I'm working with an area around a couple miles of road running SW to NE. I have metric lidar tiles in GeoTiff format. The tiles are 1,500 meters on an edge. I'm working in US Survey feet.
I originally tried creating a Surface of all 9 tiles I needed and then cropped the resultant Surface. But that was too large for my computer to handle. So I created 9 drawing each with one tile Surface in it. Now I've created a Data shortcut Reference for them all in one drawing. I got all the Surfaces in but noticed that there were gaps between the borders.
I was wondering if it was a coordinate system conversion problem. But all the settings check out. Now I think that this may be an expected behavior. I measured the gaps between the Borders and found that they all ran very close to 6.56'. In metric that's almost exactly 2 meters. 2 m is the separation of points in the Lidar surface.
Has anyone had experience with this and confirm that I'm right about it being normal?
BTW I'm working this in 2014.
Allen Jessup
Civil 3D 2012 SP 3 / IDSP 2014
Dell Precision T7400, Xeon CPU 3.16 GHz
Win 7 Pro, 64-bit,12 GB RAM, Nvidia Quadro FX 4600
Allen Jessup
CAD Manager - Designer
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I have not ever noticed a gap between adjacent Lidar tiles but they do not normally overlap so if the gap is simmilar to the distance between each point then I imagine that that would be the answer.
Normally Lidar data (in the UK) is fixed on Gridded tiles based on the Ordinance survey grid so a gap the same distance as the plotted points between tiles would not be expected.
As for your issues with size of surface. Obviously this depends on the actual area you need to work on but in the past I have traced a polyline around the area I actually need to use as this is normally not the full tile(s) extents.
You can then import the data as a point cloud clipping to the polyline and create a surface from that. I have used this approach to create different surface strips from the one single cloud and on very large data sets with good results.
One thing to point out is that you can adjust the cloud density which will also effect the accurracy of the surface, altough I think this is only at the instance of import (you need to test that as I'm not sure off hand).
M.
Thanks Mike,
I used the import cloud method originally. But I got some very bad surfaces that didn't fit the field Survey well. Bringing in the GeoTiff as a DEM gets a much better Surface. Of course that could just be my inexperience with working with a Point Cloud.
Allen
Allen Jessup
CAD Manager - Designer
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The problem with using a point cloud to create a surface is that there is a limit on number of points used. I can't remember the number offhand but if the point cloud is larger then some data is ignored. I usually create a polygon and apply that as a data clip boundary before importing Lidar point files. This produces a surface that uses all available Lidar points, within a specified area.
Steve
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Bringing in the DEM is a solution, but one must remember this is a derived product. It probably was created with breaklines (one hopes).
Bringing in the raw point cloud will require some cleanup but at least is the original dataset.
I would try to get the breaklines from whoever created the geotiff and do a hybrid, making sure I was zoomed in so the 1.5 million pt limitation is avoided. Also make sure 100% of points are being viewed, not the standard 15%.
Working with the original data vs the derived DEM would be a best practice IMHO.
As to the gap, you can simply net a few extra pts in each file and add them to the surface to avoid your gap issue. I assume they are gridded pts at a 2m grid interval.
No chance on getting breaklines. I agree about working with the original data. That's why I started there. But what I was getting wasn't something that needed a little tuning. It was seriously damaged. I don't know why. But it was.
I'm not doing any design in the areas I'll be using the Lidar Surface. We have Survey for the road. There may be a little earthwork in the areas off the road. But it's mostly to show the elevation of the adjoining land. I just want it to look right.
Allen
Allen Jessup
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@mikeevans wrote:
Did you adjust the Point cloud density values to maximum before creating the surface? On the Point cloud ribbon there's a slider which controls this. This may be why the data was damaged.
I believe I did. It's been a while. The likelihood of the data being damaged is low. It was collected as part of THIS project.
Allen
Allen Jessup
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You never want to create a surface from contours except as a last resort. Better to use DEM instead if you are going to use a derived product.