#2876: r.sun (mode 1): data are not timestamped correctly
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Reporter: fabriziosossan | Owner: grass-dev@…
Type: defect | Status: new
Priority: normal | Milestone: 7.0.4
Component: Raster | Version: 6.4.5
Resolution: | Keywords: r.sun
CPU: x86-64 | Platform: Linux
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Comment (by fabriziosossan):
Hi,
This is our logic explanation for changing the time offset sign from
positive to negative.
It should be noted first that there is no agreed convention for the sign
of the Equation of Time (EoT in the following) and can appear in both the
forms shown in [1] and [2]. The EoT implemented in the code is as former,
where positive y values denotes that a wall clock is slower than a
sundial, and vice-versa.
For example, assuming to be i) at greenwich latitude ii) on the 1st
January (when EoT is approx. -3 minutes) and iii) with the sun exactly at
south (solar noon), a wall clock would appear faster and would mark 12:03
of civil time. Therefore the additive offset to pass from the solar time
to civil time should be positive, namely the EoT expression should be with
inverted sign.
Inverting the EoT sign in order to obtain, for example, a positive offset
for the 1st of January, is coherent with the correction that is applied
for the longitude displacement later in code:
longitTime = -longitude / 15.
In this case, when moving eastwise from the greenwich meridian (this means
progressively increasing positive values of the longitude), the sun
position will move in the sky to west: in this case the clock would
appear slower than a sundial and the offset, conversely to the previous
case, should be indeed strictly negative.
Finally, one might wonder if later in the code the signs of the discussion
expressions are changed: this is not the case as these two contributions
are summed up together.
[1]
https://en.wikipedia.org/wiki/Equation_of_time#/media/File:Equation_of_time.svg
[2] https://en.wikipedia.org/wiki/Equation_of_time#/media/File
:Tijdvereffening-equation_of_time-en.jpg
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Ticket URL: <http://trac.osgeo.org/grass/ticket/2876#comment:4>
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