40,037 research outputs found
Knowledge development for organic systems: An example of weed management
Despite the large amount information on weed biology and specific weed control measures produced by researchers, organic farmers still prioritise weeds as an important area for further research. A recent project investigating weed management in organic farming systems has established that knowledge and learning are key requirements for this to be effective. Development of relevant, practically useful knowledge depends on access to information generated âscientificallyâ by researchers and also to knowledge generated as a result of farmer experience with weeds. This requires that farmers, advisors and researchers take a participatory approach to collecting and processing information on weed management, using it to develop new and relevant knowledge. The appropriate framework for knowledge development is thus a collegiate one in which all stakeholdersâ value and learn from the observations and experience of others. These findings have implications for the way in which research is conducted and funded
Information extraction techniques for multispectral scanner data
The applicability of recognition-processing procedures for multispectral scanner data from areas and conditions used for programming the recognition computers to other data from different areas viewed under different measurement conditions was studied. The reflective spectral region approximately 0.3 to 3.0 micrometers is considered. A potential application of such techniques is in conducting area surveys. Work in three general areas is reported: (1) Nature of sources of systematic variation in multispectral scanner radiation signals, (2) An investigation of various techniques for overcoming systematic variations in scanner data; (3) The use of decision rules based upon empirical distributions of scanner signals rather than upon the usually assumed multivariate normal (Gaussian) signal distributions
The use of the Indwe sandstone as a stratigraphic marker in the Molteno stage of the Karroo System
Main articleIn recent years it has become increasingly apparent that some of the early
concepts of stratigraphic relationships in the Molteno Stage of the Karroo System
are not wholly acceptable. Apart from the monotonously repetitive lithology,
the lack of' fossil data make regional correlation difficult. Early attempt at
correlation were based largely on the supposedly distinctive lithology of certain
sandston beds . The Indwe Sandstone , in particular, has received special attention
and its general acceptance by stratigraphers as a lithostratigraphic marker of
regional significance is now firmly entrenched in the literature.Non
Revised stratigraphy of the Beaufort Group in the southern Karoo Basin
Main articleThe Beaufort Group in the southern Karoo Basin between Graaff-Reinet and Sutherland
has been divided into three formations based primarily on the changing ratio of sandstone to
mudstone. The former Abrahamskraal Formation is elevated to subgroup status and divided
into two new formations, the Lootskloof Formation and the Verlatenkloof Formation, whilst
the Teekloof Formation is retained but more precisely defined. The Verlatenkloof Formation
includes two members, the Jakhals Valley Member and the Paalhuis Member. The Teekloof
Formation includes the Oukloof Member in addition to the previously defined and described
Oudeberg Sandstone Member. Stratotypes are erected for the new formations and members
in accordance with the recommendations of the South African Committee for Stratigraphy.
Subdivision of the formations and their relationship to the established biostratigraphy and
facies patterns provides a means of fixing and correlating the most important uranium mineralised
units in the succession with greater accuracy. These comprise the Paalhuis Member,
the Oukloof Member and the Jakhals Valley Member, although the most important mineralised
unit is the Paalhuis Member which contains up to 90 per cent of all known uranium
occurrences in the Beaufort West area.Non
A measurement of the transverse velocity of Q2237+0305
Determination of microlensing parameters in the gravitationally lensed quasar
Q2237+0305 from the statistics of high magnification events will require
monitoring for more than 100 years (Wambsganss, Paczynski & Schneider 1990).
However we show that the effective transverse velocity of the lensing galaxy
can be determined on a more realistic time-scale through consideration of the
distribution of light-curve derivatives. The 10 years of existing monitoring
data for Q2237+0305 are analysed. These data display strong evidence for
microlensing that is not associated with a high magnification event. An upper
limit of v < 500 km/sec is obtained for the galactic transverse velocity which
is smaller than previously assumed values. The analysis suggests that the
observed microlensing variation may be predominantly due to stellar proper
motions. The statistical significance of the results obtained from our method
will be increased by the addition of data points from current and future
monitoring campaigns. However reduced photometric errors will be more valuable
than an increased sampling rate.Comment: 16 pages, including 17 figures. Accepted for publication in M.N.R.A.
