Create a buffer zone for each area object extending both outward and inward from the area's boundary the given distance. All points in the buffer zone will be within the given distance of the boundary.
Units used for the distance value in the source / argument box are taken from the projection of the drawing. If the drawing is in Latitude / Longitude projection the units will be degrees. If the drawing is projected, the units will be in meters or feet. It is therefore strongly recommended that the drawing be projected first so that meters or other linear measure can be used instead of attempting to use degrees as a measurement unit.
To see which units are defined for the drawing's projection, click on the Tracker tool and make some measurements. Whatever units are used to show the result of the tracker tool are the units currently in force. The tracker tool is also useful for estimating the desired size range of the buffer zone.
If the drawing uses degrees and meter-based creation of buffer zones are desired, first re-project the drawing into any convenient meter-based projection (such as Orthographic) so that meters will be used as the units of measure.
Example
This example shows the creation of border buffers. Exactly the same procedure but with a different choice of transform operator is used to create inner buffers or other buffer zones.
We have selected the province of Aguascalientes in a drawing of Mexican provinces. The drawing has been projected into Orthgraphic projection. We would like to create a border buffer zone extending 20 km outward from the boundary and also 20 km inward from the boundary.
To check what units of measure are in use, we use the tracker tool to measure a distance near the province.
The tracker tool reports the measurement as about 21000 meters. This confirms our drawing uses meters as the units of measure.
In the transform toolbar we choose the Border Buffers operator and enter 20000 into the source / argument box (20 kilometers are, of course, 20000 meters). We press Apply.
The result is a newly created area in the shape of the border buffer that appears in the drawing. The newly created area automatically replaces the selection. Since it is messy to keep overlapping areas in the same drawing, we will use Edit - Cut to cut this area out of the drawing and then use Edit - Paste to paste it into its own drawing.
We can then use both drawings together as layers in a map and apply Layer Opacity of 50% to the upper layer. This shows how the border buffer area overlaps the original drawing.
Buffer Zone Variations
The transform toolbar provides four different buffer zone variations that may be created using the general procedure used above.
Variations are shown based on the area shown above in blue dotted style. All variations were created by showing the resultant buffer zone area as a 50% transparent layer in a map.
The Buffers operator creates an outer buffer zone, extending 15000 meters outward from the boundary of the area and including the original area.
Inner buffer zone, comprising all parts of the original area except those extending 15000 meters inward from the boundary of the area.
Border buffer zone, extending both inward 15000 and outward 15000 from the boundary of the area.
The Common Buffer transform operator is like Buffers, but creates one buffer zone about all objects in the scope.
In the example above the green areas were created with Buffers using the lines and the blue area was created with Common Buffer. Note that there are three area objects in the case of the green buffer zones but only a single area object in the case of the common buffer.
Local Scale and Units of Measure
Stating that the units used by buffer operators are the units used in the projection is actually an oversimplification. More accurately, the parameter value is taken as the number of local drawing units. Local drawing units are normally the same as the units used by the projection. Local drawing units are the projection units multiplied by the Local scale value for the coordinate system specified in the Edit - Change Projection dialog.
Because the Local scale value is usually 1, the value used in the source / argument box for buffer operators is usually the same number in local drawing units. Entering 1500 in a meter-based projection thus means 1500 meters. However, if the Local scale value is not 1, the value in the source / argument box must be multiplied by the Local scale value to get the number of units that will be used to create the buffer. If the Local scale value is 10 in a meter-based projection, for example, then entering 50 into the source / argument box is not a command to create a 50 meter buffer, it is a command to create a 500 meter buffer.
An additional subtlety must be considered when these transform operators are used with a drawing layer within a map. If the map component's projection is degree based while the native projection for the drawing is meter based (as, for example, with the Orthographic projection) or vice versa, then the transform operator will use whatever units are used by the drawing's projection.
Tech Tip
Variations on the basic buffer zone constructions with areas may be easily created by first creating a buffer zone and then using Clip with (Intersect) or Clip with (Subtract) between the new buffer zone area object and the original area object. For example, using a Clip with (Intersect) between a border buffer and the original area will produce a zone that consists only of the defined distance within the area's boundary and that excludes the more inner regions of the original area as well as everything outside of the boundary.
The buffer distance used must be at least four times the size of the location precision factor. If precision has been set to 10 meters then the buffer distance must be larger than 40 meters.
See Also: