====== Dip test - QC2d-1 - Revision 01======

{{kvalobs:kvoss:system:qc2:requirements:algorithms:dip.png|}}

The flag fs=2 is detected on the observation A(t), the previous value A(t-1) is also flagged fs=2, and the next previous value A(t-2) is flagged fs=1.

<code>
 If  
    | A(t) - A(t-2) | < | A(t-1) - A(t-2) | 
 and 
    | A(t) - A(t-1) | > DELTA
</code>

where DELTA is dependent on the parameter

<code>    
   set fs=9 for A(t-1) 
   set fs=6 for A(t)
</code>

A(t-1) receives a corrected value by interpolation. The interpolation method can be linear or by [[Akima splines]]. 

Relevant parameters are all those subject to the step check (QC1-3a):
^ paramid ^ parameter ^ δ(X) ^
|  **1**   | AA |  |
|  **2**   | BI|  |
|  **81**  | FF| 10 or 12,5  m/s TBC |
|  **85**  | FM|  |
|  **86**  | FX|  |
|  **87**  | FX_1|  |
|  **88**  | FX_6|  |
|  **89**  | FX_12|  |
|  **90**  | FG_1|  |
|  **91**  | FG_6|  |
|  **92**  | FG_12|  |
|  **93**  | FX_X|  |
|  **94**  | FG_X|  |
|  **104**  | RA|  |
|  **105**  | RR_01|  |
|  **113**  | SS_1|  |
|  **123**  | RT_1|  |
|  **172**  | PH|  |
|  **174**  | POM|  |
|  **175**  | PON|  |
|  **176**  | POX|  |
|  **177**  | PP|  |
|  **197**  | QE|  |
|  **198**  | QL|  |
|  **199**  | QLX|  |
|  **200**  | QO|  |
|  **201**  | QOX|  |
|  **211**  | TA| 7,5 °C TBC|
|  **212**  | TAM|  |
|  **213**  | TAN|  |
|  **215**  | TAX|  |
|  **221**  | TG|  |
|  **222**  | TGM|  |
|  **223**  | TGN|  |
|  **225**  | TGX|  |
|  **227**  | TJM|  |
|  **262**  | UU| 30% TBC|
|  **264**  | UN|  |
|  **265**  | UX|  |

===== Dip test flags =====
|Flag value|Definition|useinfo(2) consequence|
|**fs=4**|Kontrollert. Mistanke om feil detektert i QC1-3. Ingen tilsvarende feil i QC2d-1. Ingen korreksjon.|**Friskmeldt.**|
|fs=5|Kontrollert. Ingen mistanke om feil detektert i QC1-3. Mistanke om feil detektert i QC2d-1.Ingen korreksjon.|Originalverdi noe mistenkelig.|
|**fs=9**|Kontrollert. Observert endring høyere enn testverdi. Korrigert automatisk.|**Originalverdi sikkert feilaktig.**|

===== Previous met.no dip tests =====

The report 24/93 KLIMA "Theoretical analysis of the dip-test in quality control of geophysical observations" by Petter Øgland describes a diptest run independent from a previous step test.

[[http://klima.dnmi.no/rapporter/Klimarapporter/1993/24_93.pdf]]

Comparison between Øgland's parameter δ and the current Kvalobs step criterium shows:

   δ(UU) = 30%  versus the Kvalobs step(UU) = 30%
   δ(FF) = 30 m/s versus the Kvalobs step(FF) = 10 or 12.5 m/s
   δ(TA) = 10.5 °C versus the Kvalobs step(TA) = 7,5 °C

For wind and temperature the Kvalobs dip test will flag more often than by Øgland's formula. For humidity will Øgland's formula gives error flags without corresponding flag from the Kvalobs dip test. This occurs in a "skew" dip when the step test criterion is met in one of the two cases.