FRAGSTATS accepts images in several forms, depending on whether the image contains background, and whether the landscape contains a border outside the landscapeboundary (Fig. 1). The distinction among background, border, and boundary and how they affect the landscape analysis and the calculations of various metrics is a source of great confusion and thus great importance. Great care should be taken to fully understand these distinctions before trying to run FRAGSTATS. We strongly suggest that you read through this section twice, once to familiarize yourself with the terminology and a second time to understand the implications.
Figure 1. Alternative image formats with regards to background (given a class value of 99 here) and border. The thick solid line represents the landscape boundary. Positive values are 'inside' the landscape of interest and contribute to the computed total landscape area; negative values are 'outside' the landscape of interest and are only utilized to determine edge types for patches along the landscape boundary.
- Landscape boundary -- Every image will include a landscape boundary that defines the perimeter of the landscape and surrounds the patch mosaic of interest. The boundary is simply an invisible line around the patch mosaic of interest. It is not given explicitly in the image; rather, it is defined by an imaginary line around the outermost cells of positively valued cells. The landscape boundary distinguishes between cells inside the landscape of interest from those outside, and thus ultimately determines the total landscape area. All positively valued cells are assumed to be inside the landscape of interest and are thus included in the total area of the landscape, regardless of whether they are classified as background (see below) or not. This is important, because many metrics include total landscape area in their calculation. Note, in most cases the landscape boundary will surround a single contiguous region of positively valued cells. However, it is possible to have disjunct regions of positively valued cells. In this case, the landscape boundary is not a single continuous line around the landscape of interest, but rather separate boundaries around each disjunct region of interest. The important point is that positively valued cells are inside the landscape of interest, while negatively valued cells are outside, and the landscape boundary is the imaginary line(s) that separates inside from outside. Hence, if the input image contains all positively valued cells, then the entire grid is assumed to be inside the landscape of interest and the landscape boundary represents an imaginary line around the entire grid. If the input image contains negatively valued cells, then those cells are assumed to be outside the landscape of interest and thus outside the landscape boundary. In this case, the edge between positively and negatively valued cells represents the landscape boundary. The landscape boundary is important in the absence of a landscape border (see below) because FRAGSTATS 4.0 needs to know how to treat the edges along the boundary in all edge calculations. In the absence of a border, the landscape boundary will be treated according to user specifications (see below).
- Background -- An image may include background - an undefined area either 'inside' or 'outside' the landscape of interest. Note that background can exist as 'holes' in the landscape and/or can partially or completely surround the landscape of interest. The background value can be any integer value. Positively valued cells of background are assumed to be 'inside' the landscape of interest; negatively valued cells of background are assumed to be 'outside' the landscape of interest. This distinction is important, as noted above, because positively valued background (interior background) will be included in the total landscape area, and thus will affect many metrics, even though it will not be treated as a patch per se (see below). Further, via the graphical user interface (see below), any class or combination of classes can be treated (i.e., reclassified) as background for a particular analysis. There are several critical issues regarding how background is handled by FRAGSTATS:
- Interior background (i.e., positively valued background) is included in the total landscape area and thus effects metrics that include total landscape area in their calculations. However, and this is quite tricky, interior background is in essence excluded from the total landscape area in a number of class and landscape metrics that involve summarizing patch or class metrics. For example, mean patch area is based on the average size of patches at the class or landscape level. If interior background is present, mean patch size as computed by FRAGSTATS will not equal the total landscape area divided by the number of patches, because the total landscape area includes background area not accounted for in any patch. Similarly, the area-weighted mean of any patch metric (i.e., distribution statistics at the class and landscape level; see FRAGSTATS Metrics documentation) weights each patch by its proportional area representation. Here, the proportional area of each patch is not based on the total landscape area, but rather the sum of all patch areas, which is equivalent to the total landscape area minus interior background. Similarly, a number of landscape metrics are computed from the proportion of the landscape in each class (e.g., Shannon's and Simpson's diversity). Here, proportional area of each class is not based on total landscape area because the proportions must sum to 1 across all classes. Instead, the proportions are based on the sum of all class areas, which is equivalent to the total landscape area minus interior background. Given the subtle differences in how interior background affects various metrics, it behooves you to carefully read the FRAGSTATS Metrics documentation pertaining to each metric you choose, assuming of course that interior background is an issue.
