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Topics - ChadGreene

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The rest / Ice sheet photography tips
« on: October 08, 2015, 08:04:32 PM »
Does anyone have any good tips for manual photography on an ice sheet?  I've googled around a bit, but most "12 Tips for Photographing Snow" type webpages are severely lacking in depth.  Many of these lists include tips like, "wear warm clothes."  Most snow photography tips seem geared toward mountain landscapes where photos contain a fair amount of color (rock outcrops, trees, creeks) throughout.

I find that on ice sheets my surroundings are quite stunning when I'm there, but the lack of color or contrast makes it difficult to capture with a camera.  What tricks do you have and when do you use them?  The two tricks I've found:

1. Always turn off the automatic white balance to prevent snow from looking gray, and

2. Use a polarizing lens to increase contrast at the horizon between snow and sky.

What else should I be doing?

2
Antarctica / Southern Hemisphere Sea Ice grid error?
« on: September 15, 2015, 10:20:35 PM »
It looks like the NSIDC's georeferencing grids for sea ice data are offset by 400 km in the polar stereographic x direction.  In Matlab you can download and import a day's gridded sea ice concentration grid and corresponding lat,lon grids like this:

urlwrite('ftp://sidads.colorado.edu/pub/DATASETS/seaice/polar-stereo/tools/pss25lons_v3.dat','pss25lons_v3.dat');
urlwrite('ftp://sidads.colorado.edu/pub/DATASETS/seaice/polar-stereo/tools/pss25lats_v3.dat','pss25lats_v3.dat');
urlwrite('ftp://sidads.colorado.edu/pub/DATASETS/nsidc0081_nrt_nasateam_seaice/south/nt_20150914_f17_nrt_s.bin','test.bin');

fid = fopen('pss25lons_v3.dat');
lon = fread(fid,[316 332],'long','ieee-le')/100000;
fclose(fid);

fid = fopen('pss25lats_v3.dat');
lat = fread(fid,[316 332],'long','ieee-le')/100000;
fclose(fid);

fid = fopen('test.bin');
ci = fread(fid, [316 332]);
fclose(fid);


Using Antarctic Mapping Tools (http://www.mathworks.com/matlabcentral/fileexchange/47638), the grid looks okay until you put it in context with any other dataset such as a Bedmap2 Grounding line:

antmap
pcolorm(lat,lon,ci)
bedmap2 'grounding line'




To be clear, this is not an issue with the polar stereographic projection.  It is a problem with the lats and lons given in the pss25lats_v3.dat and pss25lons_v3.dat files.  This can be shown by plotting raw data in unprojected coordinates. 

In the sea ice concentration grid, values of 253 correspond to the coastline and 254 is the ice sheet.  Below I'm plotting each grid point associated with the ice sheet as black dots, and each grid point associated with the coast line as blue dots.  I'm also labeling McMurdo Station which is located at (166.67E,77.85S).  McMurdo is an identifiable location on any map because it sits on the coast and it was chosen by R.F. Scott because it is the southernmost point a ship can sail.

% Get indices of coastline:
coastline = ci==253;

% Get indices of land:
icesheet = ci==254;

figure
plot(lon(icesheet),lat(icesheet),'k.')
hold on
plot(lon(coastline),lat(coastline),'b.')

% Mark and label McMurdo Station:
plot(166.67,-77.85,'ro','linewidth',4)
text(166.67,-77.85,'MCM','vert','top',...
    'horiz','center','fontweight','bold','color','red')

xlabel 'longitude'
ylabel 'latitude'




McMurdo should sit directly on the coast.  To make it a bit more clear, we can double the grid for continuity and then plot the same as above.  Here's how I double the grid:

lat = [lat lat];
lon = [lon lon+360];
ci = [ci ci];




I've explored this a bit and it looks like the offset is 400 km in the polar stereographic easting direction.  It looks like the values are incorrect here as well: http://nsidc.org/data/polar_stereo/ps_grids.html.  I contacted NSIDC about the issue and they offered a proof that their grids are correct (http://nbviewer.ipython.org/gist/hwilcox/7e533c42abd38975a83a), but that proof does not address the issue that their grid incorrectly places McMurdo Station in the western hemisphere.

If I am missing something, please let me know.  Otherwise it may be best to manually georeference your sea ice data by

[x,y] = meshgrid(-3550000+12500:25000:4350000-12500,4350000-12500:-25000:-3950000+12500);
[lat,lon] = ps2ll(x,y,'TrueLat',-70,'EarthRadius',6378273,'Eccentricity',0.081816153);


and set

ci = flipud(rot90(ci));




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