iMOD User Manual version 4.4 (html)

11.2Tutorial 2: Map Operations

This tutorial gives an introduction to several map operations using IDF-files. See for more detailed references Section 6.7.


This is what you will do:

Required Data

For this tutorial you need the following iMOD Data Files (IDF):

All these files are located in the folder: {installfolder} \tutorials \TUT_DATA_MAP_OPER.

Note: {installfolder} refers to the full path of the directory you installed iMOD in (e.g. D:\iMOD). Note: If you are a left-handed person and you converted your mouse button settings, ’left mouse button’ should be ’right mouse button’ and vice versa in these tutorials.

Getting Started

Calculate Layer Thickness

Quite often it is necessary to compute the difference between two maps. In this example we compute the thickness of a particular model layer. We start by opening the files: TOP_LAYER3.IDF, BOTTOM_LAYER3.IDF from disk.

After the files have been opened, those files will be added to the list of opened iMOD files in the iMOD Manager. iMOD does not modify the zoom extent, so we need to instruct iMOD to zoom on the loaded IDF files.

Let us compute the thickness of model layer 3.

Alternatively, you can select those files by left click your mouse and drag your mouse position over the files. If necessary, deselect those files that are undesired by clicking your left mouse button in combination with the Ctrl button.

The selected IDF-files (inputfiles) will be filled in (Map A and Map B), as well as the outputfile (Map C). On default the output file will be saved in the folder: {IMOD_USER} \TMP \DIFF.IDF. The default equation (Formulae is [C=A-B]) subtracts the first IDF minus the second IDF, in the order in which those IDF-files will appear in the iMOD Manager. Whenever your BOTTOM_LAYER3.IDF is mentioned before the TOP_LAYER3.IDF in the iMOD Manager, you have to change the Formulae into [C=B-A].

In the lower part of the window 3 options are given to define the size of the IDF to be created.

This allows the size of the THICKNESS3.IDF to be exactly the size of the first mentioned IDF in the formulae (A in this case). Whenever you need results for the current zoom extent only, click the option Window instead, this will speed up the calculation since only a part of the selected maps (Map A and Map B) will be subtracted.

The computed difference between the files TOP_LAYER3.IDF and BOTTOM_LAYER3.IDF will be saved in the TMP folder of your IMOD_USER environment by the name THICKNESS3.IDF. iMOD has added that file to the iMOD Manager, automatically.

Let us check the result.


Figure 11.19: Example of the use of the ’Map Value’ button when moving the mouse over the canvas.

Since this kind of visual inspection is rather easy to use, it is recommended to use it frequently to check any computations.

Calculating Permeability

Each resulting IDF can be used subsequently for other map operations.

All values in Map A (T-VALUE_LAYER3.IDF) will be divided by the values of Map B (THICKNESS3.IDF). If map A and map B are reversed, the equation can be entered as [C=B/A] without interchanging the IDF name next to the field Map A and Map B.

To decrease computational times, map operations can be computed for the current zoomlevel on the graphical display only. The resulting IDF will have dimensions equal to the zoom level, however, cellsizes will be copied from the first mentioned IDF in the equation.

Bear in mind that the computed legend classes are initially based on the minimum and maximum values of the IDF-file(s). Whenever you do not see much detail on the map, those minimum and maximum values might be far apart from each other. Apply a Percentile legend or click the Histogram button (  pictures/h1-h3/image207.png ) on the Stretched tab) on the Adjust Legend window. Looking at this graph can provide you with more insight in the colour distribution of your legend. Handling legends has been described in more detail in Tutorial 1.

Conditioned Map Operation

IDF Edit is a tool in which map operations can be applied for a particular selection of cells. In this tutorial a simple example is demonstrated. Suppose a map is required that shows all areas of the third model layer that have a thickness of more than 25 meter.

First we make an empty copy of THICKNESS3.IDF and name it THICKNESS3_25.IDF.

By means of the Map Calculator it is easier to make copies of IDF-files, rather than using the Windows Explorer, since the content can be blanked out and/or the resulting IDF can be resized (use the option Window or the optionx1,y1,x2,y2 where you can specify specific coordinates yourself).

Next step is to enter IDF Edit.

It is not relevant what IDF is (de)selected in the iMOD Manager, since all IDF-files that are inside the iMOD Manager can be manipulated in IDF Edit. Important is to specify an IDF that operates as a template. All mids of raster cells inside that particular IDF will be used to store any selection. Please be aware of the fact that a coarse IDF used as a template, will not make adjustments to a finer IDF.

The current selection will be displayed as filled rectangles. Especially whenever a large selection needs to be displayed it can take a while.


Figure 11.20: Example of displaying the selected grid cells using the ’Show Selection’ button in the ’IDF Edit Select’ window.

A selection is stored on disk in the {IMOD_USER} \TMP folder. For iMOD-user a specific filename will be used, {USERNAME}SELECTED{i}.DAT. These files will store the current (i) and previous selections (i-?). As long as these files exist, iMOD can undo a selection (i>1).

This window offers several functions to adjust values in IDF-files of the current selection.

The iMOD Manager can be used whenever the IDF Edit Calculate window is active. Let us check whether the computation has been carried out correctly.

As long as the IDF Edit Calculation window is active, any computation to any IDF can be undone.

Let us see what other method can be used to make a selection and/or calculation.

This option allows you to make a selection that is determined by the location that you will select on the graphical window for all cells that have values greater than the value at the selected point. The selection should be connected to each other which makes it quite different from the previous selection method. Let us do that.


Figure 11.21: Example of displaying selected cells using the Trace option.

Finally we can adjust the current selection by means of a drawing functionality that allows you to interactively draw regions to add to the selection and/or remove from the current selection.

Map Scaling One of the great options of iMOD is its ability to rescale data files. A variety of up- and downscaling algorithms have been implemented. In this example we will rescale the top elevation of a model layer from a cell size of 100 x 100 meter into a cell size of 1000 x 1000 meter.

On this particular tab, a variety of up- and downscaling options are available by the menu fields Upscale Formulae and Downscale Formulae.

This formula takes the arithmetic mean for all values that lie inside a coarsened raster of the resulting IDF-file.

Experiment with different Formulae for up- and downscaling.


Figure 11.22: Results of the upscaled TOP_LAYER3_SCALED.IDF-file (cell size 1000x1000 meter).