Guiding system



Overview

The acquired experience of the RESCOMAN project group regarding mapping of of submerged vegetation using airborne sensors is documented in these pages as guide for planning, acquiring and post-processing of the necessary information. Focus has predominately been on the acquisition and analysis of ground observations, aerial photography and airborne scanner data. Examples of satellite imagery from Landsat-5 TM and SPOT is also shown.

Selection of image acquisition method

The selection of the form remotely sensed data is to a large extent dependent on the availability of sensors, areas to be covered etc. For many years have aerial photography been the dominant and only affordable type of high resolution remote sensing data. For the last couple of years the use of airborne scanners have increased and the scanners are made available for research as well as for more operational purposes. In the table below, the qualities end weaknesses of the various methods are listed.

It is quite difficult to provide general recommendations. Potential users are thus recommended to view the details of each of the described methods. In some cases may the weakness of a method be adjusted or compensated by a specialized setup, an example of this is that the relative small image size in aerial photography, combined with the geometry of a camera and the lack of good ground control points over water makes geo-orientation of aerial photography difficult. This may however be compensated by a specialized setup where the attitude data are logged during the image acquisition, and directly ingested by a specialized post-processing module. 

The outset for the group has been the situation of mapping aquatic vegetation, being fully submerged and in the coastal zone in Denmark. Other recommendations would naturally occur if vegetation above the water surface are to be mapped, since near-infrared remote sensing is then an opportunity. This also leads to the conclusion that the currently available satellite data are not very well suited for this purpose - the major reason for this is the low sensibility for the low radiance levels found in the coastal zone in Northern Europe. Another element is that the vegetation is concentrated in a fairly narrow zone along the coast, which cannot be described satisfactory with Landsat-5 TM or SPOT imagery. Such conclusions might not be valid in other region of the world, with clear water, high solar irradiance and large difference in spectral signatures between bottom substrate and vegetation. The situation may also change with new sensors in the future.   

  Aerial photography Airborne scanner data Satellite imagery
Availability Easy and cheap to obtain from many service providers Price and availability is still very dependent on local conditions. The number of companies operating scanners is growing Relatively easy to obtain. Archives exists.
Acquisition medium Photographic film, (digital cameras are available) Digital Digital
Geometric resolution High (< 0.5 m) Medium (1-5 m) Low (10-30 m)
Spectral information None (only RGB separation) High, though dependent on type of scanner. Low (1-3 wide  channels)
Digitization levels  8-bit per color 16-bit per channel 6-8 bit per channel
Depth penetration Medium High to medium dependent on type and configuration Low
Units Relative counts Radiance Digital numbers or Radiance
Price Medium costs per sq. km. High initial cost, medium cost per sq. km. Low cost per sq. km.
Coverage per image ~1km * 1km ~ 1km width, variable length.  > 60 * 60 km
Ortho-rectification Special module Special module General method
Ground control points few needed per image many needed per image, dependent on length  relatively few
 

Prior to the selection of a specific method it is necessary to consider the type of mapping to be carried out:

  • Is the area of interest large, requiring hundreds of photos or scan lines.

In this case it is important that there is a close link between the acquisition and the post-processing, especially the geo-orientation. 

  • Is the mapping to be repeated with exact similar instrumentation.

The airborne scanners (imaging spectrometers) are not very abundant, so repetition may not always be guaranteed. On the other side does the scanners, by proper calibration information, also support a more e exact description of the data acquisition, regarding the spectral bands with and placement an the radiometric response function.  

  • Is the mapping limit corresponding to the local Secchi-depth or deeper

Due to the higher number of digitization levels and adjustable configuration airborne scanner provide a better discrimination between vegetation and bottom than aerial photography at greater water depth. 

  • Is the vegetation to be mapped situated in a wide or narrow zone, and is the colonization form homogeneous or dense "islands" in a complex matrix of bare bottom or other vegetation types.

Aerial photography is generally better suited for the higher spatial resolution. Some of the airborne scanner systems may not provide data with less than 2-3 meters resolution.

For estimating the approximate amounts of vegetation in larger areas, a low resolution (2-4 m) is sufficient. By contrast, analysis of distinct outlines of vegetation beds requires a high spatial resolution (<=1m).

Guide on aerial photography

Guide on airborne scanner data

Conclusions on satellite imagery 


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This page was updated: 19. September 1999
These pages are maintained by Michael Stjernholm, NERI

Rescoman - Quotations allowed with reference.