The Buffer method is parameterized with at least a Radius and a Minimum Patch Area.  A raster layer of habitat is analyzed to determine areas that meet the corresponding density criteria (where available habitat within circles of the buffer radius equals or exceeds the minimum patch area).  Options are provided that allow users to maximize the number of identified areas and to allow flexibility in the buffer radius.

As in the Nearest Neighbor method, Buffer begins by grouping chunks into neighborhoods if chunks are separated from each other by less than or equal to twice the buffer radius.  Separation between chunks is measured as the smallest distance from edge to edge.  A chunk isolated by more than twice the buffer radius in all directions is placed in its own neighborhood.  Once neighborhoods have been identified, each neighborhood is assessed to determine areas of sufficient habitat density, which are then assigned patch identifiers and removed from the habitat layer being analyzed.

Patch processing involves the use of stencils.  For each neighborhood, a bounding box is drawn around its member chunks.  This bounding box is expanded on each side by 4 times the buffer radius and then filled with touching but non-overlapping circles of the buffer radius (Figure 42a).  This stencil of circles is laid over a raster layer of habitat and circles that capture equal to or greater than the minimum patch area are cut from the habitat raster as patches (Figure 42b).

Figure 42.  Patch processing with the Buffer method.  First, the bounding box of neighborhood is expanded by four times the user-defined buffer radius to create the initial stencil outline (a), which is then filled with buffer radius circles (b).  Shakes, or slight repositions of the stencil, are considered (c and d) prior to cutting of patches.

The Buffer algorithm allows for iterations of stencil placement during processing.  This is done by shifting the stencil by two-thirds of the buffer radius in a user-defined number of directions called Shakes.  The first shake is always to the north of the initial position of the expanded bounding box.  Additional shakes are evenly spaced from north in a circular direction.  For example, a shake setting of 2 would check areas for the initial expanded bounding box (shake 0; Figure 42b), a shift to the north (shake 1; Figure 42c), and a shift to the south (shake 2; Figure 42d); a shake setting of 4 would use the initial position plus north, east, south, and west.  Each shake is considered separately.  The shake that yields the most patches is cut, with the corresponding patch areas removed from the layer.  When two or more shakes would yield the same number of patches, the lowest number shake is cut.  This sequence is repeated for uncut shakes and remaining habitat.  Additional cuts are done only where a patch can be made without intersecting an already cut patch circle. 

There is an additional option that allows consideration of a Flexible Radius (Figure 43).  This option has two parameters: minimum radius and number of increments.  When selected, patch processing begins with a stencil of circles of the minimum radius, shaking and cutting as described above until no more patches of that radius can be cut, then advances to the next larger radius and repeats the shaking and cutting, and so on.  The last and largest radius used is the buffer radius.  The user-defined number of increments includes the minimum and buffer radii.  For example, a run with minimum radius of 5 map units, buffer radius of 15 map units, and increment number of 3 would have 3 cycles of shaking and cutting (radii of 5, 10, and 15).

Figure 43.  Buffer method options include shaking and flexible radius.  Shaking considers different stencil positions before cutting patches.  Flexible radius uses stencils of different radii.  The original position of the expanded bounding box is applied as shake 0.

Standard output for Buffer includes displayed layers of patch circles and actual patches.  Attribute information includes patch areas, neighborhood, pass, validity, and suitable area, if the use suitabilities option was selected.  Information about the simulation, connected points, neighborhood characteristics, and chunk characteristics are stored in a geodatabase specific to each run.  When the generate detailed output option is selected, displayed output also include the connection point, neighborhood, and chunk layers.  Additional output including the sequence of rasters used for each cut and circles tested (and their center points) are stored in the geodatabase.  A pass name comprised of counters for neighborhood, radius, pass, and shake is provided for each patch to inform when it was cut during processing.  The pass component is used to document the order in which shakes were cut.

When the Use Suitabilities option is selected, chunks are split into patches based on value-weighted areas.  The complication for buffer is again related to potential scale issues associated with processing large rasters.  To account for suitability and maintain process integrity, input rasters were converted internally to point coverages.  Stencil circles that captured points whose values (suitabilities) multiplied by the raster cell area summed to at least the minimum patch area criterion were identified as potential patches during shaking and cutting.  This use of circular shapes in a matrix of points can lead to results that seem spatially inconsistent with inconsistencies becoming more apparent as circle radii decrease in size relative to raster cell size.

Figure 44 shows Buffer patch results for little minnow spawning habitat processed with suitabilities.  Patches cut with the larger radius (50 map units) tend to occur in the poorest or sparsest habitat because the larger radius was needed to cut enough habitat to meet the minimum (suitable) patch area parameter.

Figure 44.  Creating a patch raster for habitat connectivity analyses using Buffer patch method.