How buffers are created
The buffer routine traverses each of the input feature's vertices and creates buffer offsets. Output buffer features are created from those offsets.
Creating offsets around a line
Input line feature
Offsets created around the input line feature
Buffer derived from the offsets
Description of buffer distance
The buffer distance parameter can be entered as a fixed value or as a field containing numeric values.
Example 1: Fixed distance
The following shows the buffer of a line feature class using a distance of 20, an end type of FLAT, a side type of FULL, and a dissolve type of ALL.
Because the buffer distance is a constant, all features are buffered to the same width.
Example 2: Distance from field
This example illustrates the buffer of a line feature class using a numeric field with values of 10, 20, and 30 for distance, an end type of FLAT, a side type of FULL, and a dissolve type of ALL.
Because the buffer distances are dependent on the field values, various buffer widths can be applied in the same operation.
Euclidean and geodesic buffering
An important feature of the Buffer tool is the Method parameter which determines how buffers are constructed. There are two basic methods for constructing buffers, Euclidean and geodesic.
- Euclidean buffers measure distance in a two-dimensional
Cartesian plane, where straight-line or Euclidean distances are
calculated between two points on a flat surface (the Cartesian plane). Euclidean
buffers are the more common type of buffer, and work well when
analyzing distances around features in a projected coordinate systemwhich are concentrated in a
relatively small area (such as one UTM zone).
In a projected coordinate system, there are areas in the projection where distances, areas, and the shape of features are distorted; this is a fact of using projected coordinate systems. For example, if using a State Plane or UTM projected coordinate system, features are more accurate near the origin of the projection (the center of the state, or the UTM zone), but become more distorted when moving away from the origin. Similarly, if a World projected coordinate system is used, distortion is often minimal in one area, but significant in another (for the Mercator World projection, distortion is minimal near the equator but significant near the poles). For a dataset that has features in both low and high distortion areas, Euclidean buffers will be more accurate in the low distortion areas and less accurate in the high distortion areas.
- Geodesic buffers are those that
account for the actual shape of the earth (an ellipsoid, or more properly, a geoid). Distances
are calculated between two points on a curved surface (the geoid) as opposed to two points on a flat surface (the Cartesian plane). You should always consider creating geodesic
buffers when
- your input features are dispersed (cover multiple UTM zones, large regions, or even the whole globe), or
- the spatial reference (map projection) of your input features distorts distances in order to preserve other properties such as area.
The Method parameter determines how buffers are created.
- Planar is the default option. This option will automatically determine which method to use based on the coordinate system of the Input Features.
- If the input features have a projected coordinate system, Euclidean buffers will be created.
- If the input features have a geographic coordinate system and you specify a Buffer Distance in linear units (meters, feet, and so forth, as opposed to angular units such as degrees), geodesic buffers will be created.
- This option produces the same result as the Buffer Tool prior to ArcGIS 10.3.
- Geodesic creates a shape-preserving geodesic buffer regardless of the input coordinate system. The shape-preserving geodesic buffer does not assume the lines connecting vertices are geodesic curves. It instead buffers the features in the spatial reference of the input feature class in order to create buffers that more closely represent the input features shape. If you are concerned about the shape of your buffers and how closely their shape matches the original input features it is recommended you investigate using this option, particularly when your input data is in a geographic coordinate system. In some cases this may take more time than the geodesic buffer created using the Planar option, but the result is a buffer that more accurately matches the shape of the input feature.
Geodesic buffer example
The goal of this example is to compare 1,000 kilometer geodesic and Euclidean buffers of a number of select world cities. Geodesic buffers were generated by buffering a point feature class with a geographic coordinate system, and Euclidean buffers were generated by buffering a point feature class with a projected coordinate system (in both the projected and unprojected datasets the points represent the same cities).
When working with a dataset in one of the common projected coordinate systems for the whole world, such as Mercator, projection distortion may be minimal near the equator, but significant near the poles. This means that for a Mercator projected dataset, distance measurements and buffer offsets should be quite accurate near the equator and less accurate away from the equator.
The graphic on the left shows the input point locations. The equator and prime meridian are shown for reference. Both graphics are displayed in the Mercator (World) projection.
In the graphic on the right, the points near to the equator have geodesic and Euclidean buffers that are coincident. For points near to the equator, the Mercator projection does a good job of producing accurate distance measurements. However, the buffers of points far from the equator show considerably more distance distortion, as their Euclidean buffers are much smaller than the geodesic buffers; this occurs with the Mercator projection because at the poles areas are stretched (land masses close to the poles, such as Greenland and Antarctica have enormous areas in comparison with the land masses close to the equator). All 1,000 kilometer Euclidean buffers are the same size since the Euclidean buffer routine assumes that map distances are the same everywhere in the projection (1,000 kilometers in Brazil is the same as 1,000 kilometers in central Russia); this is not true since away from the equator the projection's distances become more and more distorted. With any type of analysis of distance on a global scale geodesic buffers should be used as they will be accurate in all areas while Euclidean buffers will not be accurate in high distortion areas.
The BUFF_DIST field
The values in the BUFF_DIST field of the output feature class is in the linear unit of the Input Features' coordinate system. For example, if a 50 meter buffer distance is specified in the tool, but the input dataset has a coordinate system that uses feet as the linear unit, 50 meters will be converted to feet in the output BUFF_DIST field. There are two exceptions to this:
- If the Input Features have a geographic coordinate system and the buffer distance is specified in a linear unit, such as kilometers or miles, the values in the BUFF_DIST field will be in meters.
- If the spatial reference of the Input Features is Unknown, no conversion is applied, so the value in the BUFF_DIST field is exactly the value entered.
The following table summarizes scenarios when BUFF_DIST unit conversion is and is not performed.
| Input Features Coordinate System | Buffer Distance Units | Unit Conversion |
|---|---|---|
Geographic | Angular or linear | Converted to meters |
Projected | Angular | Converted to input coordinate system unit |
Projected | Linear | Converted to input coordinate system unit |
Geographic or Projected | Unknown | Assumed to be input coordinate system unit |
Unknown | Angular or Linear | No conversion |
| Input Features Coordinate System | Buffer Distance Units | Unit Conversion |
|---|---|---|
Geographic | Angular or linear | Converted to meters |
Projected | Angular | Converted to meters |
Projected | Linear | Converted to meters |
Geographic or Projected | Unknown | Assumed to be meters |
Unknown | Angular or Linear | No conversion |
