Thursday, April 21, 2011

No Place for Survey Crews

Earlier this year, we were tasked to perform a Mobile LiDAR collection of an Interstate tunnel (2 lanes in each direction - measuring 0.8 miles in length).  The tunnel is in a heavily trafficked urban corridor where closures required by traditional surveying, even in the early morning hours, were completely out of the question.

Due to the constant, unrelenting traffic, we performed the collection shortly after midnight.  We collected each of the 4 travel lanes at or slightly below posted speed limits - slowing down or speeding up to minimize laser shadows from other vehicles.  Following the collection, our LiDAR processing staff and Applied Technology Group produced the final products.

First, our LiDAR processing staff adjusted the passes to ensure coincidence.  Then they made quick work of removing other vehicles from the point cloud.  Then, they turned over the tiled point clouds for modeling.

An image of the point cloud with vehicles removed, but tractor trailer model added.  Notice the pothole in the bottom left hand corner of the image. 

Our Applied Technology Group utilized the cleaned point cloud to create a rendering of the tunnel in MicroStation.
In the article I wrote for LiDAR News last week I made the statement "if you can’t see it, the system isn’t measuring it." The below image is directly disputing that statement - which I don't quite mind in this case.  

The image shows a cross-section of the tunnel.  Notice the arc above the "box" tunnel.  The arc represents returns from the actual tunnel ceiling (extent of bore) that were measured through small (1") joints in the false ceiling.
Perhaps what is most fascinating to me is not that the system captured this information, but how it will be exploited by our client.


Monday, April 18, 2011

Knowing your Limitations

I have recently prepared another article for LiDAR News titled Knowing your Limitations.  Understanding what a technology does not do is equally, if not more, important than understanding what it does do.  Where those limitations can affect the successful outcome of a project, other methods, tools or techniques are needed to compensate.  The article addresses some of the larger topics.  In upcoming posts, I'll provide additional items to consider when pursuing a Mobile LiDAR project.

The images presented in the article are shown below with captions:

Manholes:  In a span of 40' along a roadway, there are 6 manholes and two water valves.

Grass on Bare-Earth/DEM:  A random non-Federal levee shows the difficulty dense vegetation adds to determine bare earth along the bank.

Obscured Areas:  The area behind two parallel parked cars is obscured in comparison to the open area between them.  The shape of the voids depicted, represents the orientation of the lasers and varies based upon heading.
As always, please let me know if there are any topics you would like to see discussed.