Friday, September 28, 2012

Picture of the Week

Although my alma mater, the University of Florida, has a bye week, I thought I'd share the Mobile LiDAR picture of the week.  I've had this for some time, but in the spirit of college football season, here it is...

Go Gators!

Monday, September 24, 2012

Baker's Dozen: Laws 1 and 2

After I wrote the Baker's Dozen: 13 Laws of Mobile LiDAR , a colleague asked me the question a lot of readers were probably thinking - aren't Laws #1 (Too much is better than not enough) and #2 (Sometimes more is just more, not better) in direct contradiction to one another?

Although at first glance they may appear contradictory, the laws actually describe two uniquely different concepts.

Mobile LiDAR Law #1: Too much is better than not enough. 
The underlying principle of this Law is to develop your collection methodology around the concept that Mobile LiDAR data can be repurposed for other uses/needs; and you NEVER want to make a repeat visit to a jobsite to collect a little more you could’ve captured the first time.  Take for example an interstate project where Mobile LiDAR is used in support of engineering design services for a road surface that is to be substantially undercut for removal and replacement of unstable soils.  

Common among large, multi-staged construction projects, numerous DOT departments and contractors may be involved.  Each group, with its own budget, is likely at varying stages of design or construction, and not necessarily proactively communicating or coordinating activities between groups that could capitalize on cost-savings.  The scope of work issued by Group A may likely only require survey information for the lanes slated for reconstruction - without consideration for the engineering design work needed by Group B for the two crossovers for maintenance of traffic during construction. 

By anticipating Group B’s needs, and simply modifying the scanning trajectory or extent of capture, you effectively create a single dataset that not only meets Group A’s requirements, but also opens the door to re-purpose the data to support Group B – saving time and money for the client.  Obviously not all scenarios are this cut and dry, and you need to be cognizant that you’re not getting too carried away collecting additional data that may not be utilized, but when thoughtfully applied, everybody wins.   

Mobile LiDAR Law #2: Sometimes more is just more, not better. 
When the Laws of Mobile LiDAR were written, Baker was in the process of evaluating an upgrade to an Optech Lynx M1 system - dual 500 kHz sensor heads.  The upgrade would result in a 2.5x increase in the volume of points that we would be capable of measuring.  Although the collection rates would be staggering, would it improve our ability to identify features better? Do something we weren't capable of doing?  Eliminate another process?  After all, the range and scanner rates would not have been any different than our existing sensors, and the 400,000 measurements per second we can currently capture produce amazing resolution and rapid ability to identify features.

As you can review in my latest article on LiDAR News regarding point densities, we left nothing to chance.  Our team evaluated the densities achieved with our system under various conditions and collection practices.  We determined that as we would have more data, not necessarily better.

The Baker's Dozen was written over a year ago and I've read them countless times since.  Each of them still applies today.  After I'm finished writing about each of them, I'll step back and reexamine the list one more time before they're chiseled in granite.


Thursday, September 13, 2012

Mobile LiDAR Point Density - Part 2

I've had a brief hiatus from the blogosphere while working on a project that kept me out of town for a few weeks.  I usually do not travel with the system since our collection team is quite capable of anything I throw at them quite honestly.  However, the most recent mobilization was truly unique as you'll come to find out in future posts - can't share too much, yet.

Gene Roe, Managing Editor of LiDAR News, was successful in getting another article from me for the regular newsletter.  My latest contribution is a continuation of a previous topic and is titled:  Mobile LiDAR Point Density - Part 2.  I discuss how the following variables influence density:
  • Vehicle speed, measurement frequency, and scanner rates;
  • Angle of incidence and the impact of flat surfaces;
  • Distance to target and effect of rotated sensors; and
  • Collection paths and moving obstructions.
The images from the article are provided for a little more clarity. 

The image represents 3 passes along the same stretch of a 2 lane residential road. The data was collected at 25 mph with a scanner rate of 120 Hz. The three images show results of collecting at laser frequencies of 75 kHz, 100 kHz and 200 kHz – from left to right.
Using the vehicle speed and scanner rate, we can calculate the line spacing - the distance between lines of LiDAR points laid down on the surface.

The image above shows the data captured from one sensor along the centerline of a runway.  The data tapers off rapidly due to the limited backscatter caused by the flat surface. The point density on the pavement stripes at the extents is increased to to the material.

I will have one last article in the series next month.  It will focus on the resolution required to identify objects and tools that are available for feature extraction which utilize photography.

Feel free to leave a comment if there's a topic you would like to read about.