2.2. Cultivo de uva de mesa
2.2.1. Descripción
2.2.4.8. Fundamento fisiológico del color en la uva de mesa
more detailed sampling and higher definition mapping are generally only possible
on projects studying limited areas (e.g. Kirkman
&Reid 1979; Neverauskas
1987b; Greenway
&Fry 1988). A survey of the N.S.W. coastline looking at
se,agrass and mangrove distribution sampled a comparable number of sites to
this Tasmanian study, and chose a map scale of 1:25 000 (West et al. 1985).
There are combinations of environmental factors that make the remote sensing of underwater vegetation difficult or impossible in some circumstances. Wind disturbance must be minimal, and recent conditions should preferably have been calm and dry, leading to minimum turbidity through wave action and runoff. High levels of phytoplankton, tannins or suspended solids in the water may obscure benthic features, and the sun's elevation at the tim.e of image capture may reflect directly off the water surface. These factors are not so relevant for photography of terrestrial features, yet can render a photographic series useless for detecting benthic features. In the case of archival photographs this may cause a significant gap in the time series for a particular location. Additionally, because light attenuation in water can vary considerably, the deeper boundaries of seagrass communities are often difficult or impossible to distinguish.
Other problems arise in interpreting benthic features off photographs. Estimates of density become difficult as seagrass beds increase in depth because the tone darkens, and contrast with the surrounding substratum and water decreases. Also, deeper basins in the substratum may be indistinguishable from seagrass beds. Detritus from seagrass leaves and macroalgae also poses interpretation difficulties as it may appear identical to live seagrass (Kirkman et al. 1988).
Flights can be specifically tailored for photographing benthic communities. Various recommendations are offered by different researchers. One suggestion for ideal conditions under which to photograph seagrasses is from low altitude, at times of low cloud cover, at low tide and with the sun near or at its zenith (FAC 1992). Generally, however, cloud cover is not recommended by researchers as shadows can be confused with seagrasses. Kirkman et al. (1988) used photographs taken on a cloudless day with a sun angle 45°. Orth and Moore (1983) suggest clear skys with a sun angle of 20° to 40° to minimise surface reflectance, and Lefevre et al. (1984) provide formulae for optimising picture quality at different periods of the year given the changing sun angles. They also give guidelines on selecting the most appropriate optical filters for monochrome photography to enhance vegetation and contrast under different conditions.
Colour aerial photographs can give better water penetration and easier identification of species than monochrome images, although they are not as suitable for spectrally based analyses of species and epiphyte cover as satellite images (Bleys et al. 1991). 1n Tasmania, aerial photograph projects in colour are only available from the early 1980's onwards, and for cost reasons the Department ?f Environment and Planning generally only holds prints in black and white of
these projects. Colour prints must be ordered specially and are expensive. Thus, although they may reveal useful additional information of the benthos,
they have been little used in this project.
Examination of aerial photographs over a time series can reveal boundary movement, changes in area and density. There are often problems in having to use photographs of different scale and quality, and these must be acknowledged in presenting results. Additionally, archival photographs obviously cannot be ground truthed, although oral histories and earlier research references can be useful substitutes.
A further problem in the use of photographs are their inherent distortions. These are due to slight changes in the attitude and height of the aircraft, lens distortion, and a progressively increasing angle from vertical of features from the centre to the edge of the photograph. These factors lead to the need to transform photographs by optical or digital means in order to match adjacent photographs into mosaics. This can be achieved by GIS software such as ARC/IN'F01'M or Adobe PhotoshopTM (Bleys
et al.
1991). If such transformations are not possible, it is considered that errors are acceptable if only the central 1 /3rd of a photograph is used.3.1.2.3 Aerial video photography
Recent technology has enabled the capture of detailed analogue images from video tapes taken from aircraft. The costs involved are very favourable compared
.
to other forms of remote sensing. Since many hundreds of images are taken in the course of a transect, the most suitable ones can be chosen and processed to overlay on real-world coordinates. Such processing can be carried out on modest desktop computers. This technology promises to provide a means of regular monitoring and updating of seagrass coverage and other areas of environmental
concern.
3.1.3 Information (GIS)
A GIS can be described as a computer based system capable of capturing, storing, checking, updating, integrating, manipulating, analysing and displaying all forms of geographically referenced information (ESRI 1990; Board 1991). A GIS allows a relational database to be attached to map information, and can be manipulated to graphically represent interelationships between different sub-sets of_ the database. Data is added in 'layers' to a base map, and, once a GIS is established, the number of layers of data that can be added is only limited by the capability of the computer hardware and software to handle it. As with any
database, however, the quality of the output can only be as good as the quality of the in put.
When mapping seagrass communities, data on the species composition, their density, depth, the coastal geomorphology, the location of sample sites for ground truthing and any other parameters, can be referrenced to the mapped outlines of seagrass beds, to sample sites and the base map. The outlines of the seagrass beds from different times can be overlayed onto this base map as separate layers, and likewise the sample sites (see Figure 3.1). In this project the base map is the coastal outline of Tasmania, digitised at 1:25 000.
Each sample site has a unique identity and is related to a data base which carries the values for the measured parameters of all sites. Once entered, the data can be manipulated and cross referenced to generate a desired output. For example, the areas of the beds can be calculated and compared between different times thus providing a measure of any changes in the coverage of seagrasses in a particular place, or over the whole State. The areas occupied by different species can be obtained, and again changes over time calculated. Similarly, the occurence of a given species can be related to the type of coastal area, substratum or depth. These data analyses can be viewed on a screen at any window size, and produced as hardcopy maps at any scale to suit particular needs.
ARC/INFO: layering of data related to the coastal outline base map
Seagrass circa 1990
Other layers of data can later be added to the GIS. These may, for example, relate to human activity in related catchments such as industry, waste disposal, Chapter 3. Seagrass Mapping