- Exterior background (i.e., negatively valued background) can have a minor effect on the analysis if functional metrics are selected. Exterior background is assumed to be 'outside' the landscape of interest and thus has no effect on the area-based metrics; however, the border between exterior background and interior cells can effect the edge-based metrics (e.g., core area, edge contrast, and aggregation metrics). Thus, the "extent" of exterior background in the input landscape has no effect, but the "length of edge" between exterior background and interior landscape cells can have an effect.
- Background (both interior and exterior) cells adjacent to non-background classes represent edges that must be accounted for in all edge-related metrics. The user specifies how background edge segments should be handled in all edge-related calculations (see below).
- Interior background (i.e., positively valued background) is included in the total landscape area and thus effects metrics that include total landscape area in their calculations. However, and this is quite tricky, interior background is in essence excluded from the total landscape area in a number of class and landscape metrics that involve summarizing patch or class metrics. For example, mean patch area is based on the average size of patches at the class or landscape level. If interior background is present, mean patch size as computed by FRAGSTATS will not equal the total landscape area divided by the number of patches, because the total landscape area includes background area not accounted for in any patch. Similarly, the area-weighted mean of any patch metric (i.e., distribution statistics at the class and landscape level; see FRAGSTATS Metrics documentation) weights each patch by its proportional area representation. Here, the proportional area of each patch is not based on the total landscape area, but rather the sum of all patch areas, which is equivalent to the total landscape area minus interior background. Similarly, a number of landscape metrics are computed from the proportion of the landscape in each class (e.g., Shannon's and Simpson's diversity). Here, proportional area of each class is not based on total landscape area because the proportions must sum to 1 across all classes. Instead, the proportions are based on the sum of all class areas, which is equivalent to the total landscape area minus interior background. Given the subtle differences in how interior background affects various metrics, it behooves you to carefully read the FRAGSTATS Metrics documentation pertaining to each metric you choose, assuming of course that interior background is an issue.
- Landscape border -- An image also may include a landscape border; a strip of land surrounding the landscape of interest (i.e., outside the landscape boundary) within which patches have been delineated and classified. Patches in the border must be set to the negative of the appropriate patch type code. For example, if a border patch is a patch type of code 34, then its cell value must be -34 (negative 34). The border can be any width (as long as it is at least 1 cell wide) and provides information on patch type adjacency for patches on the edge of the landscape (i.e., along the landscape boundary). Essentially, patches in the border provide information on patch type adjacency for patches in the landscape of interest located along the landscape boundary; all other attributes of the patches in the border are ignored because they are outside the landscape of interest. Thus, the border affects only metrics where patch type adjacency is considered: core area, edge contrast, and aggregation metrics.
Under most circumstances, it is probably not valid to assume that all edges function the same. Indeed, there is good evidence that edges vary in their affects on ecological processes and organisms depending on the nature of the edge (e.g., type of adjacent patches, degree of structural contrast, orientation, etc.). Accordingly, the user can specify a file containing edge contrast weights (described in more detail in the Contrast Metrics section of the FRAGSTATS Metrics section) for each combination of patch types (classes), including adjacencies involving background if it exists. Briefly, these weights represent the magnitude of edge contrast between adjacent patch types and must range between 0 (no contrast) and 1 (maximum contrast). Edge contrast weights are used to compute several edge-based metrics. If this weight file is not provided, these edge contrast metrics are simply not computed. Generally, if a landscape border is designated, a weight file will be specified as well, because one of the principal reasons for specifying a border is when information on edge contrast is deemed important. If a border is present, the edge contrast associated with all landscape boundary edge segments is made explicit due to knowledge of the abutting patch types. If a border is absent, then all edge segments along the landscape boundary are treated the same as background, as specified in the user-provided edge contrast weight file. Note, however, that the presence of a landscape border will have no affect on the edge contrast metrics if a contrast weight file is not specified - because these metrics will not be computed.
