Domains

Overview

One of the most important aspects of geostatistics is to ensure that any data set is correctly classified into a set of homogenous "domains".  A domain is either a 2D or 3D region within which all data is related.  Mixing data from more than one domain or not classifying data into correct domains can often be the source of estimation errors.

You will learn about:

• the effect of domains on estimated values
• viewing and using domains

Requirements

In order to understand this information, you should know how to:

• display Surpac string files
• run a Surpac macro

The effect of domains on estimated values

Imagine that you are a meteorologist, and you are given three air temperatures measured at locations A, B, and C, as displayed below.  Based on the values shown, what would you guess the temperature is at location X?  Would you guess that the temperature at location X was greater than 25?

Using the information above, you may have the following thoughts:

1. Because location A is relatively distant from X, the value at A might have little or no influence on the estimated temperature at X.
2. Because locations B and C are about the same distance from X, they will probably have equal influence on the estimated temperature.
3. Given the previous two points, the temperature at X would probably be the average of the temperatures at B and C:  (18 + 32) / 2 = 25 degrees
4. Because the influence of A has not been accounted for at all, and the estimate is exactly 25 degrees, it is difficult to say with certainty if the temperature at X is above 25 degrees.

Now consider the following. Imagine that you want to go to your favourite beach, but only if the temperature is 25 degrees or more.  You have three friends who live near the beach you want to go to, and you call them up and ask each one what the temperature is at each of their homes.  You draw the map below, with the locations of each friend (A, B, and C) and the temperatures they give you.  Your favourite beach is at location X.  Note that the friend at location B lives high up in the mountains, while friends at A and C live near the beach.

Using the information above, you may have the following thoughts:

1. The data from B can be ignored, because temperatures high up in the mountains are usually not good estimates of temperatures on the beach. 
2. A and C are on the beach, so they can be used to guess the temperature at X.
3. Because X is between A and C on the map, the temperature at X will probably be somewhere between the temperature at A and the temperature at C.
4. Therefore, the temperature at X will be somewhere between 28 and 32 degrees
5. Because the temperature range of 28 to 32 degrees is greater than the minimum value of 25 degrees, you would probably decide “Yes, I’m going to the beach!”

Compare this example with the first one.  In both cases, all of the locations and temperatures are exactly the same.  However, in the second case, when you took account of the domain which contains the data, you came up with a considerably different result.  The point is that separating data into similar regions, or domains, is a very important part of making any geostatistical estimation.

Viewing and using domains

Task: View domains

1. Open gc130.swa.

This data represents some grade control samples, ore zone outlines, and a block model (constrained by two ore zones) on a single mining bench, or elevation of 130.

Task: Separate data for each domain

Using the ore zone outlines as domains, only those grade control samples which are inside each ore zone should be used to estimate blocks inside that ore zone.

1. Click Reset Graphics.
2. Open gc130.str
3. Display all strings as markers.
1. Select Display > Point > Markers.
2. Select Display > Hide strings > As lines.
4. Open gc_orezones130.str.

Notice that gc130.str contains the samples for the entire bench.



5. Choose File tools > Apply boundary string.
6. Enter the information as shown, and click Apply.



7. Choose File tools > Apply boundary string.
8. Enter the information as shown, and click Apply.



9. Click Reset Graphics.
10. Open gc_zone1_130.str.
11. Display all strings as markers.
12. Open gc_zone2_130.str.
13. Display all strings as markers.
14. Open gc_orezones130.str.

Notice that the samples within each domain are now contained within separate string files.

Note: This process applies to two dimensional domains. There is a different process for three dimensional data.

To see all of the steps performed in this task run 2d_01_apply_boundary.tcl.

Menu commands:

Select...	to...
File tools > Apply boundary string	apply a boundary to drillhole data.