In the previous installment, we examined the process of initial quality check and geometric correction of LIDAR data. I am taking a break from the processing series to discuss the value of the "I" in LIDAR.
I (pun intended) am advocating that we always write LIDAR with a capital "I" as opposed to the popular "LiDAR." Is this really a big deal? Well, I have thought so and this feeling was confirmed at a visit to land information conference last week in which LIDAR exploitation was a hot topic. GeoCue gave a workshop on the fundamentals of LIDAR data receipt, quality check and exploitation for end-users of LIDAR data. I was quite surprised to find that none of the attendees of our workshop (from a field of about 50) were aware of the "true ortho" infrared image produced by a modern LIDAR system. Furthermore, the audience was not aware that vertical objects in high quality conventional orthophotos typically suffer horizontal displacements (due to a lack of true elevation modeling) and hence cannot be used in analysis.
The most common of these are overhead electric wires. Some customers attempt to use conventional aerial orthophotos to asses distance encroachments to overhead transmission lines. Since the wires are typically not modeled in the elevation data sources used for creating the orthos, the wires do not appear in their correct geometric location. However, correctly calibrated/adjusted LIDAR data will always provide a "true" ortho rendering of object locations, regardless of vertical displacement. Thus we need to think of LIDAR as "Laser Imaging, Detection and Ranging" as opposed to other incarnations such as the erroneous "Light Detection and Ranging" (is anyone aware of a LIDAR system that uses incoherent ‘light" as opposed to a laser?).
A simple use of the intensity attribute of LIDAR data is presented in Figure 1. Here we have a mobile mapping highway project (these data are from an Optech LYNX system). The LIDAR intensity "image" is superimposed on aerial data from Bing maps. Note the very clear depiction of roadway features such as the centerline and railroad crossing marks.
As mentioned previously, LIDAR "imagery" provides a true orthographic rendering of the scene, regardless of elevation displacements. This is dramatically illustrated in Figure 2. Here we have a set of transmission line LIDAR superimposed over ortho images of the same scene. The transmission lines are rendered as red dots in the LIDAR data and as white lines in the ortho. Note the large error in the transmission line location present in the orthophoto. This is easy to see at tower attachment points. The correct location is circled in blue whereas the location indicated in the orthophoto is circled in red. The error occurs because only the ground has been modeled in the elevation data being used in the orthorectification process. The LIDAR data, on the other hand, are directly 3 dimensional and thus indicate correct locations when rendered. Figure 1: Intensity in a Mobile LIDAR project >>
A second remarkable difference between a LIDAR "image" and a conventional image is that the LIDAR system is an active sensor. This means that, rather than relying on a secondary lighting source (such as the Sun), LIDAR illuminates the object space with laser radiation and records the reflected energy. This means that LIDAR images are unaffected by acquisition parameters such as sun angle and, indeed, can be collected in total darkness. << Electric Transmission line displacement (blue is the correct location)
>> Figure 3 clearly depicts the value of a synoptic view of a larger project area. Of course, LIDAR images are not substitutes for conventional ortho photos. They typically are of much lower resolution and the illumination is very narrow bandwidth infrared. Thus one would never consider an LIDAR image for an application such as a general map backdrop. However, when the application is measurement and relative positional analysis of objects above the ground surface, the LIDAR image is the way to go! Think about this the next time you relegate LIDAR’s "I" to the lower case!