Similarly, the user can specify a file containing edge depths (described in more detail in the Core Area Metrics section of the FRAGSTATS Metrics section) for each combination of patch types (classes), including adjacencies involving background if it exists. Briefly, edge depths represent the distance at which edge effects penetrate into a patch and must be given in distance units (meters); edge depths can be any number = 0. However, when implementing edge depths for the purpose of determining core areas, FRAGSTATS is constrained by the minimum resolution established by the cell size. Thus, in effect, edge depths will be rounded to the nearest distance in increments of the cell size. For example, if the cell size is 30 m, and you specify a 100 m edge depth, the edge mask used to mask cells along the edge of a patch (i.e., eliminate them from the "core" of the patch) will be 3 cells wide (90 m), because it is not possible to use a mask that is 3.3 cells wide. Similarly, a specified edge depth of 50 m will in effect be rounded up to 2 cells (60 m). Therefore, it is generally advisable to specify edge depths in increments equal to the cell size. Edge depths are used to compute several core area-based metrics. If this edge depth file is not provided, these core area metrics are simply not computed. Typically, if a landscape border is designated, an edge depth file will be specified as well, because one of the principal reasons for specifying a border is when information on edge effects is deemed important. If a border is present, the edge depths associated with all landscape boundary edge segments is made explicit due to knowledge of the abutting patch types. If a border is absent, then all edge segments along the landscape boundary are treated the same as background, as specified in the user-provided edge depth file. Note, however, that the presence of a landscape border will have no affect on the core area metrics if an edge depth file is not specified - because these metrics will not be computed.
A landscape border is also useful for determining patch type adjacency for many of the aggregation metrics. These metrics (described in more detail in the Aggregation Metrics section of the FRAGSTATS Metrics documentation) require information on cell adjacency; that is, the abutting class values for the side of every cell. The proportional distribution of cell adjacencies is used to compute a variety of landscape texture metrics. Although a landscape border is not often designated for the primary purpose of computing these texture metrics, a border will inform the calculation of these metrics. If a border is present, the adjacencies associated with all landscape boundary edge segments is made explicit due to knowledge of the abutting patch types. If a border is absent, then all edge segments along the landscape boundary are treated the same as background and the corresponding cell adjacencies are ignored in the calculation of these metrics.
Metrics based on edge length (e.g., total edge or edge density) are affected by these considerations as well. If a landscape border is present, then edge segments along the boundary are evaluated to determine which segments represent 'true' edge and which do not. For example, an edge segment between cells with class value 5 (inside the landscape of interest) and cells with class value -5 (outside the landscape of interest; i.e., in the border) does not represent a true edge; in this case, the landscape boundary artificially bisects an otherwise contiguous patch and the edge is not counted in the calculations of total edge length. Conversely, an edge segment between class 5 and -3 represents a true edge and is counted. If a landscape border is absent, then the entire boundary is treated as background and is treated according to a user-specified proportion. For example, if the user specifies that 50% of the landscape boundary should be treated as true edge, then 50% of the landscape boundary involving background will be incorporated into the edge length metrics. In other words, regardless of whether a landscape border is present or not, if a background class is specified, then a user-specified proportion of edge bordering background is treated as true edge and the remainder is ignored.
We recommend including a landscape border, especially if edge contrast, core area, or patch type adjacency is deemed important. In most cases, some portions of the landscape boundary will constitute 'true' edge (i.e., an edge with a contrast weight > 0) and others will not, and it will be difficult to estimate the proportion of the landscape boundary representing true edge. Moreover, it will be difficult to estimate the average edge contrast weight or edge depth for the entire landscape boundary. Thus, the decision on how to treat the landscape boundary will be somewhat subjective and may not accurately represent the landscape. In the absence of a landscape border, the affects of the decision regarding how to treat the landscape boundary on the landscape metrics will depend on landscape extent and heterogeneity. Larger and more heterogeneous landscapes will have a greater internal edge-to-boundary ratio and therefore the boundary will have less influence on the landscape metrics. Of course, only those metrics based on edge lengths and types are affected by the presence of a landscape border and the decision on how to treat the landscape boundary. When edge-based metrics are of particular importance to the investigation and the landscapes are small in extent and relatively homogeneous, the inclusion of a landscape border and the decision regarding the landscape boundary should be considered carefully.
So, let's try to put all of this together. There are five types of metrics affected by landscape boundary, background, and border designations: (1) total landscape area, (2) edge length metrics, (3) core area metrics, (4) contrast metrics, and (5) aggregation metrics. Let's consider several scenarios involving various combinations of background and border, and how each of these types of metrics will be treated under each scenario.
- Scenario 1 -- Input landscape contains all positively valued cells of non-background classes (Fig. 1a). In this case, the entire grid is assumed to be in the landscape of interest and every cell belongs to a non-background class. The landscape boundary surrounds the entire grid and there is no border or background present (this is probably the most common scenario).
Total landscape area.--All cells are included in the total landscape area calculation.
Edge length metrics. - User must specify the proportion of the landscape boundary to include as edge. All other edges are explicit.
Core area metrics.--The landscape boundary is treated like background; the user must specify the edge depth for cells abutting background in the edge depth file, and this depth is applied to the landscape boundary. All other edges are explicit; their edge depths are specified in the edge depth file.
