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Home - General / All posts - Modelling user defined slopes in a DEM
LandSystems28 post(s)
#15-Jul-20 05:38

I am attempting to construct ‘as built’ flood control embankments into a DEM using information listed in a GIS layer of embankment centrelines. My purpose is to then subtract high resolution LiDAR of the same embankment from the ‘as built’ model to determine how and where the real world embankment deviates from the original design. We can use this information to identify defects and to assess the approximate volume of material needed to rectify the design.

I have got a fair bit of the way in M9, but need a bit of help with the next step which is to derive a DEM representation of the embankment slope away from the crest edge line (supplied). Within the dataset, the embankment slope is provided as a gradient rise per meter [BNK_SLOPE_GRAD_M_M] (see attribute list below).

To complicate matters, crest height of an embankment can vary along its length and is defined in the source dataset by crest a start and end attribute ([CREST_START], [CREST_END]). This means that the slope DEM needs to be derived for each segment along the embankment length to accommodate the varying crest heights ([BANK_HT]).

Embankments are also commonly asymmetrical in cross-section with a different bank slope for each side (See Clipboard-1 image for embankment cross-section schematic). I did have a method to produce a user defined DEM slope accounting for change in crest elevation on either side of a line in M8, but I can’t locate it at the moment. Ideally I would like to produce the specified DEM slope on one side of the line, but it doesn’t matter if it is on both sides initially as I can mask it later.

I want to achieve this in M9 for performance which has now overtaken M8 as my GIS of choice. I have had a couple of cracks at achieving this first in vector mode with line spacings and heights to achieve the correct slope when joined to the DEM, but my attempts didn’t deal with sharp bends in the embankment (See Clipboard-2 image)

If anyone is able to offer any suggestions of how to go about this in M9, I would be very grateful.

[BRANCH]Segment index for the embankment

[ASSET_ID]Asset identifier

[MAXID]The maximum segment index for the embankment

[CREST_M]The width of the crest of the embankment (not needed for this part)

[CREST_START]The height of the embankment crest at segment index start (0)

[CREST_END]The height of the embankment crest at segment index end (309)

[BNK_SLOPE_GRAD_M_M] The gradient of the slope in rise (m) per meter

[BANK_HT]The calculated segment crest height

Attachments:
Clipboard-1.png
Clipboard-2.png
M9 FORUM DATA.mxb

tjhb

9,320 post(s)
#15-Jul-20 05:50

What is the question Dan?

And why do you need to use slope(s)? Why not just subtract one raster from another?

LandSystems28 post(s)
#15-Jul-20 06:19

Hi Tim, when I say slope, I don't mean a grid of slope, I mean construct the sloping bank of the embankment as a DEM from the crest edge (the line supplied) to the toe of the embankment. The slope of this DEM is defined by the attribute [BNK_SLOPE_GRAD_M_M], so in the example we know that the slope of the bank is a drop of 0.4m every 1m horizontally.

My apologies I realize that I didn't explain what I am trying to create very well in my initial post and I am not sure I am doing much better now. I will see if I can find a picture to help.

Edit: I am trying to create the red part of the image as a DEM using the slope attribute

Attachments:
ClipBoard-1 Modified.png

LandSystems28 post(s)
#15-Jul-20 06:34

Here is another example from the M8 script I had which took a line with heights that varied along its length and an embankment slope value, and constructed the embankment into the DEM. I am essentially wanting to replicate this in M9 but taking the desired embankment slope and the crest height from each segment in the line feature in the M0 project.

Attachments:
Clipboard-3.png

tjhb

9,320 post(s)
#15-Jul-20 07:35

Cool, this is fun. :)

LandSystems28 post(s)
#15-Jul-20 23:00

I have managed to find a early M8 version of the code that produced Clipboard-3 image above. This version doesn't seem to correctly use the path height at each segment, though I know that I had this working.

Attachments:
M8 FILL WORKING.map

mikedufty

861 post(s)
#22-Jul-20 01:19

I don't have an answer, but very interested to see how you go. This is one of the few things I still need to use my 15 year old copy of AutoCAD civil 3D for, and I would really like to not have to use it any more. It is really good when it works, but crashes more often than not, and import and export are a pain.

mikedufty

861 post(s)
#22-Jul-20 01:45

I think Tim even offered to help me with this several years ago and I've been too slack to provide him with details of what I want to do. Usually it is quite similar to what Landsystems is doing. Either a stockpile where I specify a level surface and want to project down at a slope to the landscape, or a pit where I have a boundary and want to project down at a slope to a floor level.

I suspect that resolving inside corners is not mathematically that easy as it seems to contribute to the worst crashes, but I can do it manually so can't be that hard.

I struggle with SQL but I think yours is travelling along the line building cones from each point, which seems like it should work nicely to achieve nothing steeper than the specified slope, which is what I'm after.

LandSystems28 post(s)
#28-Jul-20 22:28

Thanks for your interest Mike, this is a bit of a side/hobby project for me at work that I believe would be of great use to our flood control engineers. Unfortunately, I have had to park it again for a while as it has been overtaken again by other tasks. Thinking about it more however, I am pretty sure that I can now fix the problems with my earlier attempt (Clipboard-2 image in the initial post), but I suspect that there will be a more efficient tile based approach available in M9.

Anyway, rest assured that I will share here if I manage to achieve my goal

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