|Convert UTM to Lat-Long # 1102 Back to Directory|
Last Review: 2013.05.14
Help Category: Trip Reports
Summary: Convert UTM Grid reference to lat-long, especially when When a paper map starts the 2 digit grid reference numbers over again at 00, you must convert waypoints in 2 batches.
To do the conversions, we wrote a <MassUTM_Trunc.asp?Example=On>UTM Mass Converter</a>. This allowed you to specify the leading digit once, and then put in a whole set of the 3 digit ones. Since the waypoints for most trip reports share the same leading digits, you could convert all references in one batch.
However, some mapsheets have grid reference numbers that make a "transition" in the middle of the sheet. Eg:
In this case you must convert your waypoints in two batches, each with a separate "leading digit" for the "easting". Below is an example, which come from Rick Collier's report titled A Wet and Unpleasant Traverse from Whistler Mountain to Mount Gladstone.
Here are the original waypoints he was trying to convert in one batch: (which didn't work)
943-669=dirt-bike scar on Castle River road 946-667=intersection with fire lookout uptrack 960-677=open area near old fire lookout 964-679=intermediate summit 968-676=summit of Whistler Mountain 989-674=first summit E of Whistler 980-665=ridge turns E 995-664=second summit to E 004-658=low col SW of Gladstone 012-657=Gladstone/North Castle col 014-666=summit of Gladstone 012-657=Gladstone/North Castle col 019-654=Mill Creek tributary, SE of Gladstone 038-672=tributary intersects Mill Creek 037-697=gas well (end of Mill Creek road)
As you can see, he crosses the "transition" right after waypoint with the easting "995" and the next waypoint has an easting of "004". He is going from west to east. The leading digit for the ones on the west is "6", and the leading digit for the ones on the east is "7" So you split them into two batches for conversion. If you try to convert them all in a single batch, with a single leading digit, half of them will be wrong.
HOW TO GET THE PROPER LEADING DIGIT:
Now look in the lower right corner, and you see 717000.
Note that the western leading digit is "6", but the one on the east is "7". Any UTM values east of the "00" which runs just 1 km west of Mount Gladstone, must be converted with a "7", whereas the ones east must be converted with a "6".
Note also that in addition to the blue grid reference, they also have a brown number 281000. Ignore that, it is the blue number you want.
You can also always get the leading digits by looking up two "reference" peaks in Bivouac, one on each side of the transition. The full UTM is shown on every mountain page of Bivouac. In this case, the easiest peaks to look up are Gladstone and Whistler, both on 82.G.08. Gladstone is east of the transition, and Whistler is on the west. So the waypoints with high numbers (before the transition) must be converted with