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Creating DEM from topo survey

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I have a DEM from LiDAR, but since the imagery was taken a new road and has been constructed, and a large site reprofiled, affecting levels. We have had a topo survey of the area done, and I am trying to interpolate a grid from the point elevations to use in place of LiDAR where necessary (and subsequently fto use to extract z-points - as well as for presentation of results).

I have tried triangulation, however along road embankments I get little "domes" protruding into the embankment, where points along the bottom of the embankment are getting priority over higher points on the road. Obviously MapInfo doesn't know there is a string line that should connect all the low points along the bottom of the embankment, and a string line connecting all the kerb points etc. This is the most noticable result of interpolation, though I'm sure it's having effects in other places where elevation changes rapidly, and I don't want to start affecting hydraulics with this false small scale topography.

Has anyone had experience in trying to do a similar thing, and any tricks or methods to tidy this up? I'm thinking I may just have to pick out important features such as top of road embankment and add them as z lines/areas...

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I'm thinking I may just have to pick out important features such as top of road embankment and add them as z lines/areas...

well, as I understand it, this is exactly what Z-lines/areas are for, but if you need to do this at the Vertical Mapper level there are a couple of things I could recommend...

If you don't have enough points to successfully define your area using a normal interpolation, you need to make some more in order to give vertical mapper a few hints about what you consider to be the important features. The best way to do this is to draw a line and apply the elevation you want that line to be at. You can then split this line using vertical mapper's poly-to-point tool to generate the points you need. For big flat areas, drawing an enclosing rectangle and splitting it using poly-to-point will have a similar effect.

This is going to be very tedious to do if your road is sloping up or down along its length (as they are wont to do), so what I would probably do to get a suitable grid into vertical mapper is use TUFLOW, Z-lines and Z-regions to generate the shape in a 2D domain and then import the zpt check file, remove the ZH points and interpolate it to generate a grid.

I hope that's of at least a little help.

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Just Further to Gerald's comments, if you can get a grid created in VM of the new terrain you may able to try the grid stamping option in VM (Grid Manager - Tools - Splicer ).

You may be able to "stamp" the new terrain over the old by setup the stamping options to use only the max or min values.

I have used this option on a small area with some success. The critical issues the on the boundaries, it may be worthwhile copy some of the original survey information into the new terrain information before you create the grid (as Gerald has already mentioned).

Good Luck. I would be interested to know how you went and what method worked best.

Regards

David

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We had a very similar problem for a flood mapping job in Melbourne. We had photogrammetry of our study area and some detailed survey along an important road with an embankment. The survey was taken as regular cross sections across the road and embankment. We created some polylines along the road crest, gutter, top of curb and top of embankment and then created a tin of the survey using the polylines as breaklines in 12D. We then created a tin of the photogrammetry and then created a super tin (merged the 2 tins in 12D). This creates a grid that you can use for mapping purposes.

For modelling in TUFLOW we used the photogrammetry breaklines as 2D zlines to ensure that the crests, gutters, pavements were represented well in the model, so we updated the breaklines using the new survey data. I understand that you have LiDAR so probably do not have breaklines, but for any small important features you probably should use the 2D zlines, very easy to extract from a well constructed tin, or digitise up manually.

Cheers

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