Grid Origin, Grid Display Lines & Grid Post in Minex

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If you are setting up the Area Definition of a grid for the very first time in GEOVIA Minex, the most convenient method is to use the Dig function in the Grid Compute menu.

After two opposite corners of an area in the 3D Design Window are digitized (and an appropriate mesh size selected), the menu may now look like this:

Many Minex users leave the values in this menu exactly as digitized and continue with the creation of the grid. However, “best practice” dictates that the values for the Origin are rounded up or down to the nearest multiple of one-half of the mesh size and that the Extent values are rounded off to the nearest multiple of the mesh size (Figure 3).

Specifically, both the X and Y Origin values should be rounded down (i.e., made smaller) while the X and Y Extent values should be rounded up (i.e., made larger to compensate for the change in the Origin).

The first question most users will ask is:

“Why the stipulation of rounding the Origin up or down to the nearest multiple of one-half of the mesh size?”

The answer: Minex stores the value of each grid node in the center of the cell, which is exactly one-half the Mesh Size away from the corner of the cell.

Many other software packages that deal with grids store the value of each grid node at the lower left corner of the cell. Thus, when a Minex grid is exported, the coordinates of the grid nodes in the output file are easily recognized as representative of a regularly-spaced surface and are compatible with other software packages (for example, MineSched). This is especially true if the grid surfaces will eventually be converted into a block model.

The figure below shows a portion of the export of a Minex grid for which the Origin has been rounded-down (non-NULL values only). Note that the coordinates are even multiples of the Mesh Size.

To fully understand how Minex stores the grid node values, plot this grid in the 3D Design Window as Lines and query the intersection of two of these grid lines (Figure 5):

From the Output Window, you can see that the grid node is exactly at the intersection of a horizontal and a vertical grid line.

Now let’s plot the actual values of these grid nodes using the Grid Post function (Figure 6):

Again, the Z value displayed is at the intersection of grid lines. Now let’s plot the actual values of these grid nodes using the Grid Post function along with the grid cell sides (Figure 7):

The grid cell sides are the heavy, solid black lines as well as the heavy, dashed black lines in the figure above. This time it can be seen that the grid cell sides surround the grid node and are offset one-half the Mesh Size from the Grid Display – Lines.

In summary, Minex grid nodes are not stored at the corners of the grid mesh, but at a distance equal to half of the Mesh Size in both the X and the Y directions from the specified Grid Origin.

Displaying the grid as Lines as well as using the Grid Post function conclusively proves that this is the case. And even though at first it may seem strange to specify the Grid Origin at a value that is a multiple of one-half of the Mesh Size, it is worth the extra effort performing the rounding as described earlier as the resultant exported data will be much easier consumed by other software programs.

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Dereck Prince

Dereck Prince

Senior Solution Consultant, GEOVIA at Dassault Systèmes
Dereck has over 30 years of experience in consulting to and training GEOVIA Minex clients worldwide. He is an expert in all of the modules of Minex, and specializes in SQL macro writing and workflow automation using Tcl and VBA. Based in Florida, USA, Dereck is fluent in Spanish and English, and enjoys photography and travel. Dereck also enjoys Reggae, Calypso and Jazz music.