LiDAR data

LiDAR data are gaining importance in the field of high resolution topographic mapping. One of the major developments in Remote Sensing technology is the emergence of LiDAR (Light Detection And Ranging) technology. LiDAR is the Remote Sensing technology that can find the range and other information about a particular distant object by the means of measuring the properties of scattered lights.

Methods of LiDAR data Collection:

The LiDAR (Light Detection And Ranging) sensors emit 5,000 to 50,000 laser pulses per second in scanning array. The LiDAR data are collected from individual points over the ground surface. The spacing of these points depend upon the flying height and scan angle. The whole process by which a LiDAR (Light Detection And Ranging) sensor collects LiDAR data is called LiDAR Survey. There are several technologies that works side by side for performing an accurate LiDAR (Light Detection And Ranging) Survey. More accurate the survey, more accurate the LiDAR data. Thus the qualitative nature of the LiDAR data depends upon the accuracy of the LiDAR survey.

The technological assistance which a LiDAR Survey needs to perform accurately are as follows:

An airborne Global positional System (GPS) works simultaneously with the LiDAR sensor in order to generate the x coordinate, y coordinate and z coordinate. The Inertial Measuring Unit (IMU) is there to determine the amount of roll, pitch and heading of the aircraft. This is done by establishing the angular orientation of the LiDAR sensor around their x. y and z axes of the flight.

Nature of LiDAR data:

From a standard LiDAR survey, basically two types of LiDAR data can be generated. One is the scanned image of the part of the earth surface and other is the x, y and z coordinates of the points over the earth surface. The former is called intensity image and the later is also called as the raw LiDAR point file.

From this basic two data, various secondary data are generated by means of calculation. They are as follows:

• First Return DEMs: These are the first returned values which measures the elevation of the top of the building, canopy and other obstructed surface.
• Bare Earth DEMs: These are obtained by subtracting the height of the canopy and the building structures from the first return DEMs.

Format of LiDAR data:

The LiDAR data are available in UTM11 projection, NAD83 datum or IDTM projection. The first return DEMs and the bare earth DEMs are in available in the ESRI float grid format. The raw LiDAR point files are in .IBN format. The intensity Image is generally in GeoTIFF format. Basically, the LiDAR data has no standard format. The raw LiDAR data are usually delivered as Geo-referenced data in ASCII format. This ASCII format is very easy to convert it into binary raster data and then into other data formats like GRID, TIFF, IMG etc. This conversion of ASCII LiDAR data format reduces the size of the file and also facilitates in creation of the file formats that are compatible to different LiDAR software packages.

The LiDAR data are very important in the analysis of spatial data, specially in the field of GIS (Geographical Information System).