Engineering handbook on the atmospheric environmental guidelines for use in wind turbine generator development
The guidelines are given in the form of design criteria relative to wind speed, wind shear, turbulence, wind direction, ice and snow loading, and other climatological parameters which include rain, hail, thermal effects, abrasive and corrosive effects, and humidity. This report is a presentation of design criteria in an engineering format which can be directly input to wind turbine generator design computations. Guidelines are also provided for developing specialized wind turbine generators or for designing wind turbine generators which are to be used in a special region of the United States
Interpretation of the OGLE Q2237+0305 microlensing light-curve
The four bright images of the gravitationally lensed quasar Q2237+0305 are
being monitored from the ground (eg. OGLE collaboration, Apache Point
Observatory) in the hope of observing a high magnification event (HME). Over
the past three seasons (1997-1999) the OGLE collaboration has produced
microlensing light-curves with unprecedented coverage. These demonstrate
smooth, independent (therefore microlensing) variability between the images
(Wozniak et al. 2000a,b; OGLE web page). We have retrospectively compared
probability functions for high-magnification event parameters with several
observed light-curve features. We conclude that the 1999 image C peak was due
to the source having passed outside of a cusp rather than to a caustic
crossing. In addition, we find that the image C light-curve shows evidence for
a caustic crossing between the 1997 and 1998 observing seasons involving the
appearance of new critical images. Our models predict that the next image C
event is most likely to arrive 500 days following the 1999 peak, but with a
large uncertainty (100-2000 days). Finally, given the image A light-curve
derivative at the end of the 1999 observing season, our modelling suggests that
a caustic crossing will occur between the 1999 and 2000 observing seasons,
implying a minimum for the image A light-curve ~1-1.5 magnitudes fainter than
the November 1999 level.Comment: 11 pages, 15 figures. Accepted for publication in M.N.R.A.
Limits on the microlens mass function of Q2237+0305
Gravitational microlensing at cosmological distances is potentially a
powerful tool for probing the mass functions of stars and compact objects in
other galaxies. In the case of multiply-imaged quasars, microlensing data has
been used to determine the average microlens mass. However the measurements
have relied on an assumed transverse velocity for the lensing galaxy. Since the
measured mass scales with the square of the transverse velocity, published mass
limits are quite uncertain. In the case of Q2237+0305 we have properly
constrained this uncertainty. The distribution of light curve derivatives
allows quantitative treatment of the relative rates of microlensing due to
proper motions of microlenses, the orbital stream motion of microlenses and the
bulk galactic transverse velocity. By demanding that the microlensing rate due
to the motions of microlenses is the minimum that should be observed we
determine lower limits for the average mass of stars and compact objects in the
bulge of Q2237+0305. If microlenses are assumed to move in an orbital stream
the lower limit ranges between 0.005 and 0.023 solar masses where the the
systematic dependence is due to the fraction of smooth matter and the size of
photometric error assumed for published monitoring data. However, if the
microlenses are assumed to move according to an isotropic velocity dispersion
then a larger lower limit of 0.019-0.11 solar masses is obtained. A significant
contribution of Jupiter mass compact objects to the mass distribution of the
galactic bulge of Q2237+0305 is therefore unambiguously ruled out.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Society. New version has improved presentatio
Predicting caustic crossing high magnification events in Q2237+0305
The central regions of the gravitationally lensed quasar Q2237+0305 can be
indirectly resolved on nano-arcsecond scales if viewed spectrophotometricly
during a microlensing high magnification event (HME). Q2237+0305 is currently
being monitored from the ground (eg. OGLE collaboration, Apache Point
Observatory), with the goal, among others, of triggering ground and spacecraft
based target of opportunity (TOO) observations of an HME. In this work we
investigate the rate of change (trigger) in image brightness that signals an
imminent HME and importantly, the separation between the trigger and the event
peak. In addition, we produce colour dependent model light-curves by combining
high-resolution microlensing simulations with a realistic model for a thermal
accretion disc source. We make hypothetical target of opportunity spectroscopic
observations using our determination of the appropriate trigger as a guide. We
find that if the source spectrum varies with source radius, a 3 observation TOO
program should be able to observe a microlensing change in the continuum slope
following a light-curve trigger with a success rate of >80%.Comment: 17 pages, 16 figures, accepted for publication in M.N.R.A.
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