Buffer Inaccuracy

[Ben]

My 2.5km buffer around Snowbasin has turned out to be pretty far off. Both the measure tool and the scale bar I tried to add show that the buffer is more like 3.5km.  Is this possibly a glitch, or did I do something incorrectly when I buffered? I hope it's not the latter, because that would completely change the results of my analysis!

[Ben]

From some searching around, it appears that the difference has to do with the buffer tool in arctoolbox calculating geodesic distance, while the measure tool and scalebar are measuring planar distance. This can cause inaccurate scale bars and measure tool results, particularly when used with data with large spatial extents. The solution I found in my search was just to convert the scalebar to graphics and resize it to match the known distance, such as the buffer in this case.  Hopefully this is an acceptable fix, since I really do need to move on....

[Shannon Wing Belmont]

Hi Ben,

I appreciate the effort you put into resolving this. Did you, by chance, just run the buffer again?

Did you have similar issues creating a buffer for the contaminated section of Emigration Creek? I don't recall so...

My guess is the buffer calculation just went awry.

At the scale of 2.5 km, the scale bar and measure tool should reflect the correct diameter of the buffer ring.  I would imagine that the references you found to the geodetic distances simply means that the projection is taken into consideration when Arc does calculations and can effect the measurement. Curvature of the earth can be accounted for in all Arc calculations, something you can set up in the environment settings.  But at the scale of 2.5 km, shouldn't impact your results. 

Don't redo the lab, of course, but if you are curious, try rerunning the buffer tool again, verify your coordinate systems (display [data frame] et al) are in UTM and see what happens.  I can maybe buy that there could be a pretty sizable discrepancy if the display data frame coordinate system were in the Web Mercator auxilliary sphere of the basemap?? 

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Shannon Belmont

Quinney College of Natural Resources
Utah State University
5210 Old Main Hill
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[Aaron Sisson]

when you create your buffer soit needs to be added at 2500 m. you migh thave missed a key stroke or something like that.

On Sun, Apr 21, 2013 at 6:54 PM, Ben <suga...@gmail.com> wrote:

My 2.5km buffer around Snowbasin has turned out to be pretty far off. Both the measure tool and the scale bar I tried to add show that the buffer is more like 3.5km.  Is this possibly a glitch, or did I do something incorrectly when I buffered? I hope it's not the latter, because that would completely change the results of my analysis!

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Aaron R. Sisson
Soil Conservationist
South Dakota NRCS
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[Ingrid Campbell]

No, wouldn't it be 250 m?? or you can just keep it 2.5 and use km as the unit

[Aaron Sisson]

kilo = thousand. 2.5 thousand meters = 2.5 kilometers. that is the buffer from Snow Basin.

the road buffer is 250. i may have misunderstood the context. 

[Ingrid Campbell]

yup, you are right :)