How to convert Latitude Longitude to UTM - conflicting UTM Zone codes

Question

Just as in this question, I download a Sentinel TIFF file:

from pystac_client import Client

catalog = Client.open("https://earth-search.aws.element84.com/v0")

mysearch = catalog.search(collections=['sentinel-s2-l2a-cogs'],
                          bbox=[-72.5,40.5,-72,41],
                          query =  {"eo:cloud_cover":{"lt":1}},
                          datetime='2020-01-01/2021-03-31',
                          max_items=10)
print(f"{mysearch.matched()} items found")

resdict = mysearch.get_all_items_as_dict()

Select one:

resdict['features'][3]['assets']['B08']

This is a dict:

{'eo:bands': [{'center_wavelength': 0.8351,
   'common_name': 'nir',
   'full_width_half_max': 0.145,
   'name': 'B08'}],
 'gsd': 10,
 'href': 'https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/18/T/YL/2021/3/S2A_18TYL_20210330_0_L2A/B08.tif',
 'proj:shape': [10980, 10980],
 'proj:transform': [10, 0, 699960, 0, -10, 4600020, 0, 0, 1],
 'roles': ['data'],
 'title': 'Band 8 (nir)',
 'type': 'image/tiff; application=geotiff; profile=cloud-optimized'}

Let's see what Rasterio can tell us about the file:

src = rasterio.open('https://sentinel-cogs.s3.us-west-2.amazonaws.com/sentinel-s2-l2a-cogs/18/T/YL/2021/3/S2A_18TYL_20210330_0_L2A/B08.tif')

Then, if I do:

src.crs.wkt

I get:

PROJCS["WGS 84 / UTM zone 18N",GEOGCS["WGS 84",DATUM["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],AUTHORITY["EPSG","6326"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4326"]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",-75],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",0],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Easting",EAST],AXIS["Northing",NORTH],AUTHORITY["EPSG","32618"]]

This seem to suggest that for Latitude - Longitude conversion, I need to use UTM zone 18N. Lets plot the image:

plt.figure(figsize=(8,8))
src = rasterio.open(resdict['features'][3]['assets']['B08']['href'])
plt.imshow(src.read(1), cmap='pink')

This is the tip of Long Island. On Google Maps, lets plot a point 41, -71.5 on this area:

The coordinates 41, -71.5 seem to be on my image as well. I then use this website to convert 41, -71.5 to UTM, I get this result:

Ie it seem to think that UTM Zone 18T is used.

Using Wikipedia's map, the 18T Zone seems to be correct.

Shouldn't an image showing the tip of Long Island have the same UTM Zone as a coordinate corresponding to the tip of Long Island? Why is this discrepancy?

I am concerned about this because it was suggested in the comment section of this question that to find pixel coordinates from Latitude-Longitude, first I need to convert to UTM. My UTM Easting and UTM Northing value might be wrong if I use the wrong Zone, at least that is my understanding.

---

(I am new to GIS.)

Answer 1

There are, confusingly, two ways of naming UTM zones. One is using the latitude band from MGRS, which is how you get 18T. The other is to simply use "N" for north of the equator and "S" for south of the equator, which is how you get 18N.

Answer 2

There's a common misconception that latitude bands serve a purpose in the UTM system. The UTM standard does not operate with latitude bands, but many notations include a hemisphere designator (N or S) after the zone number. The confusion likely stems from the Military Grid Reference System (MGRS), which derives from the UTM and UPS systems.

In the UTM system, all you need for a non-ambiguous coordinate is an Easting, a Northing, a UTM zone number, and which hemisphere the point is in.

This is not really an issue in the Northern Hemisphere, because the Northing is the same in all latitude bands from N to X. The letter S, however, can either refer to the Southern Hemisphere or the latitude band S, which would be in the Northern Hemisphere. Since the latitude bands don't really serve a purpose, but rather cause confusion and ambiguity, their use should be avoided.