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AI-GEOSTATS: forests on slopes

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  • remko duursma
    Dear listmembers, I have a question: How much of the world s forests are located on slopes greater than 10%? Ideally, an answer would consist of a frequency
    Message 1 of 4 , Dec 4, 2000
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      Dear listmembers,

      I have a question:

      "How much of the world's forests are located on slopes greater than 10%?"

      Ideally, an answer would consist of a frequency distribution of forests by slope class.

      Thanks,

      Remko Duursma


      Ph.D. student
      Department of Forest Resources
      University of Idaho


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    • Patrick van Laake
      ... Remco, I suggest you browse to the website of the EROS Data Center of the USGS. There you can pick up GTOPO30, a 30 arc-second DEM from which you can
      Message 2 of 4 , Dec 4, 2000
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        remko duursma wrote:

        > Dear listmembers,
        >
        > I have a question:
        >
        > "How much of the world's forests are located on slopes greater than 10%?"

        > Ideally, an answer would consist of a frequency distribution of forests by slope class.
        >

        Remco,

        I suggest you browse to the website of the EROS Data Center of the USGS. There you can pick up GTOPO30, a 30 arc-second DEM from which you can extract slope. While you are there, you can pick up the SLCR (for
        Seasonal Land Cover Region) data set which indicates different forest types a.o. land use classes. Both data sets are global and have a cell size of about 1km.

        With this data (and the right GIS software) you can yourself derive statistics for any slope class, slice, correlate with other data sets, differentiate by continent, ecoregion, forest type, and many more things.

        Much more fun doing it yourself than just getting a frequency distribution.

        Enjoy,
        Patrick van Laake

        >
        > Thanks,
        >
        > Remko Duursma
        >
        > Ph.D. student
        > Department of Forest Resources
        > University of Idaho


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      • Chuck Ehlschlaeger
        ... Before you spend time downloading files, you should ask yourself the following questions: 1) What is MY definition of slope? Do you want to use slope
        Message 3 of 4 , Dec 4, 2000
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          >> "How much of the world's forests are located on slopes greater than 10%?"
          >
          >
          >> Ideally, an answer would consist of a frequency distribution of forests by slope class.
          >>
          >
          > I suggest you browse to the website of the EROS Data Center of the USGS. There you can pick up GTOPO30, a 30 arc-second DEM from which you can extract slope. While you are there, you can pick up the SLCR (for
          > Seasonal Land Cover Region) data set which indicates different forest types a.o. land use classes. Both data sets are global and have a cell size of about 1km.
          >
          > With this data (and the right GIS software) you can yourself derive statistics for any slope class, slice, correlate with other data sets, differentiate by continent, ecoregion, forest type, and many more things.
          >

          Before you spend time downloading files, you should ask yourself the
          following questions:

          1) What is MY definition of slope?

          Do you want to use slope values of land that has been aggregrated to 30
          arc seconds? At the equator, 30 arc seconds is about

          111km 1deg 1min 30sec 2
          ----- * ----- * ----- * ----- = .925km
          1deg 60min 60sec


          The slope values of 30 arc second data will be vastly different than 3
          arc second data, which is different than the slope values at 30m data.
          One of my cartography exercises is to have my students download a 7.5'
          DEM and do a slope analysis (at 30m resolution). They then do a slope
          analysis with the same data aggregated to 60m. There is always a difference.

          2) What is MY definition of forest?

          The Modifiable Areal Unit Problem (which I provided an example of
          earlier in this email) also exists for question 2. A 30 arc second
          square cell called forest is likely not to be the same thing as a 3 arc
          second cell called forest.

          In summary, find or build the data that will answer the question. Don't
          use whatever data is available.

          sincerely, chuck

          PS. If you are interested in using geostatistics to represent the
          uncertainty of overly generalized data on applications, the following
          articles might be of interest:

          "Representing Uncertainty of Area Class Maps with a Correlated Inter-Map
          Cell Swapping Heuristic," by Charles R. Ehlschlaeger. Submitted to
          Computers, Environment, and Urban Systems on May 5, 1999, accepted
          January 20, 2000. Published Vol. 24, No 5, pp 451-69. URL:
          http://geography.hunter.cuny.edu/~chuck/urban/urban.html.

          "Visualizing Spatial Data Uncertainty Using Animation," by Charles R.
          Ehlschlaeger, Ashton M. Shortridge, and Michael F. Goodchild. Submitted
          to Computers in GeoSciences in September, 1996. Accepted October, 1996.
          Published Vol. 23, No 4, 1997, and at URL:
          http://geography.hunter.cuny.edu/~chuck/CGFinal/paper.htm and
          http://www.elsevier.nl/locate/cgvis/

          --
          Chuck Ehlschlaeger 212-772-5321
          Dep. of Geography fax: 914-407-2029
          Hunter College chuckre@...
          695 Park Ave. secure: chuckre@...
          New York, NY 10021 http://geography.hunter.cuny.edu/~chuck/

          "Everybody is in favor of the First Amendment, but we'd have a hell
          of a time ratifying it today." Senator (VT) Patrick J. Leahy


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        • Patrick van Laake
          ... Chuck, I completely agree with you. Too many people already use data that is not suitable for the kind of analysis they are trying to perform, either
          Message 4 of 4 , Dec 5, 2000
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            Chuck Ehlschlaeger wrote:

            > >> "How much of the world's forests are located on slopes greater than 10%?"
            > >
            > >
            > >> Ideally, an answer would consist of a frequency distribution of forests by slope class.
            > >>
            > >
            > > I suggest you browse to the website of the EROS Data Center of the USGS. There you can pick up GTOPO30, a 30 arc-second DEM from which you can extract slope. While you are there, you can pick up the SLCR (for
            > > Seasonal Land Cover Region) data set which indicates different forest types a.o. land use classes. Both data sets are global and have a cell size of about 1km.
            > >
            > > With this data (and the right GIS software) you can yourself derive statistics for any slope class, slice, correlate with other data sets, differentiate by continent, ecoregion, forest type, and many more things.
            > >
            >
            > Before you spend time downloading files, you should ask yourself the
            > following questions:
            >
            > 1) What is MY definition of slope?
            >
            >
            > 2) What is MY definition of forest?
            >
            >
            > In summary, find or build the data that will answer the question. Don't
            > use whatever data is available.
            >

            Chuck, I completely agree with you. Too many people already use data that is not suitable for the kind of analysis they are trying to perform, either because there is no better data, but more likely so because they do
            not know how to evaluate improperly documented data and therefore know no better.

            Remco's question, however, explicitly stated global coverage. Apart from the computational limitations in choosing a finer granularity of the data, there are two reasons why I so casually mentioned these two data
            sets:

            * They are the best data sets available with "uniform" global characteristics (GTOPO30 is actually constructed from many different sources with varying degrees of fidelity). For instance, both data sets are being used
            by the Food and Agriculture Organization of the UN to construct their Forest Resources Assessment 2000 analysis, if that is any indication.

            * At a global scale the problem generalization and data aggregation that are generally applied do not merit or even support using better data. Remco's request for a frequency distribution to me seems coincident with
            this.

            Still, the call on using this data is Remco's.

            Regards,
            Patrick


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