Transform - Inner Buffers
Create a buffer zone for each area that is within and smaller than the area by
the given distance. Buffer zone distances must be specified using integer
values. 23 is thus a valid value but .06 is not acceptable.
Units used for the distance value in the parameter 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.
See the Transform - Border Buffers topic for a detailed example illustrating the creation of buffer zones.
Buffer Zone Variations
The transform toolbar provides four different buffer zone variations.
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 - Projection dialog.
Because the Local scale value is usually 1, the value used in the parameter 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 parameter 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 parameter box is not a command to create a 50 meter buffer, it is a command to create a 500 meter buffer.
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.
See Also:
Transform - Buffers
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