I am curious about the use of 100 realizations to generate a probability
map. is this a standard approach? if so, is a "small" p-value (such as
.05) used? if so, it would seem like 100 iterations might be a smallish
sample size for distinguishing, say, .05 (ie 5 outcomes out of 100) from,
say, .01. is 100 used because it seems like it is a reasonable number or
because of the computer time restrictions?
do geostat folks treat these as realizations or as pseudo-realizations?
brian
****************************************************************
Brian Gray
USGS Upper Midwest Environmental Sciences Center
575 Lester Avenue, Onalaska, WI 54650
ph 608-783-7550 ext 19, FAX 608-783-8058
[EMAIL PROTECTED]
*****************************************************************
Chaosheng Zhang
<Chaosheng.Zhang@nui To: [EMAIL PROTECTED]
galway.ie> cc: Dave McGrath
<[EMAIL PROTECTED]>
Sent by: Subject: AI-GEOSTATS: Risk
Assessment with Gaussian Simulation?
ai-geostats-list@uni
l.ch
04/27/2002 10:25 AM
Please respond to
Chaosheng Zhang
Dear list,
First, I would like to say thank you to Gregoire for keeping this list
alive.
I'm trying to do "risk assessment", and I have some questions about risk
assessment with Gaussian Simulation:
(1) How to produce a probability map?
With Gaussian simulation, we can produce many maps/realisations, e.g., 100.
Based on the 100 maps, a probability map of higher than a threshold can be
produced. I wonder how to produce such a probability map? My understanding
is that for each pixel, we just count how many values out of the 100 are
>threshold, and the number is regarded as the "probability". Am I right? It
seems that this is a time consuming procedure with GIS map algebra. Are
there any suggestions for a quick calculation?
(2) Is a probability map better than a Kriging interpolated map for the
purpose of risk assessment?
(3) Is "PCLASS" function in IDRISI 32 Release 2 better/easier than the
probability map from Gaussian simulation?
>From the online help of IDRISI 32 R2, Section "Kriging and Simulation
Notes", it says "If the final goal of simulated surfaces will be to
directly reclassify the surfaces by a threshold value, and calculate a
probability of occurrence for a process based on that threshold,
conditional simulation may be unnecessary. Instead kriging and variance
images may be created and then used together with PCLASS." Any comments?
(4) How to carry out "PCLASS"?
Following the above question, I have a problem in doing PCLASS. I cannot
input the file name of Kriging variance to the field of "Value error" of
the documentation file. It seems that this field only accepts a "value",
not an "image file name" or anything in text. Anyone has the experience?
Cheers,
Chaosheng Zhang
=================================================
Dr. Chaosheng Zhang
Lecturer in GIS
Department of Geography
National University of Ireland
Galway
IRELAND
Tel: +353-91-524411 ext. 2375
Fax: +353-91-525700
Email: [EMAIL PROTECTED]
[EMAIL PROTECTED]
Web: http://www.nuigalway.ie/geography/zhang.html
=================================================
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