[Esip-documentation] Request for ACDD global attributes for geospatial data resolution

[Mary Jo Brodzik]

Dear Esip-documentation members,

Thank you for the discussion of my request at Monday's telecon, I 
apologize for missing the telecon.  I just listened to the youtube 
recording of it, which was useful.

In the time since my request, I had to start producing data, so I took the 
existing geospatial_lat/lon_resolution practices as my model.

I actually created my files with the attributes you suggested at the 
meeting, e.g.:

geospatial_x_resolution = "3125.00 meters" ;
geospatial_y_resolution = "3125.00 meters" ;

It did not occur to me to populate and set geospatial_x_min/max or 
geospatial_y_min/max; I reasoned that these were covered by the x/y 
dimension variable bounds.  However, in the spirit of giving the user of 
my data set some intelligible information in the global attributes, I also 
populated the lat/lon bounds like this:

geospatial_bounds_crs = "EPSG:6931"
geospatial_lat_min = 0.
geospatial_lat_max = 90.
geospatial_lon_min = -180.
geospatial_lon_max = 180.

I realized that this mixed mensurations a bit, since I did not populate 
attributes for (nonsensical, in my case) geospatial_lat/lon_resolution.

I think that your proposed new attributes 
geospatial_x/y_resolution/min/max would definitely meet my use case.

As for definitions for your minutes page at:

http://wiki.esipfed.org/index.php/Documentation_Cluster_Minutes_2017-01-23

may I suggest the following definitions for a beginning point (modified 
from the analogous ACDD lat/lon attributes):

geospatial_x_min: Describes leftmost limit for projected data x 
dimension in a left-handed Cartesian plane; specifies the lowest x 
dimension value covered by the dataset.

geospatial_x_max: (replace leftmost/lowest with rightmost/highest)

geospatial_y_min: Describes bottommost limit for projected data y 
dimension in a left-handed Cartesian plane; specifies the lowest y 
dimension value covered by the dataset.

geospatial_y_max: (replace bottommost/lowest with uppermost/highest)

geospatial_x_resolution: Information about the targeted spacing of 
projected data points in x dimension. Recommend describing resolution as a 
number value followed by the units. Example: '3125.00 meters'

geospatial_y_resolution: (replace x with y)

Regarding your question about how similar the projected data case is to 
the swath data case:  I would say that a rectangular array of projected 
data is different from swath data, because the spacing between adjacent 
pixels across the array is fixed in map coordinates, e.g. from one pixel 
to the next in my data, the spacing is always 3.125 km in the map 
coordinates (the projected plane).  Depending on the projection, the 
correspond location distances can and will be different on the sphere, and 
in terms of latitute or longitude, but in map coordinates it is a single 
number and does have meaning for a user.  I'm using the term "map 
coordinates" as in the left-handed Cartesian plane in the second figure of 
this document:

https://nsidc.org/support/41976964-Points-Pixels-Grids-and-Cells-A-Mapping-and-Gridding-Primer-

Thank you for your time and consideration, I will be happy to answer any 
other questions about my use case if they arise.

Mary Jo

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Mary Jo Brodzik, Senior Associate Scientist, 303-492-8263
NSIDC/CIRES, Univ. of Colo. at Boulder, 449 UCB, Boulder, CO 80309-0449
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"We cannot solve our problems with the same thinking we used
when we created them."  --Albert Einstein