Model Benchmarking

[Phil Hurvitz]

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
> 
> 
>