Monday, November 23, 2009

Measuring Systems Part 1 - Positioning

In traditional Surveying & Mapping, points are determined by measuring an angle and distance to an object from an instrument over a known point (X, Y, Z or elevation).  We have all seen, at one point or another, a person at a total station locating a rod held by another field crew member - often while we zoom past at highway speeds. 

Now, with Mobile LiDAR, we are Surveying & Mapping at highway speeds.

In order to better understand how accurate measurements are collected from our vehicle, it is important to know the instruments onboard and the function of each.  As individual sensors, they do not provide an adequate solution.  But, when used as part of a system, they provide the foundation for accurate information. 

The purpose of this post is to provide a basic overview of our system.  Additional parts will cover the lasers, cameras and other components and processes which provide a complete solution.  If you have questions, please leave a comment.

Global Positioning System (GPS)
The primary method of location for the vehicle is GPS.  We utilize 2 onboard units to provide the position of the vehicle as well as the heading (having a known baseline, distance and direction, between antennas provides a measure of direction of the vehicle).  The fundamental issue with GPS as a sole (hence I used primary) source of positioning is that we measure a position one time per second.  For those of you breaking out your calculator, that equates to 88 feet per second when traveling at 60 mph.  In that second, our lasers could have measured 400,000 points (covered in part 2, stay tuned).  Therefore, we rely on our second measurement instrument.

Inertial Measurement Unit (IMU)
The IMU measures the attitude (roll, pitch and yaw) of the vehicle, much like an aircraft, at a rate of 200 times per second.  By measuring those changes in direction about the X, Y and Z axis, we are able to calculate the vehicle position at those increments.  Using interpolation, we are able to further refine intermediate positions.

Distance Measurement Instrument (DMI)
The often overlooked member of the measurement family is the DMI.  Barely noticeable and not mounted on the "LiDAR Wing", the DMI has two distinct purposes: determine the distance traveled by measuring the revolutions of the wheel 1,024 times per second and tell the system when the vehicle is stopped.  Since there is drift in GPS and the IMU, the DMI basically determines when the wheel stops revolving.  By measuring the diameter of the wheel and calculating circumference, we also know the distance traveled per partial revolution.

For NASCAR fans:  since we measure the diameter of the wheel at rest, the circumference of the wheel is calibrated while we're driving and GPS provides a good solution.

For non-NASCAR fans: as we drive, the tire will begin to heat and will build pressure thereby increasing tire circumference and impact the distances measured by the DMI.

In a nutshell... one system helps calibrate another system.  In a later post I'll cover what happens when we lose GPS!!! 

Comments are moderated, so if you have a question, comment or suggestion please let me know.

Thanks for following!

Target of Opportunity

Great Bridge Bridge

Some time ago, we were in Chesapeake, Virginia performing a collection and demonstration.  While driving, we saw a target of opportunity - the Great Bridge Bridge - and collected roughly 10 seconds of data.  There was no purpose for the collection, but after processing I thought it would make an excellent example for showing point cloud samples - given the joint in the split drawbridge.

Above, I've render the point cloud two different ways.  On the right hand side are point colorized by height/elevation only. There is nothing to differentiate one point from another as far as composition.  On the left hand side I've included the intensity of the return in the colorization.  From the intensity, you can begin to differentiate the striping, road and materials. Just as in traditional surveying where measured points will have an associated description, we can begin to see how intensity is used in point classification - more to come on that subject.

In addition, I've compiled a brief video of the bridge. Feel free to post comments and suggestions.

Tuesday, November 17, 2009

Subscribe to our Blog

First, I would like to welcome you to the new and improved blog.  I figured after spending the money to invest in a high tech piece of equipment, our blog should have the look and feel similar to our vehicle - not a default template.

I've been asked by several people how to go about following the blog without having to check the site daily.  Therefore, here are two simple ways to get updated postings.  Perhaps the easiest is to submit your email in the upper right-hand corner of the home page. You will need to follow a few steps to receive emailed blog posts.

Alternatively, you can consume the blog posts directly in Microsoft Outlook.  First, right click RSS Feed in the Mail Items; then click "Add a New RSS Feed"; finally, type in and click Add.  The blog posts will appear similar to emails.

If you have any comments or recommendations for blog posts, please leave a comment.

Thursday, November 5, 2009

2009 Texas GIS Forum

The Presentations from the 2009 Texas GIS Forum are now available to download.  Check out Mobile LiDAR: Surveys at the Speed of Business presented by Stephen Clancy.  (Link to all other presentations)

(*Edited to remove dead hyperlinks)

Pipe Ladder Animation

A few weeks ago, our Mobile LiDAR crew collected several roads around a heavy industrial area - including refineries and a railyard.  Adjacent to and above our survey area was a complex labyrinth of pipes, fencing, railroad tracks and utilities. Making sense of all of the data could be quite daunting if looking at the entire collection. Utilizing animation, I've been able to illustrate how clipping the information can yield a new perspective. The video is a simple animation compiled showing a pipe ladder that we drove under. All of the information depicted was captured in seconds. The photograph shows the pipe ladder as taken with one of the onboard cameras.

Note:  If you're not able to view YouTube videos and would like a copy,  leave a comment with your contact information.  Comments are moderated and your information will not be published to the board.