Contrast metrics.--The landscape boundary is treated like background; the user must specify the edge contrast for cells abutting background in the edge contrast weight file, and this weight is applied to the landscape boundary. All other edges are explicit; their edge contrast weights are specified in the edge contrast weight file.
Aggregation metrics.--The landscape boundary is treated like background and is simply ignored since there is no information available on patch type adjacency.
- Scenario 2 -- Input landscape contains all positively valued cells, but includes a background class (Fig. 1b). In this case, the entire grid is assumed to be in the landscape of interest , but some cells belong to a background class. Here, the background is interior because it is positively valued and thus inside the landscape of interest. The landscape boundary surrounds the entire grid and there is no border present.
Total landscape area.--All cells are included in the total landscape area calculation.
Edge length metrics.--User must specify the proportion of the landscape boundary and background edges to include as edge. All other edges are explicit.
Core area metrics.--The landscape boundary is treated like background; the user must specify the edge depth for cells abutting background in the edge depth file, and this depth is applied to both the landscape boundary and background edges. All other edges are explicit; their edge depths are specified in the edge depth file.
Contrast metrics.--The landscape boundary is treated like background; the user must specify the edge contrast for cells abutting background in the edge contrast weight file, and this weight is applied to both the landscape boundary and background edges. All other edges are explicit; their edge contrast weights are specified in the edge contrast weight file.
Aggregation metrics.--The landscape boundary and background are treated similarly; both are simply ignored when evaluating adjacencies since there is no information available on patch type adjacency in either case.
- Scenario 3 -- Input landscape contains a mixture of positively valued cells and negatively valued background cells (Fig. 1c). Note, it doesn't matter whether the negatively valued background cells are located entirely on the periphery of the positively valued cells (i.e., outside the landscape of interest) or located as holes in the interior of the landscape, or a combination of the two. In all cases, the positively valued cells are is assumed to be inside the landscape of interest, whereas the negatively valued background cells are assumed to be outside the landscape of interest and thus outside the landscape boundary. In figure 1c, the background is entirely exterior because it is all negatively valued and thus outside the landscape of interest (even though some of the exterior background patches are embedded as holes within the landscape). The landscape boundary separates contiguous regions of positively valued cells from the negatively valued cells and there is no border present. Alternatively, the exterior background could be considered border, but the use of border is generally reserved for situations involving negatively valued non-background cells.
Total landscape area.--All positively valued cells are included in the total landscape area calculation; negatively valued cells (here, all background) are ignored.
Edge length metrics.--User must specify the proportion of the landscape boundary and background edges (in this case, they are the same) to include as edge. All other edges are explicit.
Core area metrics.--The landscape boundary is treated like background; in this case, the entire landscape boundary is in fact also background. The user must specify the edge depth for cells abutting background in the edge depth file, and this depth is applied to the background edges (in this case, all on the landscape boundary). All other edges are explicit; their edge depths are specified in the edge depth file.
Contrast metrics.--The landscape boundary is treated like background; in this case, the entire landscape boundary is in fact also background. The user must specify the edge contrast for cells abutting background in the edge contrast weight file, and this weight is applied to the background edges (in this case, all on the landscape boundary). All other edges are explicit; their edge contrast weights are specified in the edge contrast weight file.
Aggregation metrics.--The landscape boundary and background (in this case, they are the same) are treated similarly; they are simply ignored when evaluating adjacencies since there is no information available on patch type adjacency.
- Scenario 4 -- Input landscape contains a mixture of positively valued cells, including some positively valued background cells, and negatively valued background cells (Fig. 1d). Note, as in scenario 3, it doesn't matter whether the negatively valued background cells are located entirely on the periphery of the positively valued cells (i.e., outside the landscape of interest) or located as holes in the interior of the landscape, or a combination of the two. In all cases, the positively valued cells are is assumed to be inside the landscape of interest , whereas the negatively valued background cells are assumed to be outside the landscape of interest and thus outside the landscape boundary. Here, the background is a combination of interior and exterior background. The landscape boundary separates contiguous regions of positively valued cells from the negatively valued cells and there is no border present. As noted in scenario 3, the exterior background could be considered border, but the use of border is generally reserved for situations involving negatively valued non-background cells.
Total landscape area.--All positively valued cells, including the 'interior' background, are included in the total landscape area calculation; negatively valued cells (here, all background) are ignored.
Edge length metrics.--User must specify the proportion of the landscape boundary (in this case, all background) and interior background edges to include as edge. All other edges are explicit.
