Hi, @chaitanyamalla.

Thanks for your question. I’ll be happy to guide/assist you.

Before testing your scenario out, if you can, could you provide your datasets or a workspace template? It’ll help to see if there's a way to approach this. If not, not to worry, I can try to gather some resources/recreate your scenario.

All the best! 

HI @AliAtSafe, Unfortunately, i am not allowed to share the data, due to sensitive information. CAD cable dats is small, but Pointcloud LAS is very big data. 

I was able to solve it partially by using QGIS & FME, but not fully.

DXF can be opened into QGIS directly or by converting into QGIS Geopackage (import export dxf) tool. I have taken points on cable network every 1m interval, and extracted Z values of cables onto points. I have converted LAS to RASTER DEM  .tif format so that every pixel has elevation value. Now I extracted pixel value to overlying Cable points. 
Now our points data has both Z values(from cable and from elevation pixel values), just creating new attribute with difference of Z values. can give me points which are 0-50cm by filtering them. I can now tell approximately, the length of cable below ground tll 50cm by multiplying points with 1m.  

Hi @chaitanyamalla,

Please send the point cloud data to a TINGenerator transformer to created a TIN surface, then use an Extruder to extrude the TIN surface by -50 cm. Next add a Clipper, with the extruded TIN as the Clipper and the DXF cable lines as the Clippees. The Clipped output should be all the cable segments that are between 0 and -50 cm below the ground.