|
|
 |
 |
 |
 |
 |
 |
|
FRAGSTATS Frequently Asked Questions |
|
This page contains answers
to frequently asked questions, including questions and answers extracted
from the FRAGSTATS list serve that warrant permanent posting.
|
|
|
|
| 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.
|
| 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? |
|
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 |
| Q5. How do I install FRAGSTATS for use with Arc Grids? |
|
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). |
| 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. |
| 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? |
| 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. |
For more information, please contact:
Dr. Kevin McGarigal
Department of Natural Resources Conservation
University of Massachusetts
304 Holdsworth Natural Resources Center
Box 34210
Amherst, MA 01003
Fax: (413) 545-4358
Phone: (413) 577-0655
Email: mcgarigalk@nrc.umass.edu