Lidar 10 cm contour lines

[Vasily_BC]

Hello,

I need to generate contour lines for area of 25 sq. km. Problem is that it should be 10 cm contour lines.

I have Lidar but for me it looks almost impossible to work with such amount of data, even through Data Shortcut.

Also I see problem in number of tiny spots of terrain, see image below.

Only way to solve this for me right now is to generate cropped surfaces (let's say 1 sq. km).

To "contour line noise" I would probably generate after DEM with accuracy of 10 cm and after that create 10 cm contour lines.

Any suggestions?

Thanks

[john.mckenzie]

That is a surface of 2500 hectares or approx 6200 acres. a 10 cm interval is approximately equivalent to a 4" contour interval. Stay with me, I am translating to english units to help my poor english brain relate to the issue at hand.

First.

A 10 cm interval implies a high degree of accuracy is required. So, it will be impossible to avoid the contour noise as you describe it if you are truly interested in the accuracy implied.

So you have a couple of options as I see it.

A mosaic of smaller surfaces with high accuracy or

A larger surface built from filtered lidar data (thin the data points out) or simplify the surface once built to make it manageable. thinning out the lidar or simplifying the surface will both assist in making the surface manageable but may not give the degree of accuracy a 10cm contour interval implies.

A general rule of thumb is that a 1 meter contour interval results in mapping of approx. 1/2 meter accuracy. therefore a 10cm interval implies a mapping accuracy of 5cm (2 inches to our english friends). This was in the day of hand drafting, but making contour intervals smaller on a surface will yeild "funkier" results the smaller the interval which demonstrates the concept of implied accuracy. What looks good at 1m interval will start to show bogus linework when you get down to the .1m level unless you have the survey data to support the smaller/tighter interval.

[gccdaemon]

I deal with LIDAR as well and it is very data heavy. Once your point cloud is created, i would create several surfaces with seperate boundaries to break the data up. Chances are that with that much land, you're going to need to phase it out anyway.

You can remove a percentage of points from the point cloud based on percentage or elevation fuzz distance when you import the LIDAR data, but you'll lose accuracy.

Good Luck, and be careful labeling the contours because the redraw time on the topo lines will eat up productivity. Save that for last.

[Vasily_BC]

Unfortunatly removing percentage of points will reduce accuracy. Moreover, actual Lidar have been processed and what I have is bare earth filtered points. It means that density of Lidar much less then original survey. So splitting surface to parcels is the only way so far.

[Vasily_BC]

That's why I was thinking about DEM that can create grid of certain accurasy based on original TIN. And as an option after I could querry spots let's say less then 0.5-1 sq.m. (5-10 sq.ft).

[gccdaemon]

I know about the point type filtering, but this is an additional step.

Once added to the surface, you can go to surfaces->Edit Surface->Simplify surface and remove additional point data.

I usually export the surface to landXML to get rid of the point cloud data as well.

[gccdaemon]

You also might want to create break lines for creeks before you use the DEM so you don't lose too much accuracy.

[john.mckenzie]

I tend to agree. Split to parcels and maintain the accuracy.

Another option would be to  generate the smaller surfaces and export the linework to acad entities.

XREF the cad entities into your dwgs for existing.

Create a simplified but large (areawise) surface for your early design work until you hone into your solution. then resolve against the tighter but smaller surfaces for final presentation.

Just an idea.

[john.mckenzie]

Of course I am assuming that you are designing a surface from the one you are creating. I may be assuming too much. LOL

[ksorsby]

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@john.mckenzie wrote:

therefore a 10cm interval implies a mapping accuracy of 5cm (2 inches to our english friends).

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In England, we always use metric for civil engineering work, John.

Just sayin'  

Kevin

[john.mckenzie]

I did mean Imperial units and I knew someone was going to call me out on it. LOL

[ksorsby]

Haha! I do find it odd though that non-British people tend to think that in Britain we work in imperial for everything when in fact (civil) engineering is exclusively metric and has been as long as I've been alive. In the 20 years I've been engineering I think I've had only a couple of old drawings in feet and inches and I've sent them back as 'unusable' within the project!

As far as I can see, pints for beer, miles for driving, stones when finding out how heavy you are, feet for how tall you are, everything else metric.

Kevin