Large Scale Mapping: State-of-art technology in Aerial LIDAR and High Resolution Digital Camera imaging

What’s Large Scale Mapping?
Concept of large scale mapping keep changing with time and technology; In the early 1990s it was 1:10K, in the early 2000 it is better than 1:5K. in early 2010; it could be better than 1:1K!

What are the essential inputs / data for Large Scale Mapping?
The essential inputs for large scale mapping are: High accuracy image pixels of 5-40 cm; 10-50 cm height information, Corresponding Ground control points; related to the each defined scales.

How do we get these essential inputs/ data?
Conventional Ground Surveys, Aerial surveys and Satellite platforms are the prime data sources.

Current Status on LIDAR. Digital Imaging Cameras:
With the recent technology developments in Satellite imaging it is a proven fact that from a polar orbit of about 600 KM altitude a 60cm GSD imagery in panchromatic band and a 2 M GSD in MSS mode are being used by the mapping and GIS community. The next generation satellite slated for launch in 2007 is likely to provide with 40-cm PAN GSD imagery and 1.6M GSD in MSS will be available to the global users. The satellite data has proven its worth in large scale map updating and specially the Thematic Applications.

However, for a conventional base map preparation and compliance with ASPRS standards, the Aerial imaging, DEM and DTM supplemented with the Ground Control Point (GCP) with a very high accuracy are essential to prepare large scale maps.

Aerial Digital Cameras:
The invention of aerial Digital Imaging camera with CCD technology has resulted ground sample distance of 3cm-6cm imaging in pan and MSS mode. This is the state of art technology available to meet the very large scale mapping requirements.

While Satellite CCD imaging is based on push broom line scanning, aerial imaging adapts frame scanning for Photogrammetric applications.

Typical Aerial Digital Frame Camera consists of eight telescopes imaging onto corresponding CCDs. The center four telescopes image onto 9 CCDs that are assembled in processing into a single image frame. The surrounding four telescopes image corresponding four color bands onto a single CCD in each telescope. All CCDs are positionally calibrated to ensure sub-pixel band-to-band registration. All color bands collected simultaneously with panchromatic at a ground sample ratio of 3.5 to 1. To create color images at the same GSD as the panchromatic, color images are fused with the corresponding panchromatic image. There are no fringing effects due to mis-registration since color arrays are fixed and calibrated. The result is a color image experiencing no artifacts such as fringing due to imaging of moving vehicles.

The camera has a high signal-to-noise ratio due to its large dynamic panchromatic dynamic range of 14 bits and the ability to adjust exposure time and f-number to lighting conditions. The radiometric resolution or dynamic range of each color band is 12 bits or 4096 levels. This resolution is maintained through processing until the final deliverable is prepared where conversion into 3-band 24 bit images is accomplished.

The camera imaging platform is supported with and IMU and GPS unit to get a very high accuracy attitude and coordinate information.

The digital image data along with the IMU and GPS data are recorded onboard a high density recording systems such as magnetic disks, tapes and then the data is processed for map preparations through Aerial Triangulation, compilation, Ortho corrections, Stereo and GIS map preparation.

Aerial LIDAR ( Light Detection And Ranging)
To measure the accurate height information and derive the DEMS and DTMS, LIDAR is the state of art technology. Light Detection And Ranging (LIDAR) is a powerful technology aerial survey sensor becoming popular in deriving Digital Elevation Models (DEM) and Digital Terrain Model (DTM ) with a stunning accuracy of 10-15 Cm. LASER beam is the light source employed hence, LIDAR is monochromatic and has no radiometric imaging advantage.