Oh, this looks like an exciting and fun project. Although certainly a lot of work!
Just so I understand, in the attached screenshot, the big red arrow that I added, that's pointing to, essentially, sort of a ridgeline on the green? It slopes down on both sides?
In this tutorial, down through step #4, it shows how to use the Stitch SubPath LPE (sorry, I called it Interpolate Subpaths before - sorry for the confusion). That's what came to mind about tickmarks which have a variable length to indicate elevation. However, now that I see your examples, I see that you use long-short-long-short tickmarks. So you'd have to overlay 2 sets of subpaths - a set of longs and a set of shorts. I'd have to play with that a bit, to find out if it would be a time saver or not, or if I could find shortcuts to make it less labor intense.
https://forum.inkscapecommunity.com/index.php?action=articles;sa=view;article=13The Interpolate extension is similar, except there's no "live" component. While it's not obvious how to make them variable, as opposed to just going from longer to shorter, I have found a way to do that. But again, I'll have to experiment a bit.
When you said you were using a clippath, when I first read your message (without the image to look at) I thought you meant you were using a clipping path for each tickmark or maybe each few tickmarks. But now I see you mean for a whole line you make a clippath.
The most I could muster was using a clipping path on the ruler ticks. This limiits length of the major ruler ticks but will not influence the small ruler ticks
For that, you could do the same thing I suggested for stitch subpaths. Just make another clipping path for the short ticks. You could even duplicate the clipping path of the "longs" and just move over the relevant portion of the path, to use for the "shorts". Although you'd have to create the long ones and the short ones separately....or separate them after you make them.
One option that I haven't mentioned is Markers. But at least so far, I can't think of a way to use them for variable length of the tickmarks or tick-lines, as they kind of turn out to be.
Let me play with your file for a little while. At the moment, I think clipping may turn out to be the best option. But often when I start to actually work on the canvas, I think of new things.
You mentioned arrows in your first message, but I don't see anything like that in the file you provided. Well, not a line of variable size arrows anyway.
OH!! Just thought of something. There's an Inkscape extension for orienteering, which I think is something like "trail-blazing". It creates maps - I guess something like hiking maps. I think it uses all kinds of symbols and similar type of things. Let me find a link to that. I don't know if it works with the current version of Inkscape (probably not) but it probably works with some version (which might be better than nothing, if it turns out to be helpful).
Ok, there are 2 links for it:
http://www.nopesport.com/news/1343-o-scape-free-orienteering-mapping-software and
https://sourceforge.net/projects/o-scape/ It's said to work with version 0.48. Unless one of those links has more current info. I don't think there are many LPE's in 0.48, if any. My old brain, I just can't remember. But being an extension, I would guess it makes some of the work more or less automatic. In any case, definitely worth taking a look.
And I'll still do some experimenting in your file, and see what I can come up with. I've been curious about that orienteering extension for a long time, and I might just download it and 0.48 just to see what it can do. Although I might be running out of time for today, but if I do, I'll be back tomorrow.
Edit
Oops! Not Interpolate Subpaths. I meant Stitch SubPaths! Sorry about that! Editing my comments above.