This library converts coordinates in latitude/longitude to UTM and from UTM to latitude/longitude.

Here's a sample run showing all off essentially all of the functionality.

    * (ql:quickload 'utm)
    To load "utm":
      Load 1 ASDF system:
        utm
    ; Loading "utm"
    
    (UTM)
    * (utm:lat-lon-to-utm 39.264657358 -105.396267073)
    
    465814.36361674307d0
    4346221.50650324d0
    13
    * (utm:utm-to-lat-lon 465814.36361674307d0 4346221.50650324d0 13)
    
    39.264654456410966d0
    -105.39627074290249d0
    * (utm:lat-lon-to-utm 39.264657358 -105.396267073 :ellipsoid "WGS72")
    
    465814.3743203891d0
    4346220.137838842d0
    13
    * (utm:utm-to-lat-lon 465814.3743203891d0 4346220.137838842d0 13 :ellipsoid "WGS72")
    
    39.26465442463012d0
    -105.39627074272353d0
    * (utm:ellipsoid-names)
    
    ("NAD83" "WGS84" "GRS80" "WGS72" "Australian1965" "Krasovsky1940"
     "International1924" "Hayford1909" "Clake1880" "Clarke1866" "Airy1830"
     "Bessel1841" "Everest1830")
    *

It is based heavily on information from Steve Dutch's UTM info website and UTM formula website.

It does not make any attempt to correct for the polar regions, so the results for those areas may be incorrect. To learn why UTM breaks at the polar regions, see here.

The calculations seem to be fairly accurate. Converting from lat/lon-> utm -> lat/lon returns the same latitude and longitude within 6 or 7 decimal places, which is probably more accurate than coordinates coming from a regular GPS unit. The results also match up with several online converters, such as this very good one on the University of Montana's website.