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Incomprehensible Shape_Length and Shape_Area Field Values

Question asked by joincto on Apr 7, 2016
Latest reply on Apr 7, 2016 by Dan_Patterson

I notice that on many geoprocessing operations (eg joining polygons to points), and with certain geoprocessing tools (eg Multiple Ring Buffer) then the created datasets have two new fields in them (Shape_Area, and Shape_Length)

 

My questions are two:

 

1. Why are these fields automatically created and populated when geoprocessing? I neither need nor want them.

    They just clutter up my data.

2. And,more important, the values for these fields do not always make sense

    (actually I wonder if they ever make sense: I have yet to find an example when they do)

 

Let me give you a simple example of item#2. I use the "Multiple Ring Buffer" tool, and create a single circular polygon

of radius one mile. One would think, therefore, that the Shape_Area would come out as pi sq miles: 3.141592...

But no, I see a value of 0.000936. (Screen shot attached as .png)

 

BTW: I am using the so-called "Equidistant Conic" projection.

 

Ok, so maybe the units are sq ft or sq meters.... No! The value would be much bigger.

Could they be in steradians: No! The radius of the earth is about 4000 miles and so the value would be pi divided by radius squared: approx 2e-7: much smaller than the value I see. Same comment applies to shape length. I see a value of 0.111. So, if I divide 2*pi*r by pi*r**2 I get 2/r=0.111/0.000936  =>  r=0.0169?????

 

So nothing seems to make any sense.

 

Can anyone shed some light on this??

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