New answers tagged length
If I understand correct, your question consists of 2 parts: 1. How to get many linestring to merge to 1 2. How to get the point 1000 km further down the road from point 1 Part 1 needs some more explanation from you. It seems you already did a linemerge and that works for me as well with the data you give. Maybe you can post a new stackexchange question ...
Maybe you could try saga-gis software for creating LS. I remember that there was a module called LS Factor to create that. Here is documentation for that module: http://www.saga-gis.org/saga_module_doc/2.1.3/ta_hydrology_22.html
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