Core area metrics.--The landscape boundary is treated like background; in this case, the entire landscape boundary is in fact also background. The user must specify the edge depth for cells abutting background in the edge depth file, and this depth is applied to all background edges (in this case, both on the landscape boundary and interior). All other edges are explicit; their edge depths are specified in the edge depth file.
Contrast metrics.--The landscape boundary is treated like background; in this case, the entire landscape boundary is in fact also background. The user must specify the edge contrast for cells abutting background in the edge contrast weight file, and this weight is applied to all background edges (in this case, both on the landscape boundary and interior). All other edges are explicit; their edge contrast weights are specified in the edge contrast weight file.
Aggregation metrics.--The landscape boundary (in this case, all background) and interior background are treated similarly; both are simply ignored when evaluating adjacencies since there is no information available on patch type adjacency in either case.
- Scenario 5 -- Input landscape contains a mixture of positively valued non-background cells and negatively valued non-background cells (i.e., a true border; Fig. 1e). In this case, the positively valued cells are is assumed to be inside the landscape of interest , whereas the negatively valued cells are assumed to be outside the landscape of interest and thus outside the landscape boundary. The landscape boundary separates contiguous regions of positively valued cells from the negatively valued cells; no background exists; and there is a true border present. This is unquestionably the ideal scenario because every cell is classified into a real class (i.e., no background) and a border is included to inform all edge, core, and adjacency calculations.
Total landscape area.--All positively valued cells are included in the total landscape area calculation; negatively valued cells are ignored.
Edge length metrics.--Because a border is present and there is no background, all edges are explicit; that is, the image provides explicit information on whether every edge segment along the boundary is a true edge or not. In this case, the user does not need to specify the proportion of the landscape boundary to include as edge. In fact, any user specification in this regard via the user interface will be disregarded.
Core area metrics.--Because a border is present and there is no background, all edges are explicit; that is, the image provides explicit information on the abutting patch types along the boundary. In this case, all edge depths are specified in the edge depth file.
Contrast metrics.--Because a border is present and there is no background, all edges are explicit; that is, the image provides explicit information on the abutting patch types along the boundary. In this case, all edge contrast weights are specified in the edge contrast weight file.
Aggregation metrics.--Because a border is present and there is no background, all edges are explicit; that is, the image provides explicit information on the abutting patch types along the boundary. In this case, all boundary edge segments are included in the adjacency calculations.
- Scenario 6 -- Input landscape contains a mixture of positively valued cells, including both background and non-background classes, and negatively valued cells, including both background and non-background classes (Fig. 1f). This is the most complex scenario involving a complicated mixture in interior and exterior background and a border. In this case, all positively valued cells (including interior background) are is assumed to be inside the landscape of interest, whereas the negatively valued cells are assumed to be outside the landscape of interest and thus outside the landscape boundary. The landscape boundary separates contiguous regions of positively valued cells from the negatively valued cells. A true border is present, but it includes some background class. This is perhaps also an ideal scenario, like scenario 5, but contains a realistic, sometimes unavoidable, situation in which some areas must be classified as background, either because there is no information available from which to classify them, or because it is deemed desirable ecologically to treat these areas as undefined background.
Total landscape area.--All positively valued cells, including the interior background, are included in the total landscape area calculation; negatively valued cells are ignored.
Edge length metrics.--Because a border is present but contains some background and there is interior background, only a portion of edges are explicit; that is, some edges abut background (either interior or exterior) and it is not explicit whether they represent true edge or not. In this case, the user must specify the proportion of edges involving background to include as edge.
Core area metrics.--Because a border is present but contains some background and there is interior background, only a portion of edges are explicit. Here, all boundary edges involving background and interior background edges are treated the same. The user must specify the edge depth for cells abutting background in the edge depth file, and this depth is applied to all background edges (in this case, both on the landscape boundary and interior). All other edges are explicit; their edge depths are specified in the edge depth file.
Contrast metrics.--Because a border is present but contains background, and there is interior background, only a portion of edges are explicit. Here, all boundary edges involving background and interior background edges are treated the same. The user must specify the edge contrast weight for cells abutting background in the edge contrast weight file, and this weight is applied to all background edges (both on the landscape boundary and interior). All other edges are explicit; their contrast weights are specified in the edge contrast weight file.
Aggregation metrics.--Because a border is present but contains some background, and there is interior background, only a portion of edges are explicit. In this case, edge segments involving background (both on the landscape boundary and interior) are ignored in the adjacency calculations.