How to link time-dependent data tables ?

[coho66]

what's up doc

thanks for the contribution there doc...

I think I have the logic figured out now:

assuming a horizontal cutoff and no upper threshold...i think i have 3 possible scenarios:

1)Tmax & Tmin < base temp (low threshold)-> # DD = 0
2)Tmax & Tmin >= base temp-> #DD = ((Tmax+Tmin)/2)-base temp
3)Tmax > base temp AND Tmin < base temp-> # DD = formula # 4 in the pdf by Zalom et al posted by Alex, OR one could less accurately approximate it by assuming base temp as Tmin and using formula # 2 listed above and then one gets: #DD = ((Tmax+base temp)/2)-base temp

 

[Alex E.]

The lower cut off or Tb is critical in several ways:
(a) it defines when certain bugs start getting active (emerge from dormancy) – see pest management tables for exact temperatures for specific pests.
(b) it determines what values you are calculating. The lower your Tb the bigger your numbers will become. If your Tb is totally wrong, then the degree day curve (cumulative) becomes meaningless. For instance, if a perennial flower is supposed to flower between 1000 and 1100 degree-days and your calculations are way off then your predictions can be off by as much as a month. Same goes for the poor farmer who may be wasting his expensive pesticide at a time when the eggs for the next generation of bugs have long hatched or the other extreme, spraying when there are no eggs yet.
(c) when Tb is above the daily minimum temperature then the basic formula calculates zero degree-days which is wrong. There are/were still a number of hours in the day when plants grew happily – they did not “ungrow” after the minimum temperature dropped below Tb.
The actual degree-days (fractions) missed do add up over time; particularly since you are plotting a cumulative curve. The single sine method takes care of this situation.

 

[Alex E.]

Thanks for your patience doc...

Your detailed instructions of how to work with time points as well as time intervals are quite intriguing. I certainly will try your approach.

Unfortunately, I still have this nagging question about linking (or not linking) my tables. So far, I had assumed that I must link tables for queries to work. You may have to forgive me if I am wrong, but I am getting the impression from your latest posting that I could run two queries – one on each of two non-linked tables – and then run a join query. This thought is almost too nice to be true.

Linking tables is critical for maintaining data integrity over several tables. In my case, I believe I can get away without rigorous data integrity enforcement. My data are static (historical).

 

[The_Doc_Man]

I believe it is possible to do this with a two-source query and a where clause, whether or not the keyword JOIN is used. If using two queries as the source won't allow a JOIN, just use a SELECT a.x, b.y, etc etc FROM a, b WHERE a.x = b.y clause etc etc