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AI-GEOSTATS: Summary to nested models?

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  • Carolina Garcia Imhof
    Thank you very much to all people who answered my question. It was very helpful. Here’s a summary of the responses. Cheers, Carolina Dear list members, I
    Message 1 of 1 , Oct 24, 2002
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      Thank you very much to all people who answered my question. It was very
      helpful. Here’s a summary of the responses.
      Cheers,
      Carolina

      Dear list members,
      I have started using geostats only this year and therefore I appologise that
      my

      questions are too basic for this discussion list...
      I am using Variowin 2.2 and GeoEAS 1.2.1 to analise my data, and an
      early
      version of Surfer to contour the distribution. However, I've encountered
      various

      problems:
      1. When I use latitude and longitude in my DAT file, in what units is the
      variogram x-axis (h)?
      2. Surfer doesnt understand the grid file created by GeoEAS KRIGE. How
      do I
      transfer my results to surfer (until now, I've used the estimated values
      from
      cross validation to have surfer generate a grid file)?
      3. My variogram shows completely different behaviour from distance A to
      B, than

      for distance B to C. I've tried fitting various nested models, but I havent
      been

      able to come out with a good fit for the whole variogram. Is it possible to
      analise A-B data separately from B-C data?



      Hi Carolina,
      I'm new at this too, so have an aswer to your first question only. :)
      Basically, you can't use latitude and longitude to indicate the location of
      your samples in a geostatistical analysis because the distance between
      locations is not euclidean. When you calculate variograms, you are
      assuming
      that you are on a plane X-Y coordinate system. Lat and log account for the
      curvature of the earth. So unless you are dealing with very short distances
      you should first convert your lat and long locations into UTM coordinates
      (or other plane projection). I use a program called geotrans to convert lat
      and long to UTM, you can download it at
      http://164.214.2.59/GandG/geotrans/geotrans.html
      Once you have your locations in UTM, the variogram distance are
      measured in
      meters.

      Julian

      Julian M. Burgos

      University of Washington
      School of Aquatic and Fishery Sciences
      1122 NE Boat St. (Box 355020)
      Seattle, WA 98195-5020






      1- You need to transform your latitude longitude geographical data into
      UTM coordinates (or any equivalent transformation) because you need to
      work in metres or km (or cm!)

      > 2. Surfer doesnt understand the grid file created by GeoEAS KRIGE.
      How do
      I
      > transfer my results to surfer (until now, I've used the estimated values
      from
      > cross validation to have surfer generate a grid file)?

      If you want to krige witn Geo-Eas you have to use its CONREC program to
      have your map. Otherwise, if you like to use Surfer, you just need to
      krige there, using its own grid utility.


      > 3. My variogram shows completely different behaviour from distance A to
      B,
      than
      > for distance B to C. I've tried fitting various nested models, but I havent
      been
      > able to come out with a good fit for the whole variogram. Is it possible
      to
      > analise A-B data separately from B-C data?
      What are A,B,C? any kind of different variables, or just some different
      locations?
      If there are just different points you need to know the ir reliability
      before doing any assunption on them. If there are different variables,
      you do not need to suppose they behave in the same way.

      I think there is something wrong with your variogram. If variables are
      quite the same for small distances (flat variogram) they cannot start
      being different from one each other at large distances.
      Maybe you have chosen a bad lag (h), say a lag too short. It is very
      important for you to know the precision of your data (it doesnt make any
      sense to say temperature is 10.5 if your thermometer measures
      degrees!!)

      Hope this helps,
      Carme



      Carolina

      >
      >1. When I use latitude and longitude in my DAT file, in what units is the
      >variogram x-axis (h)?
      >
      Most geostatistics programs are going to treat the coordinates as though
      they are "x", "y", i.e., rectangular. If you are using Lat,
      Long these
      are not exactly rectangular coordinates. That is, one minute of Lat is
      one nautical mile but in general one minute of Long is not one nautical
      mile, the distortion can be very large at high latitudes. You need to
      change to UTM coordinates instead.

      As a further comment, most packages will only accept numbers in
      decimal form so if you have Lat, Long in degrees, minutes and seconds
      the program will either give you an error message or completely
      mis-interpret the entries. I let my students use Lat , Long in decimal
      form in my geostatistics class but ONLY in class and I warn them that
      there will be a distortion. The only reason I do this is that they are
      learning about how to do geostatistics and not using the end results for
      anything outside of the class.



      >
      >2. Surfer doesnt understand the grid file created by GeoEAS KRIGE. How
      do I

      >transfer my results to surfer (until now, I've used the estimated values
      from
      >cross validation to have surfer generate a grid file)?
      >
      I don't use Surfer so I can't completely answer your question. I assume
      by "grid file" you mean the output file from KRIGE. This is an ASCII

      file and easily edited in any text editor. The format is exactly the
      same as the input file (different column names of course). If you only
      estimated one variable then there will be four columns; two for
      coordinates, one for the kriged estimate and one for the kriging
      standard deviation. The header identifies the columns. You might have to
      strip off the header for use in Surfer.


      >3. My variogram shows completely different behaviour from distance A to
      B,
      than
      >for distance B to C. I've tried fitting various nested models, but I havent
      been
      >able to come out with a good fit for the whole variogram. Is it possible to

      >analise A-B data separately from B-C data?
      >
      I assume that A, B, C are points?? If so, your statement doesn't quite
      make sense. The sample variogram is not defined or dependent on a line
      segment. The plotted values of the sample variogram (or the computed
      values of the model variogram) are determined by lag distances. In that
      case, there is no reason to expect that the value for two different lag
      distances would be the same. Perhaps you are referring to different
      directions? In that case you would be describing an anisotropy, this
      would show up (assuming that you are using PREVAR and VARIO or
      using
      VARIOWIN) when you compute and plot directional sample variograms.
      For a
      geometric anisotropy, this would show as a change in the range of the
      variogram with respect to distance.

      Finally is it possible you are asking whether the region of interest can
      be split into two separate parts?? This is certainly possible, you
      would have to split the data set into two different parts estimate and
      model the variograms separately. Krige the two subregions separately
      using the respective variograms and data sets. It is likely that the
      grids (generated by the software) would overlap and you would have to
      check to see which grid nodes should be associated with which
      subregion
      and use the appropriate kriged value. If you were going to do this, you
      would want to ask if there is some property that changes between the
      regions to justify/motivate splitting the region into two parts.

      Donald E. Myers
      http://www.u.arizona.edu/~donaldm




      Carolina Garcia Imhof
      Marine Mammal Research Group
      Marine Science Department
      310 Castle Street, PO Box 56
      University of Otago
      Dunedin, New Zealand

      Fax: 64 3 479 8336
      Phone: 64 3 479 5476
      e-mail: garca917@...
      caro_gar@...

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