Dear Kristian,

Believe it or not, what you’re seeing is intended behavior. But this design is certainly non-intuitive and should be changed, or at least a warning should be issued about what will happen.

Aviso publishes their data on a 0-to-360 grid. Rather than being centered on the Prime Meridian, it is centered at the 180^th Meridian. When you enter an Xmin of -10, MGET determines that the smallest X coordinate on this grid is 0 and silently clips the grid at 0 instead of -10. The intended design of the underlying clipping code is to allow the user to specify a clipping extent that is beyond the edges of a non-global dataset and automatically return the largest extent possible. This was modeled after ArcGIS’s own clipping tool, and I can see an argument for it being reasonable default behavior. But it does not really make sense for global datasets: if someone specifies a coordinate beyond the extent of a global dataset, it is not valid, or perhaps it should “wrap around” to the other side of the planet. I will see what I can do to improve this behavior. Because the clipping code is utilized in many places, I will have to do it carefully and probably won’t get to it for a while…

In any case, the way you can solve this problem is to tell MGET to rotate the 0-to-360 grid to a more traditional -180 to +180 grid. Put in -180 for the Rotate By parameter. Then go ahead with your original Spatial Extent that used -10 as Xmin. Let me know if that works.

Best,

Jason

Dear MGET team,

I was wondering if you could inform me of where I might be going wrong regarding the processing of AVISO data. I am trying to extract data for a given extent along the West/Central Coast of Africa (Xmin -10.0000; Xmax 32.000; Ymax 15.000; Ymin -18.000) as indicated by the bounding box in the attached screenshots. This extent works for data extracted from NASA JPL PO.DAAC and NOAA Oscar. However when I run this with any of the Aviso products (MSWH, MWIND and MADT) the resulting outputs do not cover the required extent – see screen shot 1 and 2 for example with MADT product. When I change the extent (Xmin -15.0000; Xmax 1.000; Ymax 15.000; Ymin -18.000) I still encounter the same problem – however it is more pronounced – see scree shot 3 and 4. Any suggestions would be greatly appreciated.

Best wishes,

Kristian

Dr Kristian Metcalfe

Centre for Ecology & Conservation

University of Exeter
Penryn Campus
Penryn
Cornwall
TR10 9FE
UK

Tel: +44 (0) 7792828705

Twitter: @_KMETCALFE

Skype: kristian.metcalfe

Web: http://www.kristianmetcalfe.com/