Ticket #511 (closed Defect: fixed)
All Aviso products are erroneously offset to the north and east by 1/2 grid cell
|Reported by:||jjr8||Owned by:||jjr8|
|Component:||Tools - Data Products - Aviso||Version:|
Description (last modified by jjr8) (diff)
Here is the email thread describing the problem:
From: AVISO [mailto:firstname.lastname@example.org] Sent: Tuesday, January 03, 2012 3:49 AM To: email@example.com Subject: RE : [aviso] In the gridded data, to which point does lat/lon refer to? CRM:01213193 Dear Jason, indeed, you are right: in the gridded data, the data are stored as points and not boxes that means that the lat/lon refers to the exact point of the given value (and not the SW corner as mentionned in the FAQ). This is explained in the section 4.3 of the handbook named: "Ssalto/Duacs User Handbook: (M)SLA and (M)ADT Near-Real Time and Delayed Time Products" that you can find at http://www.aviso.oceanobs.com/en/data/tools/aviso-user-handbooks/index.html So, we will correct the FAQ asap. Furthermore, the definition of the Mercator grid is defined in section 4.3.2 of the same handbook. We apologize for not answering your question some years ago. Thank you for releving the mistake and hoping this has answered your question Best wishes to you and your family Françoise Mertz Aviso User Services ------------------- Message d'origine ------------------- De : ROBERTS, JASON Date d'envoi : 12/12/2011 À : AVISO Objet : [aviso] In the gridded data, to which point does lat/lon refer to? Dear Aviso, In the FAQ (http://www.aviso.oceanobs.com/en/services/faq/index.html), there is this question: Question: In the gridded data, to which point lat/lon refer to? Answer: The lon/lat given is the SW corner of the box, as specified in the metadata of the NetCDF file Are you sure that answer is correct? Does the lat/lon refer to the center of the box instead? I have been comparing geostrophic currents for recent years of Aviso DT-MSLA Upd to OSCAR (http://www.oscar.noaa.gov/). The two products produce similar estimates and have the same 1/3º resolution I have noticed that they appear to be "off" by 1/2 cell from each other. When I shift the Aviso image by 1/2 cell to the southwest, the alignment is much better. You can observe this effect by rapidly switching between the attached images. You will see that OSCAR.png and AVISO_Center.png correspond fairly well, while AVISO_SW_Corner.png appears to be "off" from both of those. After observing this, I reminded myself that the Aviso FAQ was not necessarily wrong; it could be that I misaligned the OSCAR data. But after further investigation, I believe the Aviso FAQ is wrong and that the OSCAR data are correctly aligned. I have two reasons: 1. The OSCAR coordinate system is simpler than Aviso, and therefore it is less likely that I misunderstand it. OSCAR uses an equirectangular projection (sometimes called plate carrée), with clear latitude boundaries of 80 ºN to 80 ºS. I am very confident I have understood their coordinate system and applied it correctly. Aviso uses a Mercator projection but does not anywhere specify the parameters of this projection for the merged satellite product (the FAQ does specify some reference ellipsoid parameters, but not for the merged product, and omits the other characteristics of the Mercator projection). As you may recall, I asked about the details of the Mercator projection two years ago and, unfortunately, I never received a satisfactory answer. (This is not your fault; perhaps if I spoke French I could communicate my confusion better.) 2. If I apply the rule of the FAQ-that lat/lon refers to the SW corner of the box, rather than the center-to the Mediterranean data, is obviously poorly aligned with shoreline datasets such as GSHHS. It seems unlikely that the Med product would use coordinates of cell centers while the global product would use coordinates of the southwest corner. In conclusion, I believe the FAQ is wrong. It should say that the lat/lon refer to the center of the box, not the southwest corner. Can you please confirm or deny this? Thanks for your help, Jason
All MGET tools that work with Aviso data were based on the erroneous answer in the Aviso FAQ. As a consequence, they improperly shift the data 1/2 cell to the north and east from the correct position. The solution is to remove the 1/2 cell shift.