0

Two methods of selection in ArcMap that I thought would be the same are in fact not equal. I created a 3 mile buffer around a single line feature (approx 6mi in length). I'm working with parcels, as points, that should be within 3 miles of the line feature.

Method 1) Create buffer of 3 miles based on line. Select by attribute > target = parcels > source = buffer > selection method = intersect. This yielded 77374 parcels.

Method 2) Select by attribute > target = parcels > source = line > selection method = within a distance: 3 miles. This yielded 77399 parcels.

The missing 25 parcels are found close to the boundary of the buffer along curves.

So, how does ArcMap build a buffer around a line?

Buffers around a point result in a curve with a single vertex. The picture shows the vertices of the buffer. The parts of the buffer that are made from the endpoints are a series of connected points about 1k feet apart instead of a curve. Is there a way add curves or decrease spacing of vertices as environments in the tool?

enter image description here

I ended up buffering all the vertices in the line feature, dissolving them, union with original buffer, dissolve those. Then selecting by intersection yields the correct count and has a difference in area of ~600k sqft.

  • Try the intersect command resources.arcgis.com/en/help/main/10.2/index.html#//… to get constant results (note: generalise you line if too many vertices) – Mapperz Feb 5 '15 at 19:24
  • Are you in a projected coordinate system or geographic? If you're not in projected, that could explain the discrepancy between the two methods (you need to be). Also, shapefile vs geodatabase will have differences in the way curves are handled. – Chris W Feb 5 '15 at 19:24
  • @Chris W Everything is in the same projected coordinate system in a file gdb. – gm70560 Feb 5 '15 at 19:29
  • There are some notes in the Buffer help file that might help explain the discrepancy you're seeing. In particular on that page is a link to another page, How Buffer Works. What's interesting to me is they're opposite of my experience in that trying to use a buffer in an unprojected CRS has always given me fewer results than the select by location - they only match when in a projected CRS. – Chris W Feb 5 '15 at 19:38
  • @ChrisW - put the "how it works" link and the picture at the top and I'll take it as an answer. That is what I was after. – gm70560 Feb 5 '15 at 19:50
4

The ArcGIS help files for the Buffer tool include a How Buffer Works page with a full description. There, it is explained that offsets are created from the line and its vertices, then connected to form the buffer.

Esri's explanatory image

But the page also goes into detail about the difference between Euclidean and Geodesic buffering, which comes into play when working in projected vs geographic coordinate systems respectively. This includes some gotchas about how those curves around the ends are formed and distortions caused by projection. The help file specifically mentions using the Densify tool to increase vertices around the end curves in some cases (rather than an environment setting).

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.