Class Metrics
Class metrics are computed for every patch type or class in the landscape; the resulting class
output file contains a row (observation vector) for every class, where the columns (fields)
represent the individual metrics. The first two columns include header information about the
class:
(C1) Landscape ID.--The first field in the class output file is landscape ID (LID).
Landscape ID is set to the name of the input image obtained from the input file (see Run
Parameters).
(C2) Patch Type.--The second field in the class output file is patch type (TYPE).
FRAGSTATS contains an option to name an ASCII file (class properties file) that
contains character descriptors for each patch type. If the class descriptor option is not
used, FRAGSTATS will write the numeric patch type codes to TYPE.
There are two basic types of metrics at the class level: (1) indices of the amount and spatial
configuration of the class, and (2) distribution statistics that provide first- and second-order
statistical summaries of the patch metrics for the focal class. The latter are used to summarize the
mean, area-weighted mean, median, range, standard deviation, and coefficient of variation in the
patch attributes across all patches in the focal class. Because the distribution statistics are
computed similarly for all class metrics, they are described in common below:
Class Distribution Statistics.--Class metrics measure the aggregate properties of the
patches belonging to a single class or patch type. Some class metrics go about this by
characterizing the aggregate properties without distinction among the separate patches that
comprise the class. These metrics are defined elsewhere. Another way to quantify the
configuration of patches at the class level is to summarize the aggregate distribution of the patch
metrics for all patches of the corresponding patch type. In other words, since the class represents
an aggregation of patches of the same type, we can characterize the class by summarizing the
patch metrics for the patches that comprise each class. There are many possible first- and second-order statistics that can be used to summarize the patch distribution. FRAGSTATS computes the
following: (1) mean (MN), (2) area-weighted mean (AM), (3) median (MD), (4) range (RA), (5)
standard deviation (SD), and (6) coefficient of variation (CV). FRAGSTATS computes these
distribution statistics for all patch metrics at the class level. In the class output file, these metrics
are labeled by concatenating the metric acronym with an underscore and the distribution statistic
acronym. For example, patch area (AREA) is summarized at the class level by each of the
distribution statistics and reported in the class output file as follows: mean patch area
(AREA_MN), area-weighted mean patch area (AREA_AM), median patch area (AREA_MD),
range in patch area (AREA_RA), standard deviation in patch area (AREA_SD), and coefficient
of variation in patch area (AREA_CV).
|
MN (Mean) equals the sum, across all patches
of the corresponding patch type, of the
corresponding patch metric values, divided by
the number of patches of the same type. MN is
given in the same units as the corresponding
patch metric.
|
|
AM (area-weighted mean) equals the sum,
across all patches of the corresponding patch
type, of the corresponding patch metric value
multiplied by the proportional abundance of the
patch [i.e., patch area (m2) divided by the sum of
patch areas].
|
|
MD (median) equals the value of the
corresponding patch metric for the patch
representing the midpoint of the rank order
distribution of patch metric values for patches of
the corresponding patch type.
|
|
RA (range) equals the value of the
corresponding patch metric for the largest
observed value minus the smallest observed
value (i.e., the difference between the maximum
and minimum observed values) for patches of
the corresponding patch type.
|
|
SD (standard deviation) equals the square root
of the sum of the squared deviations of each
patch metric value from the mean metric value
of the corresponding patch type, divided by the
number of patches of the same type; that is, the
root mean squared error (deviation from the
mean) in the corresponding patch metric. Note,
this is the population standard deviation, not the
sample standard deviation.
|
|
CV (coefficient of variation) equals the standard
deviation divided by the mean, multiplied by
100 to convert to a percentage, for the
corresponding patch metric.
|