If I understood you correctly, you have a 2D Point Shapefile. Horizontally, these points spatially do not coincide with computational nodes of the mesh. Instead, these points are located within triangles. Based on this setting, you want to derive the Z-elevation at the locations of the Points stored within the Shapefile.
If this is true, you nmay develop a plugin (C/C++) or a script (Python) using the FEFLOW API (Application Programmers Interface). Use the following API function:
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#include <ifm/document.h>
double IfmGetZValueAtXYSlice
(
IfmHandle pDoc,
double x,
double y,
int slc,
IfmBool *ine
);
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Alternatively, you can easily do this task in GIS. Export the elevations of the Slice of interest into a Point Shapefile by right-clicking on Elevations in the Data Panel ==> Export Data. Load this file in GIS and generate a TIN (Triangulated Irregular Network). After that, use your initial Point Shapefile (the one you did not exported from FEFLOW) and write/project the elevations from the TIN into the attribute table of the shapefile. The Toolbox function in ArcGIS is called [b]AddSurfaceInformation[/b]. Older ArcGIS versions also called this function [b]SurfaceSpot[/b].
As a last step in GIS you need to generate a 3D Shapefile based on the new column of the attribute table storing the Z-values.
Load the 3D Shapefile into the Maps Panel of FEFLOW. Right-click on that and choose Convert to – Location Set Points from the Context Menu. These Points are now listed in the Entities Panel as 3D Point Set. Activate this 3D Point Set and generate your field lines as usual.
As a general alternative you can simply start with the particle tracks from a Slice. In this way, you do not need GIS or the FEFLOW API.