[netCDF #SJY-200306]: Support understanding COARDS data

Hello Tony,

Thanks for the interesting question! I suspect your suspicion is right 
regarding the Mercator projection.  Calculating the longitudinal index via the 
slope/intercept formula works because the longitudinal data are uniformly 
spaced.  Because the latitudinal values are Mercator spaced, you cannot use the 
same formula.  Unfortunately, I am unable to tell you what formula you *should* 
use.  You may need to find an alternate dataset which contains uniform 
projections, or create a new dataset in which you've applied an 
inverse-mercator projection to the latitudes contained within this file.

I feel silly supplying you with a link to the Mercator page on Wikipedia, as 
I'm sure you have already looked at it; however, since I'm am going to refer 
you to it, I include the link for completion.

* https://en.wikipedia.org/wiki/Mercator_projection

I suspect the answer to your question may be found in the mathematics on this 
page, but again, perhaps the simpler answer would be to locate a different 
dataset which contains uniform latitude and longitudes.

I'm sorry I can't provide a more directly useful answer, but hopefully this 
information will be of some use to you.

Have a good day,

-Ward



> Hi,
> 
> I downloaded smith_sandwell_topo_v8_2.nc from
> https://www.unidata.ucar.edu/software/netcdf/examples/files.html, and am
> trying to understand the data it contains.  I've been able to read the
> values, and have come up with a fairly complete map of the data within.
> However, my calculations to determine the latitude are yielding
> apparently incorrect values, with latitude increasingly wrong as I
> approach the poles.  This sounds like something to do with projetion
> issues, but I'm not sure where my math is wrong?  Can you help?
> 
> As a sample data point, I'm using the southernmost point of Africa.
> Google Maps reports this to be -34.837352,19.980005.  The same point
> within the smith_sandwell file appears to be at 591,2060.
> 
> I've read the following values from the smith_sandwell file:
> longitude = 10800 ;
> latitude = 6336 ;
> longitude:long_name = "Uniformly spaced longitudes (.01667E -> 359.9833E)
> latitude:long_name = "Mercator spaced latitudes (72.0009S -> 72.0009N)" ;
> 
> To calculate that point's longitude, I'm running the following calculation:
> 591/10800 * (359.9833 - .01667) + .01667 = 19.714843919
> 
> This seems more or less correct.  I'm using the same calculation to
> determine latitude, and coming up with:
> 2060/6336 * (72.0009 - -72.0009) + -72.0009 = -25.182132955
> 
> This is wrong - I assume I'm not accounting for the Mercator projection
> somehow, but I'm not sure how.  Can you tell me what I need to do to
> calculate the correct latitude given the y axis origin, range and
> point's y location?  Thanks.
> 
> Tony Howard
> 
> 
> Hi,
> 
> I downloaded smith_sandwell_topo_v8_2.nc from
> https://www.unidata.ucar.edu/software/netcdf/examples/files.html, and am
> trying to understand the data it contains.  I've been able to read the
> values, and have come up with a fairly complete map of the data within.
> However, my calculations to determine the latitude are yielding
> apparently incorrect values, with latitude increasingly wrong as I
> approach the poles.  This sounds like something to do with projetion
> issues, but I'm not sure where my math is wrong?  Can you help?
> 
> As a sample data point, I'm using the southernmost point of Africa.
> Google Maps reports this to be -34.837352,19.980005.  The same point
> within the smith_sandwell file appears to be at 591,2060.
> 
> I've read the following values from the smith_sandwell file:
> longitude = 10800 ;
> latitude = 6336 ;
> longitude:long_name = "Uniformly spaced longitudes (.01667E -> 359.9833E)
> latitude:long_name = "Mercator spaced latitudes (72.0009S -> 72.0009N)" ;
> 
> To calculate that point's longitude, I'm running the following calculation:
> 591/10800 * (359.9833 - .01667) + .01667 = 19.714843919
> 
> This seems more or less correct.  I'm using the same calculation to
> determine latitude, and coming up with:
> 2060/6336 * (72.0009 - -72.0009) + -72.0009 = -25.182132955
> 
> This is wrong - I assume I'm not accounting for the Mercator projection
> somehow, but I'm not sure how.  Can you tell me what I need to do to
> calculate the correct latitude given the y axis origin, range and
> point's y location?  Thanks.
> 
> Tony Howard
> 
> 
> Hi,
> 
> I downloaded smith_sandwell_topo_v8_2.nc from
> https://www.unidata.ucar.edu/software/netcdf/examples/files.html, and am
> trying to understand the data it contains.  I've been able to read the
> values, and have come up with a fairly complete map of the data within.
> However, my calculations to determine the latitude are yielding
> apparently incorrect values, with latitude increasingly wrong as I
> approach the poles.  This sounds like something to do with projetion
> issues, but I'm not sure where my math is wrong?  Can you help?
> 
> As a sample data point, I'm using the southernmost point of Africa.
> Google Maps reports this to be -34.837352,19.980005.  The same point
> within the smith_sandwell file appears to be at 591,2060.
> 
> I've read the following values from the smith_sandwell file:
> longitude = 10800 ;
> latitude = 6336 ;
> longitude:long_name = "Uniformly spaced longitudes (.01667E -> 359.9833E)
> latitude:long_name = "Mercator spaced latitudes (72.0009S -> 72.0009N)" ;
> 
> To calculate that point's longitude, I'm running the following calculation:
> 591/10800 * (359.9833 - .01667) + .01667 = 19.714843919
> 
> This seems more or less correct.  I'm using the same calculation to
> determine latitude, and coming up with:
> 2060/6336 * (72.0009 - -72.0009) + -72.0009 = -25.182132955
> 
> This is wrong - I assume I'm not accounting for the Mercator projection
> somehow, but I'm not sure how.  Can you tell me what I need to do to
> calculate the correct latitude given the y axis origin, range and
> point's y location?  Thanks.
> 
> Tony Howard
> 
> 

Ticket Details
===================
Ticket ID: SJY-200306
Department: Support netCDF
Priority: Normal
Status: Closed