Model Benchmarking

[Phil Hurvitz]

I think the rule is 50% of the cell, but it might be the cell center.  I
asked ESRI recently on the behavior of ExtractByCircle, and the answer was
something to the effect of "I tried ... and the result I got was
...." which didn't really tell me anything about definitive rules.

-P.

******************************************************************************
Phil Hurvitz, MFR | GIS Specialist | College of Forest Resources | 355 Bloedel
Box 352100 | University of Washington, Seattle, Washington  98195-2100, USA
 tel: 206.685.8179 | FAX: 206.685.3091 | e-mail: phurvitz at u.washington.edu 
             WWW: http://lobo.cfr.washington.edu/~phurvitz/
******************************************************************************

On Tue, 23 Jan 2001, Keven Bennett wrote:

>     Figuring out what is 'reasonable' is another vagueness: statistically
> speaking, 95% confidence would probably be enough, and I guess this means doing
> the sampling you suggest in numerous selected areas and get a statistacally sound
> result, and an error bar for 95% (I forgot how many STTDEV's away from the mean it
> is for a two tailed test, I can look that up).
> 
>     As far as AV, then, I do lose the polys w/in cell boundaries with less than 50
> sq. meters?
>     Keven
> 
> 
> 
> Phil Hurvitz wrote:
> 
> > Hi Keven, what is considered 'reasonable?' I'd take the approach of using
> > a smaller section of the study are and using smaller cell size, then
> > progressively moving up in cell size.  You might get a feel for a minimum
> > size to properly model the phenomenon.  If you can answer the question is
> > "what is the smallest patch of spartina that can be considered a source
> > for spread?"  That might drive your initial choice of cell size.
> >
> > The problem with AV-SA's rasterization
> >
> >         anFTab.AsGrid ( aPrj, aField, gridSize)
> >
> > is that it doesn't give you any choice on what to do with partial cells,
> > whereas ArcInfo uses
> >
> >         POLYGRID <in_cover> <out_grid> {value_item} {lookup_table}
> >         {weight_table}"
> >
> > the weight table lets you choose how to handle partial cells).  So you are
> > stuck with small polys that disappear rather than creating single cells,
> > if that is what you wanted to do.  I'm sure other spatial modeling
> > software gives you this option as well.
> >
> > -P.
> >
> > ******************************************************************************
> > Phil Hurvitz, MFR | GIS Specialist | College of Forest Resources | 355 Bloedel
> > Box 352100 | University of Washington, Seattle, Washington  98195-2100, USA
> >  tel: 206.685.8179 | FAX: 206.685.3091 | e-mail: phurvitz at u.washington.edu
> >              WWW: http://lobo.cfr.washington.edu/~phurvitz/
> > ******************************************************************************
> >
> > On Mon, 22 Jan 2001, Keven Bennett wrote:
> >
> > >     I am working with ONRC in developing an iterative spread model of
> > > Spartina in Willapa bay.  The intent of the model is to give one the
> > > ability to 'see' how Spartina is distributed on the ground in a given
> > > time in a given year at 10 meter resolution.
> > >     We have the model running with perameters we believe are reasonable
> > > representations of the primary factors that affect the spread (with the
> > > exception of magnitude and direction of streamflows).
> > >     We want to benchmark our output with 1994 and 1997 data from DNR
> > > which is fairly accurate high resolution vector data derived from CIR
> > > aerial photography, but we have issues to address first:
> > >     We can't directly compare the two years with the two years
> > > corresponding output because the vector data captures polygons with
> > > areas much smaller than 100 sq. meters (10 meter cells).  The resonable
> > > assumption is that the model output should have less total area than the
> > > '94 and '97 data we want to accept as true.  The question is how much?
> > > Any suggestions on arriving at a figure for this?
> > >     If it is safe to make the assumption that any polygon less than 50
> > > sq. meters using ArcView's rasterization algorithms (convert to grid in
> > > correct analysis window), then can we justify that a match of our model
> > > output to the area arrived at by rasterizing the DNR data as 'reasonably
> > > accurate'?
> > >     These are kind of vague questions, but this is the kind of issues we
> > > are confronted with.
> > >     Keven Bennett
> > >
> > >
> > >
> 
>