- Apr 30, 2002Dear 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

more comments.

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:

alive.

>

> Dear list,

>

> First, I would like to say thank you to Gregoire for keeping this list

>

assessment with Gaussian Simulation:

> I'm trying to do "risk assessment", and I have some questions about risk

>

can

> (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

> be produced. I wonder how to produce such a probability map? My

Am

> 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".

> I right? It seems that this is a time consuming procedure with GIS map

directly

> 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

> 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?

the

>

> (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

> 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

> =================================================

>

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