How to calculate the difference in elevation between two datasets of points?

I have two datasets of different points in the same coordinate system, which describe the same line (road), so I want to see the difference in elevation between them.

I've been told that I should transform them to lines and then find the intersection points and calculate the difference in elevation there. But how can I do it in QGIS? Is there any plugins that I can use to somehow present the points as a route and then compare the two routes?

The first dataset derived from NRTK (vrs) measurements during a kinematic positioning on a road (1518 points). The second dataset derived from measurements by total station 30 years ago (100 points) so there is no common point between the datasets. I need to describe the elevation difference of the road, so I have to find a way to compare the datasets. Both datasets represented as E,N (projected), H (orthometric).

• What you have been told can lead to arbitrary and even ridiculous results. In extreme cases, there may be no intersection points at all. Do you have elevations at these points or are you picking them up from some DEM? Commented Mar 21, 2017 at 16:28
• i have elevations for both datasets. One is from NRTK-GPS measurements and the other from total station measurements 30 years ago, so i wanted to compare them. Both datasets are derived from the same part of a road so I guess there will be some common points on their lines! No?
– Dino
Commented Mar 21, 2017 at 16:40
• It all depends on how the points on the road are represented. There are many ways in which they could be slightly displaced from one another in a way that forms no intersections--a change from the NAD27 to the NAD83 datum will do that in many cases, for instance. It's not at all clear you even need to create routes--maybe you only need to find nearby GPS points to the total stations. If you could give more details about the data, you would improve your chances of getting a good answer. Commented Mar 21, 2017 at 19:09
• Thank you very much, I will edit my post and add more details.
– Dino
Commented Mar 21, 2017 at 20:10

I think I'd try a few ways and see how it looks.

My first way, and the easiest, is to use FME to create a 3D TIN out of each. Maybe clipped to a small buffer of the original line. Then you can difference the two TINs and view the 'change' surface.

Another way would be to somehow intersect the two lines and interpolate (linearly, along the line?) the height of the other second line onto points of the first line. This is not a standard GIS operation, so it's probably going to require programming.

Creating surfaces is the simplest, though.