EXTRACTION OF DEM FROM LIDAR DATA

Extraction of DEM from Lidar data



Extraction of DEM from Lidar data

Introduction

Q: With reference to specific example mention the advantage of Airborne Lidar data for producing digital elevation model compared to traditional group survey. Describe processing model to generate DEMs from raw Lidar data. Provide flow diagram for various processes involved, describing any limitations of the processes involved. Clearly mention the assumptions made in this processes and any other alternative approaches (provide reference).

A Digital Elevation Model (DEM) is defined as a file or database containing elevation points over a contiguous area (Miller, 2004; Ma, 2005). In sampler words, it is a digital model or a 3D representation of a terrain's surface. The model is mostly used for planets created from terrain elevation data. The geo processing tools have been used with increasing frequency for the study of integrated rural ecology variables, allowing for faster and more efficient analysis. One of the products generated by GIS, and one of great importance for analyzes considering the actual surface of the land, is the Digital Elevation Model (DEM). The consideration of the actual surface is essential for the correct calculation of volumes, areas and distances, crucial parameters for a series of geo morphological indicators. There are several methods for the generation of these models and there is no consensus on which one is more accurate, since this will depend on characteristics related to the study area. DEMs may be subdivided into:

1. Digital surface model (DSM) that contain elevation information about all features in the landscape, such as vegetation, buildings, and other structures; and

2. Digital terrain model (DTM) that contain elevation information about the bare-Earth surface without the influence of vegetation of man-made structure.

Four major technologies are used to obtain elevation information, including (Bossler et al., 2002):

- In situ surveying,

- Photogrammetry

- Interferometric Synthetic Aperture Radar (IFSAR); and

- Light detection and Ranging (LIDAR)

In situ surveying using conventional surveying or GPS instruments can yield accurate x,y,z information. However, field surveys are time-consuming and expensive on a per-point basis. Even with GPS instruments, it is often difficult for surveyors to obtain points in thick undergrowth. Due to these obstacles, the density of x, y, z observations obtained in an area is sometimes low. It then becomes necessary to interpolate between distant points to produce a digital terrain model of the area.

LIDAR offers an alternative to in situ field surveying and photogrammetric mapping techniques for the collection of elevation data (Maune, 2001). LIDAR technology can be used to provide elevation data that is accurate, timely, and increasingly affordable in inhospitable terrain (McGlone, 2004). LIDAR does not, however allow the analyst to control the placement of individual x, y, z measurements on ridgelines or break lines.

Methodology

Figure: Flow diagram to processing DEM from raw LIDAR data

Creating a TIN and DEM from the Lidar Return

Figure1: TIN for last return pulses, Last return DEM, DEM for first returned point cloud data

Q: What is the Difference between the tin and DEM?

A ...