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.

Cheers!
Stephen

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.

Cheers!

Stephen

Defining Accuracies

I have written a third article for LiDAR News  which addresses accuracies of Mobile LiDAR information.  It may be accessed through this link.  

The images in the article illustrate how we constrain our collections to ground control, much in the same was as aerotriangulation (AT) in Photogrammetry.

The table shown below (and included in the article) illustrates the difference between our check points (labeled Control) and the LiDAR point elevation at the corresponding locations.  This methodology is used to calculate the accuracy per the National Standards for Spatial Data Accuracy.

Should you have a topic for discussion, please leave as a comment.

Thanks,

Stephen

Lots of Dots

The other day, a friend and business acquaintance addressed a letter to me as "Mr. Lots of Dots."  I thought it was fitting given that my professional life is all about dots, dots that we measure and dots showing where we've been.

That evening as I was reading my son his favorite story - There's a Map on My Lap* - the words took on a entirely new meaning.  Even though I've probably read the book 200 times, one particular rhyme stood out:

"Dot maps, like this one,
are covered with dots.
Some have a few dots,
but some others have lots."


Last week, our Mobile LiDAR vehicle surpassed 40,000 miles (several more miles if you include it taking a ride on a barge).  With each new mobilization as we're collecting more dots, we're also adding new dots for the places we've been.  If it wasn't for the wrap we have on the truck, I would have one of those sticker maps you see on the back of RV's because,

"You can always use maps.
They will help you in knowing
where you have been 
and just where...
...you are going."


If you are a geospatial professional and have children (or a wife that doesn't understand what it is you really do), the book There's a Map on My Lap is a great way of explaining what you do. I've bought copies for several friends that have become new parents. Also, it makes a great read for younger students for GIS Day.

Enjoy!

Stephen

*References to: There's a Map on My Lap! By: Tish Rabe; Dr. Suess Enterprises, LP Copyright 2002

Mobile LiDAR: Projects at a Glance

My second article for LiDAR News is now available.  It may be accessed through this link.  

The image in the article is shown below.  It is an intersection of SR30A in Panama City, FL - collected May 2010.  You can clearly make out the extraction of pavement markings, sidewalks, utilities, pavement edges and other features.

Should you have a topic for discussion, please leave as a comment.

Thanks,

Stephen