If a groundwater contour map is to be interpolated, a simple interpolation of the measurement values, as a rule, leads to unsatisfactory results. In spite of this, the model files often contain data which can be taken into account during the interpolation. These include:
fixed potentials (POTE),
source/sink terms (KNOT),
receiving water course (VORF and idealised leakage data LERA resp. LEKN ),
areas of equal unknown potential heads (GLEI),
minimum values for distance between ground and groundwater-level,
Q=0 boundary conditions and
transmissivities (TRAN)
After activating the check box "additional interpolation points" more check boxes appear where the user can define which additional data should be considered during the interpolation.
Additional data for the interpolation of a groundwater contour map
After each interpolation some potential heads are generated for a new interpolation job. They are used as additional auxiliary points in the local sphere of influence of the listed data. The results of the first interpolation with measured data (file including data for interpolation) are used for the calculation of these auxiliary points
As „Additional interpolation points“ appears only data which is assigned as attribute in the model file.
Example: If instead of the attribute „LEKN“ only the attribute „LERA“ is assigned in the model file, the check box „receiving waters with leakage“ does not appear. („LEKN“ has to be assigned explicitly!)
(1) fixed potentials: All nodes assigned to POTE are used as additional auxiliary points (equal to measured data) for a new interpolation.
(2) funnels around sources and sinks: For nodes assigned to KNOT and inside the estimated range additional auxiliary points will be created with the help of the initial k-values. Then these nodes are assigned to the calculated reduced or increased potential head for another interpolation.
For this k-values (KWER) and element thicknesses of the aquifer have to be available global after a previously performed model checking.
The thicknesses need not necessarily be defined on the data type MAEC, but can also result from the
Calculation of the initial and maximum thickness .
(3) Receiving waters with leakage:
The qualitative influence of an aquifer on the groundwater can be simulated through the definition of a substitute leakage coefficient for all nodes with defined water course level (VORF). For this purpose you must assign the data LEKN to all nodes with defined water course level. Dependent on their connection to the groundwater they get values between 0 (without connection) and 1 (total contact). For these nodes now additional auxiliary points are generated. The potential head is calculated linear between the water course level and the interpolated potential.
Attention: The above described substitute leakage coefficient overrides the for the groundwater simulation needed and defined leakage. It is only useful for the interpolation of water table contours with additional information. For all other calculations you must reinstall the original leakage coefficients!
Therefore, the original leakage values have to be restored or deleted from the model file using an editor or via the menu: Attributes 
Delete…
(4) Regions with equal potential heads: Taking into account regions of unknown equal potential heads GLEI is equivalent to the interpolation of nodewise data. But in contrast to the interpolation of nodewise data here the spatial effect is taken into account.
(5) Minimum distance between ground level and groundwater level: With the help of the data KKKK (general node data) you can control keeping the nodewise potential head at a minimum distance between ground level and ground water level. For this you have to assign a specific minimum distance greater than 0 to the affected nodes. These values are checked before and after each interpolation step.
(6) Boundaries without in- and out flow: For taking into account this boundary condition you can assign the data KNOT = 0.0 to the affected parts of the boundaries. The element centres of the so created boundary poly-lines are reflected and you get constraint points at both sides of the boundaries. The potential head at these points is calculated with the help of another interpolation with max. 5 iteration steps.
(7) Transmissivities: The influence of the permeability (KWER) and the initial thickness (MAEC) can be taken into account while determining the element transmissivities between two points of measurement. Additional auxiliary points are only created in aeras where no other auxiliary points exist and then only when the transmissivities between two measuring points differ more than 10%.
Priorities:
When taking into account all values of influence (1) - (4) it is possible, that different influences create auxiliary points at the same FE-node. Since this causes local contradictions you have to specify clear and complete priorities for these special cases. Number 1 means greatest priority.
The given priority results in the order of selection of the parameters of influence. She can be changed later. Giving priorities to the other parameters of influence is not required, because the there created auxiliary points base on the other values. The parameters of influence (5) - (7) are dealt with in the order of their window position.
The interpolation with additional auxiliary points creates the data file <stuetzTMP.txt> (standard text format). The file includes all created auxiliary points with their potential heads, additional to the file with measuring data.