buffer distance

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06-07-2016 01:50 PM
DavidStockdale
New Contributor II

Is there an easy way to determine for an irregularly shaped polygon, what the buffer distance would need to be to create a buffered polygon of a specified size? For example I have a polygon of 500 acres, and I need to create a polygon of 10% greater area, i.e. 550 acres. Is there a way to calculate what the buffer size would need to be in such a  case?

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12 Replies
RobertBorchert
Frequent Contributor III

The issue with multiplying 1.1 times the number of years would be huge.  multiply 1.1 times 5 years and that would be an increase of 5.5 times the size.  Or rather 550% larger

In excel we can calculate the new area and parameter and get it pretty close. I just tried it and the 5th year was very very close to doing the whole thing five times.

3 columns.

Row 1  Len(A1)     Area(B1)     Buffer(C1)   The first values for Len and Area come from you polygons.  the first buffer is

=((B2*1.1) -B2)/A2  this will fill in C2

A2 and B2 came from the polygosn

for cell A3           =A2 * (1 + (C2/100))  then drag it down

for cell B3          =B2*1.1 drag it down

for cell C3 drag C2 down

I did this with my example.   By creating 5 buffers in ArcMAP the final increase was 3.939114292  by running this trough Excel I came  up with 3.939114202

So I would think if you export your polygons with ObjectID, shape length and shape area you can calculate the 5 and 10 year buffer sizes rather quickly.  then you can bring the table back in and with a join populate the 5 and 10 year buffers.

However, I would say in Access we could build a calculation that would add up the new area and length and in a single iteration calculate the final buffer for 5 and 10 year.

DavidStockdale
New Contributor II

I tried using your Excel idea and compared it with manually doing 5 buffers in ArcMap, using your formula for the buffer distance. Doing it in Excel got quite different buffer distances than doing it in ArcMap, but the end result was extremely close, 99+ percent.  So I'm rather puzzled as to why that should be.

YearLenAreaBuffer DistanceAcresArcMap/ExcelLen (ArcMap)Area (ArcMap)Buffer Distance(ArcMap)Acres(ArcMap)
0582477.51447218070.038.1064116681.000000582477.51447218070.038.106411668
1629695.58451939877.038.2484128340.998870569034.05551881183.419.117412820
2681635.46157133864.738.3819141180.998816570108.55357066214.2310.009714101
3738769.32662847251.218.5070155300.998997572501.94662784212.610.966615514
4801616.57769131976.338.6241170830.999418577001.41769091740.9911.974317073
5870748.55376045173.96---187910.999744581227.09476025680.41---18786
NeilAyres
MVP Alum

Probably because the area of the buffered circle in ArcMap is not really a circle, but an "NGon", something that looks pretty much like a circle, but is in fact a bunch of vertices going around the edge. All those tiny little arcs along the edge, add up to your difference.