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FRAGSTATS
Frequently
Asked Questions
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This page contains answers
to frequently asked questions, including questions and answers extracted
from the FRAGSTATS list serve that warrant permanent posting.
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| Q1.
We ran some known cases through the new FRAGSTATS to gain a better appreciation
for the nearest neighbor distances. The new documentation says that it is
measured edge-to-edge. It appears that what is measured is the centroid-to-centroid
distance of the two nearest grid cells. So for example if we have two 30
x 30 m grid cells that are one cell apart, the expected nearest neighbor
distance is 30 m. The actual distance reported is 60 m. We have tested this
out with multiple patch sizes and I think that the deviation is due simply
to the distance from the centroid to the edge of the cell that is
closed to the neighboring patch. Is this correct? |
| This is one of the important
differences between the old version of FRAGSTATS (2) and the newest version
of FRAGSTATS (3). In the new version, nearest neighbor distance is calculated
from the center of the cell on the patch edge (i.e., centroid of the cell)
to the center of the nearest cell in another patch of the same class. So
this is cell-centroid to cell-centroid of the two nearest cells. This has
the effect of adding a single cell width to all nearest neighbor distances.
We implemented this for several reasons. In part, this was necessary to
accommodate the new similarity index so that you don't have a zero denominator
(i.e., the distance between adjacent patches). In addition, we decided that
it makes more sense ecologically than cell edge-to-edge distance, since
it better reflects the error associated with the raster data representation.
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| Q2. Are there any
limits to class ID values (i.e., the unique integer value assigned to each
distinct class or patch type)? |
| All classes must have non-zero integer values; that is,
zero should not be used as a class value because it is impossible to assign
a negative value to zerio to distinguish cells of that class that are outside
the landscape boundary. Otherwise, there are no limits to the class values
that can be used, although it is logical to use the smallest numbers possible. |
| Q3. Why are some features
(e.g., metrics) inactive in the FRAGSTATS interface? |
| FRAGSTATS is still in a state of development. Unfortunately,
we created the interface in anticipation of implementing a large number
of new features, but have not yet fully implemented or tested several features.
These features are inactive in the graphical user interface. Please note,
that some features will become inactive during the parameterization process
based on user selection of certain other features. For example, if you select
the moving window analysis option, then "patch metrics" will become
inactive in the run parameters dialog box, because patch metrics are not
suited for a moving window analysis. |
| Q4. How do I cite
FRAGSTATS for publication? |
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Unfortunately, we can't provide a proper journal or book citation for
the current version of FRAGSTATS. Currently, this website is the sole means
of "publication" of the software. We have a FRAGSTATS book in
the making, but it will be some time before it is finished. In the meantime,
I suggest the following citation:
McGarigal, K., S. A. Cushman, M. C. Neel, and E. Ene. 2002. FRAGSTATS:
Spatial Pattern Analysis Program for Categorical Maps. Computer software
program produced by the authors at the University of Massachusetts, Amherst.
Available at the following web site: www.umass.edu/landeco/research/fragstats/fragstats.html
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| Q5. How do I install
FRAGSTATS for use with Arc Grids? |
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As noted in the user guidelines under data formats, if you wish to use
FRAGSTATS 3x with ESRI Arc Grids, you must have a system PATH variable
in your environmental settings (Windows NT, 2000, XP) that points to a
necessary dll file that comes with the Spatial Analyst Extension or with
ArcGIS. This file is typically found in the bin32 folder (ArcView) or bin
folder (ArcGIS). This allows you to install and run fragstats from any
directory on your hard disk. Alternatively, you can run fragstats from
the bin32 or bin folder. Detailed instructions are provided in the Help
documentation (User
Guidelines--Overview--Data Formats).
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| Q6. Why is FRAGSTATS
unable to allocate sufficient memory? |
| FRAGSTATS is a compute-intensive program; its performance
is dependent on both processor speed and computer memory (RAM). Ultimately,
the ability to process an image is dependent on the availability of sufficient
memory; the speed of processing that image is dependent on processor speed.
Of particular note is the memory constraint. FRAGSTATS loads the input grid
into memory and then computes all requested calculations. Thus, you must
have sufficient memory to load the grid and then enough leftover for processing
and other operating system needs. Memory requirements are affected by grid
size, number of patches and number of classes. Of partiuclar
importance is the number of patches, which can vary by orders of magnitude
from one application to another. To determine whether you have enough memory
based solely on grid size, you can use the following formula: #cells*4bytes.
Thus, if you have a 256 rows by 256 columns grid, the minimum memory needed
is 256 kb (256*256*4/1024 bytes/kb). However, you also need additional
memory that increases with the number of patches and classes, and this
can be extremely constraining when working with complex (patchy) images.
