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Re: Double precision data into caldat [message #91884 is a reply to message #91879]

Tue, 08 September 2015 13:58

Paul Van Delst[1]
Messages: 1157
Registered: April 2002

Senior Member

Hello,

On 09/08/15 15:37, Larry H. wrote:
> Hi,
>
> I'm trying to convert some Julian dates back into standard dates
> using caldat. (In fact, I am testing what I got out of julday in the
> first place.) There should be hours and minutes in the results. If
> I take the output of julday as a variable, say TEST, and put it into
> caldat as
>
> caldat, test, m, d, y, h, mm, s
>
> I get the right answer. However, if I just use the actual value of
> test, I get odd results. So, for the Julian day 2456658.56250000, I
> should get
>
> 2014 1 1 1 30 0
>
> in year, month, day, hour, min, sec form. If I use
>
> caldat, 2456658.56250000D, m, d, y, h, mm, s
>
> the results are correct, but if I do the type conversion using
> double(), it doesn't. So I have
>
> caldat, double(2456658.56250000), m, d, y, h, mm, s
>
> and
>
> p = double(2456658.56250000) caldat, p, m, d, y, h, mm, s
>
> both giving the result
>
> 2014 1 1 0 0 0
>
> Does anyone know why this is? As far as I know, all of those input
> values are the same.

Nope. They are not.

IDL> p=2456658.56250000
IDL> help, p
P FLOAT = 2.45666e+06
IDL> print, p, format='(f25.10)'
2456658.5000000000

Single precision has about 7-8 significant digits.

So for a value you write as "2456658.56250000" -- which is a single
precision literal constant -- you've only got "2456658.5" of useful digits.

When you use DOUBLE, as in:

IDL> p=DOUBLE(2456658.56250000)

you are taking a SINGLE PRECISION literal constant (2456658.56250000)
and converting it to double. So you still lose the extra information
(the "X.X625") stuff, e.g.

IDL> p=DOUBLE(2456658.56250000)
IDL> help, p
P DOUBLE = 2456658.5
IDL> print, p, format='(f25.10)'
2456658.5000000000

Now, if you declare a double precision literal constant, like so:

IDL> p=2456658.56250000d0
IDL> help, p
P DOUBLE = 2456658.6

you are "declaring" a double precision literal so you have about 15
digits of precision-y goodness which you see when you print it out:

IDL> print, p, format='(f25.10)'
2456658.5625000000

Basically, if you ever declare literal constants in code where those
values can have more than 8 useful digits, always use double precision
declarations. Converting a single precision 8+ sig fig number to double
will have no effect.

Hope that helps.

cheers,

paulv

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[Message index]

		Double precision data into caldat By: laura.hike on Tue, 08 September 2015 12:37
		Re: Double precision data into caldat By: David Fanning on Tue, 08 September 2015 13:01
		Re: Double precision data into caldat By: laura.hike on Tue, 08 September 2015 13:38
		Re: Double precision data into caldat By: laura.hike on Tue, 08 September 2015 13:43
		Re: Double precision data into caldat By: Jim Pendleton on Tue, 08 September 2015 14:18
		Re: Double precision data into caldat By: Lajos Foldy on Tue, 08 September 2015 13:54
		Re: Double precision data into caldat By: Paul Van Delst[1] on Tue, 08 September 2015 13:58
		Re: Double precision data into caldat By: laura.hike on Tue, 08 September 2015 15:22
		Re: Double precision data into caldat By: David Fanning on Tue, 08 September 2015 15:43

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