Ever since CalTopo integrated LiDAR into their slope angle shading, I've been using it to suss out potentially difficult terrain, to avoid or to seek, depending on the activity. Lacking scalable LiDAR-based contour overlays (notwithstanding the excellent work done by one forum member to show this is possible), the remaining challenge has been in determining the height of the cliffs.
I assume others figured this out long ago, but I just discovered a way to measure them last year and thought I'd share, partially in hopes someone will point out a much simpler method (this one isn't hard, it's just an unobvious application of slope angle shading).
So here's an example: A drop in Corbett Creek outside Ouray at 38.04388, -107.70588:
Generally, the black (60+ deg.) bands are cliffs until proven otherwise. Blue (51-59 deg.) bands are going to be pretty steep but may be 3rd-classable depending on features.
In the above image, you can see this drop fits inside a single 40-foot contour, from which--in the pre-LiDAR topographical Dark Ages--one could conclude it's less than 40 feet tall. MapBuilder topos use one of the less-granular contour sets, but we can check the USGS topo to see if it agrees:
Often the USGS topos will better indicate areas of steeper terrain (like that cliff immediately south), but in this case it mostly aligns with the known suspect contours.
To figure out how tall this drop really is, draw a profile (Shift+E, or click "ruler" icon > Profile, or right-click > Profile), placing the 2 endpoints on the bottom and top of the cliff. Avoid a profile that traverses diagonally across the face of the cliff--your line should be perpendicular to it (aligned with its aspect). Double-click to finish the profile, then look at the gross elevation delta in the rendered profile.
Here, instead of a profile, I drew a line and then selected Profile in its context menu, but the profile method is quicker:
LiDAR measures this drop at 112' (it's actually taller but my endpoints were conservative). Having visited this location and many others measured this way, I can speak to the accuracy of this method. Note that ropes will need to be (sometimes quite) a bit longer depending on the steepness of the drop, which you can also measure using maths (the run in feet is shown at top just before you finish a profile, or when you hover a line).
LiDAR vs. Cliffs
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LiDAR vs. Cliffs
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Re: LiDAR vs. Cliffs
Clever! CalTopo is sufficiently complex that I'm always discovering new ways to use it.
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Re: LiDAR vs. Cliffs
Wow the elevation sampling got way better with the Lidar update, huh? It used to bottom out at like 25' samples, making what you just demo return suspicious results.
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Re: LiDAR vs. Cliffs
Yeah! This is true wherever Caltopo is using LiDAR DEMs (still not quite everywhere in CO). Not sure if they generate their own or source them, but either way the data is readily available. I'd still love to see what Eli Boardman did over here offered as a digital overlay with selectable (and/or zoom-dependent) intervals. My simplistic view is this could be done programmatically with the DEMs or LPC without too much trouble, but probably interpolating contours at fixed intervals gets complicated.
Here's a good one: In this case, although the old contours show the steeper area, they drastically misrepresent the actual terrain.
Here's a good one: In this case, although the old contours show the steeper area, they drastically misrepresent the actual terrain.
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Re: LiDAR vs. Cliffs
Ah, ok: the sampling interval can be so small, since the line is so short. Gets chunkier pretty fast. Caltopo's graph is really misleading if you look at little segments like this:
.02 miles is 105.6 feet, so the x axis is not in proportion with the y, even though it's labeled as, "0x" vertical exaggeration. That's *technically* correct, but the distance is being exaggerated by 4x. I guess something to be aware if you don't do this "math" thing you talk about in your head. I also don't know how accurate 1:1 is, as this isn't a 90 degree slope, this is near Scenic Cruise in the Black,
I feel I'm using the tool incorrectly, though I may open up a ticket about the distance exaggeration on the graph.
(this has little now to do with LiDar, I understand)
.02 miles is 105.6 feet, so the x axis is not in proportion with the y, even though it's labeled as, "0x" vertical exaggeration. That's *technically* correct, but the distance is being exaggerated by 4x. I guess something to be aware if you don't do this "math" thing you talk about in your head. I also don't know how accurate 1:1 is, as this isn't a 90 degree slope, this is near Scenic Cruise in the Black,
I feel I'm using the tool incorrectly, though I may open up a ticket about the distance exaggeration on the graph.
(this has little now to do with LiDar, I understand)
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Re: LiDAR vs. Cliffs
The width of the graph is just determined by your window size at the time you generate the profile. Ignore the slope, or you could get a more realistic-looking graph this way (lol): Also, the results won't be great if one of the endpoints is in the middle of a cliff--since the horizontal resolution of LiDAR is 1m, it could be off by hundreds of feet on a very steep feature. I don't know Scenic Cruise, but unless it stops somewhere in the middle of the wall, it's best to ignore the old contours and use the black color band in the slope angle to approximate the top/bottom of the cliff. I get ~1500' for the height of the wall at that location. Oh, and the "exact" horizontal distance in feet for these short segments can be seen when you hover a line (at the top), or in the context window when you click it. This probably isn't how they expect people to use the profile feature haha.justiner wrote: ↑Tue Apr 02, 2024 9:48 am Ah, ok: the sampling interval can be so small, since the line is so short. Gets chunkier pretty fast. Caltopo's graph is really misleading if you look at little segments like this:
.02 miles is 105.6 feet, so the x axis is not in proportion with the y, even though it's labeled as, "0x" vertical exaggeration. That's *technically* correct, but the distance is being exaggerated by 4x. I guess something to be aware if you don't do this "math" thing you talk about in your head. I also don't know how accurate 1:1 is, as this isn't a 90 degree slope, this is near Scenic Cruise in the Black,
I feel I'm using the tool incorrectly, though I may open up a ticket about the distance exaggeration on the graph.
(this has little now to do with LiDar, I understand)
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