Unambiguous determination of position using the UTM grid system generally requires specifi- cation of the following five elements:
• horizontal datum,
• UTM longitude zone,
• UTM easting coordinate, and
• UTM northing coordinate.
Example: Conventional Format for UTM Coordinate Specification
Problem: Consider the following UTM coordinate specification for a point: NAD 83, UTM Zone 11, N. hemis., 450300mE, 5291192mN
What is the horizontal datum?
In which UTM longitude zone is the point located? In which hemisphere is the point located?
Solution:
The horizontal datum is the North American Datum of 1983 (NAD 83). The location is within UTM longitude zone 11.
The location is in the northern hemisphere.
In some situations it may be acceptable to omit the datum, zone or hemisphere from the speci- fication, but only if the omitted elements are clearly implied by the context.
Why is it necessary to specify all five elements of the UTM coordinate specification? Let’s consider each element in turn.
The horizontal datum effectively defines the position and orientation of the graticule, relative to which each UTM grid system is defined. Therefore, the horizontal datum is an essential element in the definition of the UTM grid system. UTM coordinates defined with respect to one horizontal datum differ from those defined with respect to another horizontal datum. For instance, the UTM grid system defined with respect to the North American Datum of 1927 (NAD 27) and that defined with respect to NAD 83 are different grid systems. The two grid systems bear a superficial resemblance to one another because they’re structured similarly (i.e., both use 6◦ longitude zones and 8◦ latitude zones, etc.). However, they’re different grid systems because the positions and orientations of the graticules for the two systems differ. This distinction isn’t a mere technicality. For instance, according to the Department of the Army [2001], UTM coordinates for the same point, but corresponding to different horizontal datums, may differ by as much as 900 m.
If the hemisphere designation (or latitude-zone designation) is omitted from the UTM co- ordinate specification, then the point’s position is effectively determined only to within two possible locations – one in each of the northern and southern hemispheres. The one exception to this is points that lie on the equator. The northing coordinates of such points will be either 0 mN (if referenced to the northern hemisphere) or 10 000 000 mN (if referenced to the southern hemisphere). In either case, it would be clear that the point lies on the equator because the minimum UTM northing coordinate in the southern hemisphere is greater than 0 mN and the maximum northing coordinate in the northern hemisphere is less than 10 000 000 mN. Therefore, locations of points on the equator can be specified as either northern hemisphere or southern hemisphere without introducing ambiguity.
If the longitude zone is omitted from the UTM coordinate specification, then the point’s longitudinal position is effectively determined only to within 60 possible locations – one in each longitude zone.
If the easting coordinate is omitted from the UTM coordinate description, then the point’s easting position is effectively determined only to within a longitude zone. Refer to Table2.3. Each UTM longitude zone is almost 668 000 m wide at its widest part (i.e., at the equator). In the northern hemisphere, each UTM longitude zone is almost 70 000 m wide at its narrowest part (i.e., at 84◦N. lat.). In the southern hemisphere each longitude zone is over 116 000 m wide at its narrowest part (i.e., at 80◦S. lat.). That’s a lot of imprecision.
If the northing coordinate is omitted from the coordinate description, then the point’s nor- thing position is effectively determined only to within a hemisphere. As shown in Table 2.4, UTM longitude zones in the southern hemisphere are over eight million meters long, while those in the northern hemisphere are over nine million meters long (measured from northern to south- ern boundaries). Again, that’s a great deal of imprecision. The precision can be increased substantially by specifying the latitude zone (see example below).
Example: Increasing the Precision of the Northing Coordinate by Specifying the UTM Latitude Zone
Problem: How imprecise (roughly) is the UTM northing coordinate if the UTM latitude zone is included in the coordinate specification, but the numerical value of the northing coordinate is omitted? Assume the earth is spherical.
Solution: If the shape of the earth is approximately spherical, then each minute of latitude is approximately equivalent to one nautical mile, or 6076 feet [U.S. Air Force, 2001]. Actually the length of a minute of latitude varies somewhat with latitude, because the earth is more closely approximated by an ellipsoid than by a sphere. However, to model the earth as an ellipsoid requires substantially more mathematical effort than this example requires.
Latitude zones C through W are 8◦ wide in the direction of the northing coordinate, so within those zones the imprecision of the northing coordinate is approximately
(8◦ lat.) 60’ lat. 1◦ lat. 6076 ft 1’ lat. 0.3048 m ft ≈ 890 000 m
UTM latitude zone X is 12◦ wide in the direction of the northing coordinate, so within zone X the imprecision of the northing coordinate is approximately
12◦ lat. 60’ lat. 1◦ lat. 6076 ft 10 lat. 0.3048 m ft ≈ 1 300 000 m
Example: Preliminary Screening of UTM Coordinate Specifications for Out-of-Bounds Errors
Problem: Quickly check each of the following UTM coordinate specifications for out-of- bounds errors:
NAD 27, UTM Zone 10, N. hemis., 150300mE, 9951192mN NAD 83, UTM Zone 12, S. hemis., 751334mE, 1116907mN NAD 83, UTM Zone 19, N. hemis., 833980mE, 192mN NAD 27, UTM Zone 17, N. hemis., 150300mE, 34602mN NAD 27, UTM Zone 18, N. hemis., 237811mE, 9328904mN NAD 83, UTM Zone 63, N. hemis., 623300mE, 9328904mN
Specify which element or elements, if any, are incorrect and explain why.
Solution: For a quick, preliminary screening of UTM coordinate data, use the “0◦N./S.” section of Table 2.3, and Table 2.4. Beware, however, that this approach will detect only extreme cases of out-of-bounds errors. More refined screening may be required to detect all out-of-bounds errors.
“NAD 27, UTM Zone 10, N. hemis., 150300mE, 9951192mN” is incorrect. The easting coordinate is lower than the lower limit given in the “0◦N./S.” section of Table2.3, and the northing coordinate exceeds the upper limit listed in Table2.4. “NAD 83, UTM Zone 12, S. hemis., 751334mE, 1116907mN” is incorrect. The northing coordinate is out of range (too low).
“NAD 83, UTM Zone 19, N. hemis., 833980mE, 192mN” is incorrect. The easting coordinate is out of range (too high).
“NAD 27, UTM Zone 17, N. hemis., 150300mE, 34602mN” is incorrect. The easting coordinate is out of range (too low).
“NAD 27, UTM Zone 18, N. hemis., 237811mE, 9328904mN” is incorrect. The northing coordinate is out of range (too high).
“NAD 83, UTM Zone 63, N. hemis., 623300mE, 9328904mN” is incorrect. The zone number is too high (i.e., UTM zone 63 does not exist).