Ground bare earth points can be used as a Digital Terrain Model (DTM) or converted to contours or, as we will see later, a relief model. The vegetation can be used to determine the heights of trees. In addition, the intensity feature allows the brightness of the reflected return to be recorded. This can then be rendered to produce an image of what is on the ground which is similar to an infrared photo. While this is not close to photographic colour quality it does allow interpretation of what is on the ground. LiDAR missions are planned very similarly to aerial photo missions. However, the LiDAR aircraft is usually flying much lower (between 1000-3000 metres) and lines are spaced closer together as the beam width is relatively narrow. LiDAR data can (and often are) used together with standard air photo or a more advanced CCD camera to produce digitally rectified images or othophotos. The DTM is used to rectify the image taking out the distortions caused by relief. This saves time and money compared to collecting a terrain model by photogrammetry. However, it should be noted that it is rare that a LiDAR system and a precision aerial camera are flown at the same time, as the swath width covered by the camera is not the same as that covered by the LiDAR. But Lasermap GPR can fly LiDAR and CCD camera at the same time on the same platform. The following is a list of the main advantages of using LiDAR as a large scale survey technology:
Airborne LiDAR surveys can be applied in a multitude of applications requiring large scale mapping with most of them related to infrastructure development or maintenance. The following table presents some of the main applications areas of using LiDAR followed by some mission results executed by Lasermap during the 2004-2006 period.
LIDAR SURVEY FOR ROAD CONSTRUCTION The first airborne LiDAR survey in Thailand was completed on July 2005 by Lasermap Asia. The LiDAR data, acquired over a road segment South-East of Bangkok, aim to demonstrate the usefulness and practicality of using this new high-tech method of large scale topographical surveys for road construction and maintenance projects in Thailand. The LIDAR survey was carried out over a 300-square kilometers area on the west side of Suvannaphum Airport and along the 50 kilometer corridor of Motorway.  Fig 2. Sample data set of 3-D LiDAR data over Bangkok Generally, survey and design for a common highway requires about 7-8 months while major projects can require at least 12 months. The LiDAR acquisition over an area totalizing 300 square kilometers took less than three days while data processing took less than a month. Thanks to the flexibility of flight planning by Lasermap Asia, data could be successfully captured even during the beginning of the monsoon season since the aircraft was able to fly below the cloud cover. By adjusting the forward speed of the aircraft and overlapping lines of data capture to form a block of data, the resulting terrain model is made up of data points that are approximately one meter apart. Even though each and every single laser impulse does not necessarily reach the ground as it may hit vegetation, the survey provides nonetheless a very detailed ground model necessary for large scale mapping up to 1:1,000 as shown in this thematic map shown in Figure 3 which was derived from the infrared laser intensity map only.  Fig 2. Sample data set of 3-D LiDAR data over Bangkok Fig. 3. LiDAR derived thematic map used for road design in Bangkok |