dem users manual - asprs

Chapter 6: IFSAR
Chapter 6 explains the capabilities and limitations of Interferometric Synthetic Aperture Radar (IFSAR), both
airborne and spaceborne. This technology sees through clouds, is less weather dependent that other technologies,
and acquires elevation data sets of large areas at relatively low costs. It is important that DEM users understand that
elevations derived from x-band IFSAR are initially the elevations of top surfaces (treetops and rooftops) and not the
bare-earth terrain, so if you need DSMs of large areas, this technology may be best for you. DTMs can be produced
from IFSAR DSMs, and a sample IFSAR data set, including an orthorectified radar image, is included on the DVD
that accompanies this 2 nd edition. Figure 5 helps explain how Synthetic Aperture Radar (SAR) works.
Figure 5. The 3-D world is collapsed to 2-D in conventional SAR imaging. After image formation, the radar return
is resolved into an image in range-azimuth coordinates. This figure shows a profile of the terrain at constant
azimuth, with the radar flight track into the page.
Chapter 7: Topographic and Terrestrial Lidar
Chapter 7 is improved significantly from the 1 st edition, especially in describing different laser systems and how
they work, modern sensors with very high pulse repetition rates, and explaining what a lidar point cloud is. This 2 nd
edition includes entirely new sections on lidargrammetry and ground-based lidar (terrestrial laser scanning) for
which there are nearly 2000 systems in use worldwide today, providing high resolution scanned images of
infrastructure so detailed that every pixel has its own 3-D coordinate. Many different sample lidar datasets are
provided on the DVD included with this 2 nd edition, including topographic and terrestrial lidar and fly-throughs.
Figure 6a shows a point cloud from an airborne lidar sensor, color-coding different lidar returns from the trees and
ground. Figure 6b shows a terrestrial lidar dataset (terrestrial laser scan) what looks like a photograph, except that
every pixel has 3-D coordinates so that as-built measurements are quickly and accurately surveyed.
Figure 6a. Topographic lidar “point clouds,” colorcoded
for 1 st through 4 th returns. Many 1 st returns
(yellow) reach the ground, whereas some of the 3 rd
returns (red) are still in the trees and don’t penetrate.
Figure 6b. High-resolution laser image of a fluid catalytic
cracker (FCC) in a refinery captured with a phased-based
scanner. Each pixel has 3-D x/y/z coordinates.
MAPPS/ASPRS 2006 Fall Conference
November 6 – 10, 2006 * San Antonio, Texas