The solution to this problem if it arises is to work with as much memory
as possible, simplify the grids, reduce grid size (e.g., resampling), subdivide
the grids into separate sections, etc. A long-term solution is in the works
that should greater reduce or even eliminate memory constraints. In the
meantime, you will have to work around the memory constraints. Note, the
memory requirement is especially constraining in the moving window analysis
because FRAGSTATS requires enough memory for the input grid plus one output
grid, plus enough leftover for other processing needs and system needs.
If the moving window analysis is selected, FRAGSTATS checks to see if it
can allocate enough memory for three grids (i.e., 1 input grid + 1 output
grid + enough leftover to insure performance). In the above example, you
would need at least 768 kb of memory to conduct a moving window analysis
at a minimum based on grid size, but an unknown amount of additional memory
depending on numbers of patches and classes. |
| Q7. Why does the Windows
task manager report that FRAGSTATS is "not responding"? |
| During a long FRAGSTATS run (e.g., a batch file), it may
be unclear whether FRAGSTATS is actually running or not. For reasons that
can't be explained here, the Status Bar may not report "processing"
when in fact the program is running. The surest way to confirm that FRAGSTATS
is actually running is to open the Windows Task Manager (for NT-based systems).
However, the Application tab will show that Fragstats is "Not Responding".
Open the Processes window and check to see if FRAGSTATS is using CPU and
memory. If it is, then your O.K., let it continue running. Unfortunately,
if you try to return to the FRAGSTATS window while it is still running,
the window may be blank. That's O.K. Just wait for the process to end. |
| Q8. I can't get FRAGSTATS
to run under Windows XP. It downloads and seems to install with no problems,
but when I double click on the icon or select it in my Start menu program
list, the hour glass appears for a few seconds and then dissappears, and
FRAGSTATS never starts up. |
| The symptoms you are experiencing are usually generated
by the ESRI license manager. The mechanism is simple, when FRAGSTATS starts
it looks in the system path for a grid library to load. If one is found,
it is loaded and initialized. During the initialization phase there are
two possible scenarios. If you have the ArcView companion avgridio.dll,
it does a few checks to see if it is part of a legitimate installations
and then allows running. If you have ArcGIS, then aigridio.dll calls the
license manager to verify legitimacy. In both cases, if the legitimacy is
not validated due to a variety of reasons (e.g., dll copy present in a folder
that is not part of the installation, misconfigured license manager, or
simply all licenses already in use) the library aborts the entire application.
Why they do that instead of returning an error is beyond me, but the result
is that the user sees an hourglass as part of the loading process and the
application is killed before even showing its main window. The solution
to the problem depends on which libraries are available on a particular
machine. If only ArcView is present, make sure there is only one copy of
avgridio.dll and that it is in the bin folder of the installation. If both
ArcView and ArcGIS are installed then, ...arcview\bin should be in front
of ...arcgis\bin as listed in the system's PATH variable. The first encountered
library is always loaded, and the one that came with the standalone ArcView
(up to version 3.3) + Spatial Analyst is easier to get going. If you only
have ArcGIS and/or ArcINFO Workstation, then recheck the license manager
configuration, available licenses (especially for the grid module), and
whatever else that can result in denying the right to execute the library,
including file system access rights (read, execute) for the user under which
Fragstats is trying to run. |
| Q9. What does the
error message "bad parameters internally passed" mean? |
| Unfortuntately, in version 3.3 this error message is generated
in 12 source files for several reasons. The most common reason is an empty
(or missing) input or output filename, but also zero rows or zero columns
in the input dataset, or no metric selected at any level. In the absence
of any additional info (input file type and name, metric selection, extra
messages prior or following the one in question...) it is impossible to
tell what exactly went wrong. If you get this error message after checking
and correcting the above mentioned problems, then please send a detailed
message with all relevant information to Kevin McGarigal. |
| Q10. Why can't I
load my ERDAS images? I have been experiencing some problems when trying
to run the Fragstats program, version 3.3. Specifically, I have repeatedly
received the message (Error Already Logged) when using Urdas files. I have
tryed multiple formats to get the program to run and I'm not having much
luck.
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| The culprit is likely the lack of suitable
units in the file's metadata. Fragstats expects cells to be in meters and
for this information to be in the file's header. If you re-generate the
file(s) with the cell size specified and meters given as units, the problem
will likely dissapear. If this doesn't solve the problem, you may want to
use the tool LANDISVIEW
to convert your ERGIS gis files into ASCII grids for compatibility with
FRAGSTATS. If you still have problems, then please send a detailed message
with all relevant information to Kevin McGarigal. |
| Q10. Why can't I
run Fragstats under ArcGIS 9.2 or greater?
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| Several users have reported problems trying to run Fragstats
under ArcGIS 9.2 or greater, when it ran fine under version 9.1. The long-term
solution requires work on the software. The short-term solution is to put
the Fragstats.exe and Fragstats.chm files in the ArcGIS Bin folder on your
hard disk and run it from that location. |
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