iMOD User Manual version 4.4 (html)

7.13Define Startpoints

iMOD offers the possibility to trace particles throughout the model extent. For these simulations it is necessary to define the starting locations (startpoints) for these particles (pathlines).

The starting location for particles is defined inside polygons, rectangle, on grids, along lines and/or on the edge of circles around specified points. All these are specified in normal 3-D-coordinates (x,y,z) and will be translated to the model at trace time. In this manner, a startpoint definition, can be easily (re)used to other model resolutions and/or model extent.

Select the Toolbox option from the main menu and then choose Define Startpoints to start the Open/Create a Startpoint Definition window.

Open/Create a Startpoint Definition window:


Select a StartPoint Definition, etc …

Enter or select an existing *.ISD file from the list. Those *.ISD files are found in the {user}\startpoints folder, see Section 9.20 for more detailed information on *.ISD files.

Open / Create …

Click this button to open the selected *.ISD file and start the StartPoint Definition window.

Help …

Click this button to start the Help functionality


Click this button to close the Open

Start Point Definition window, Shapes tab:



Displays the name of the current opened *.ISD file from the folder {user}\startpoints.


Click this button to start the Select window in which you select the shape that defines the lateral position of the startpoints.

Select window


pictures/h5/image865.png Point
Click this option to define startpoints for a single point

pictures/h5/image866.png Rectangle
Click this option to define startpoints within a rectangle.

pictures/h5/image867.png Polygon
Click this option to define startpoints within a polygon.

pictures/h5/image868.png Circle
Click this option to define startpoints on a circle.

pictures/h5/image869.png Line
Click this option to define startpoints on a line.

pictures/h1-h3/image233.png Grid
Click this option to define startpoints on a grid.

pictures/h5/image859.png pictures/h5/image860.png pictures/h5/image861.png
pictures/h5/image862.png pictures/h5/image863.png

Click these buttons to draw, open, save, delete or rename a shape. More detailed information can be found in Section 2.6.2


Click this button to draw the spatial location of the startpoints of the selected shapes. This is different for each shape type. It is also a preview and might be different whenever the option Snap to Cell centres of to be used Model Grid is selected. In that case, the particles are “snapped” to the model grid centres of the current selected model.

Example of startpoints created by rectangle, polygons, circles, point and/or lines

Help ...

Click this button to start the Help functionality.

Close ...

Click this button to close the StartPoint Definition window. You will be asked to save the adjustments to the opened *.ISD file.

StartPoint Definition window, Definition tab, (1) for polygons (2) on circles and (3) along lines:

pictures/h5/image871.png pictures/h5/image872.png pictures/h5/image873.png

Distance X,
Distance Y

Enter the lateral distances in X and Y direction for those startpoints inside the selected Polygon or Rectangle shape.

Snap to Cell centres of to be used Model Grid

Click this item to snap the startpoints location to the nearest model centroid. For Circle shapes the centre of the circle is snapped to the nearest model centroid. The Line and Grid shape do not support this snapping.


Enter the radius of the circle. The is editable for the Point shape, for the Circle shape the radius can be modified interactively by modifying the node on the circle.


Enter the distance between the startpoints on the radius of the circle (Circle shape) or along the line (Line shape).

Top-Level (enter
IDF/numeric value)
Bottom-Level (enter
IDF/numeric value)

Enter a IDF that represents the top and/or bottom elevation of the startpoints, e.g. D:\TOP_L8.IDF and D:\BOT_L8.IDF. For each lateral position of a startpoint, the values will be read from the entered IDF-files. Moreover, you can enter a constant value (e.g., -1) to indicate that all values are constant.

Reference Top and Bottom by (IDF)

Check this item to enter an IDF-file as reference level. The values from this IDF will be used to add or subtract the values from the Top-level and Bottom-level. In the example in Figure 5-9d-i, the final values for the Top elevation will be those values in the D:\SURFACE.IDF minus 1 (the entered numeric value).


Open IDF
Select this button to select an IDF-file from the file selector.

Vertical Interval (number)

Enter the number of vertical intervals between the values from the top and bottom elevation. By entering the number:
1: iMOD will position one startpoint in-between the values for the top and bottom elevation;
2: will yield a startpoint equal to the top and a startpoint equal to the bottom elevation;
\(>\)2: yields startpoints equally distributed between the top and bottom elevation.

Note: iMOD will not check, at this time, for the non-existence of any entered IDF-file. These files will be checked at runtime for the particle tracking.

Note: It is also possible to use IPF file with x,y and z information instead of an ISD-file.