[postgis-users] day length calculation

[David William Bitner]

CREATE OR REPLACE FUNCTION daylength(real, int)
  RETURNS double precision AS
'select 24*(ACOS(1-(1-TAN(radians($1))*TAN(.409088 * COS(.0172024 *
int))))/PI());'
  LANGUAGE 'sql' VOLATILE;

usage:
select daylength(latitude,dayofyear)

so if you have a geometry column and a day of year column...
select daylength(Y(geom),dayofyear) from your table;

If you want it to just take a date instead of dayofyear, you could use this:

CREATE OR REPLACE FUNCTION daylength(real, date)
  RETURNS double precision AS
'select 24*(ACOS(1-(1-TAN(radians($1))*TAN(.409088 * COS(.0172024 *
extract(doy from $2)))))/PI());'
  LANGUAGE 'sql' VOLATILE;

I pulled these equations from here:
http://dc.metblogs.com/archives/2006/12/the_longest_nig.phtml


David

On 1/26/07, Kirk Wythers <kwythers at umn.edu> wrote:
>
> I am looking for ideas to add a daylength calculation to a postgis
> database. The database in question has a table named "sites", which
> contains lat, lon, and geometry info for a number of locations. A
> second table contains climate data including a day_of_the_year column.
>
> I would like to add the capability to calculate the length of each
> day (could be done as a view or another table). The equation for day
> length calculation takes as input day_of_the_year and latitude, does
> some trig (uses acos, tan, cos, radians, and pi) and spits out an
> estimate of the day length in hours.
>
> How would folks recommend I approach this problem?
>
> Thanks,
>
> Kirk
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>



-- 
************************************
David William Bitner
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