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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | RE: GISList: Spatial Join of sorts |
| Date: |
06/24/2003 05:50:00 PM |
| From: |
Dimitri Rotow |
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>
> I have seen posts about spatial joins before I believe. I have a point
> file with a attribute in it I need. I have a polygon file. One point
> lies within one polygon. I would like to transfer one attribute field
> from the point file to the polygon file but I have nothing other than
> its spatial location in common. I do not want to create an output
> shapefile as the built in spatial join function does. I want to append
> the attributes to the existing polygon file. Does anyone know of a
> method or script that allows me to do this in one quick swoop.
> Steve
>
>
Sure, that's trivially easy. Use the Spatial Overlay command and it does it
in one step.
Depending on how sure you are that there is one and only one point within
each area, you might want to think about what Transfer Rule you would like
to use for the columns you want to transfer. For example, if more than one
point occurs in each area you potentially have an N to 1 transfer and you
should choose how you want that done. That is, the data from N points is
being transferred to 1 area so you need to decide whether you want to take,
say, the average of values from the points and put it into the area, to take
the sum of the values and put it into the area or simply just take one of
the values and put it into the area. Which rule you choose depends on what
you are doing.
For example, if your points represents cities and each has a population
field and you want to find the total population in each area, you'd pick
"sum" as your N to 1 transfer rule. If the field is something like number
of days of sunshine for each city, you'd pick "average" as your rule if what
you are looking to do is get the number of days of sunshine for the area
that contains the cities. There are lots of rules possible, Copy, Count,
Average, Maximum, Minimum, Median, Sample and Sum, for the different
situations people want to handle. "Count" for example, is handy when what
you want is to color an area by how many customers (points) you have in it
so in a single step you can transfer the number of customer points into a,
say, "number of customers" field.
Cheers,
Dimitri
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