks very much for any time and consideration you give to my dilemna. 

Mason

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