How does a proj-string (UTM) w/o a zone number work?

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04-25-2020 07:50 AM
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New Contributor II

Software: RStudio

Data: a world elevation map, elev, which was downloaded from Manuel Gimond's web site

The original crs is as follows:

+proj=longlat +ellps=WGS84 +towgs84=0,0,0,0,0,0,0 +no_defs

I did a test as follows, just out of curiosity:

elev_projected <- projectRaster(elev, crs = "+proj=utm +ellpse=WGS84")  # the +zone= parameter not specified 

In ArcGIS Desktop, when projecting to UTM, we have to specify which specific zone to use. R, however, seems to allow this to happen w/o complaining. Even though there was a warning message, the raster was projected anyway. 

3 projected point(s) not finite

The extent of the output is as follows:

xmin       : -14587336  xmax       : 14350664  ymin       : -19985214  ymax       : 20161986

I wonder what actually happens when the zone parameter is missing from the proj string. Does this string make any sense? The output map is attached. Thanks.       

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MVP Esteemed Contributor

There are 60 zones north of the equator that are identical and 60 zones south of the equator that are identical  (albeit, for a given datum).

The 'relative' location of objects within a zone will remain the same, however, the central meridian of each, as you know is different.  So, it really doesn't matter what you do with the data that are projected in UTM as long as you don't try to tie it into real world coordinates... which requires that the central meridian (in meters) is translatable to a longitude of the appropriate datum.

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New Contributor II

Totally agreed! We could use whatever projection available as long as we don't tie it into coordinates. The R community should have made the zone parameter "required" in this proj string. Thanks. 

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MVP Esteemed Contributor