## 607Re: AI-GEOSTATS: Risk Assessment with Gaussian Simulation?

Expand Messages
• Apr 30, 2002
Dear all,

Thanks for so many interesting replies and thoughtful discussion. This is
not a summary yet, as I am expecting more to come.

Just to express my feeling about Indicator Kriging. To produce a probability
map, IK might be one of the choices. However, I always feel that too much
information is lost when doing the indicator transformation. When I see so
many "0"s in a dataset, I just feel the data quality is too poor.

Well, the other method of combination of Kriging and Kriging variance for
risk assessment has not been well discussed yet, and I would like to read

My last question "(4) how to carry out "PCLASS" " is now answered by the
developer of Idrisi. The fact that the file name of Kriging variance cannot
be entered (with Metadata command) is a bug of the program, which will be
corrected soon. At present time, a text editor may be used to modify the
image documentation file.

Now, let me discuss how I would like to make a probability map based on
Kriging and Kriging variance. For each pixel of the Kriging interpolated
map, there is a value of Kriging variance. The Kriging variance is a measure
of uncertainty (which is related to sampling density and spatial variation,
etc.???). If we assume that the value of the Kriging pixel follow a normal
distribution and the standard deviation is equal to the SQRT of Kriging
variance, the probability of any threshold can be calculated. Furthermore,
to make the risk assessment more realistic, I would like to include other
errors, such as sampling error and laboratory analysis error into risk
assessment. These errors can hardly be quantified, but if we say 10% or 20%
of the pixel value (for soil samples), perhaps there is no objection.
Therefore, the standard deviation of the pixel is increased by adding this
kind of errors.

I am not clear how to calculate the total standard deviation of the two
sources, is it:
Total standard deviation =
SQRT (Kriging Variance + SQUARE (Sampling Errors) ) ?

Any ideas and comments on this method?

Chaosheng Zhang

> On Sat, 27 Apr 2002, Chaosheng Zhang wrote:
>
> 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: Chaosheng.Zhang@...
> ChaoshengZhang@...
> Web: http://www.nuigalway.ie/geography/zhang.html
> =================================================
>

--
* To post a message to the list, send it to ai-geostats@...
* As a general service to the users, please remember to post a summary of any useful responses to your questions.
* To unsubscribe, send an email to majordomo@... with no subject and "unsubscribe ai-geostats" followed by "end" on the next line in the message body. DO NOT SEND Subscribe/Unsubscribe requests to the list
* Support to the list is provided at http://www.ai-geostats.org
• Show all 10 messages in this topic