Home     Help   Index     Login
Convert UTM to Lat-Long 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.

When getting waypoints off paper maps, you have the job of converting the UTM grid references to latlong. This is tricky because the convention was to leave out the leading two digits. Eg: Convert 943-669 to a lat-long.

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:

  95 96 97 98 99 00 01 02 03
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.

 The first method is using the paper map. Take out ANY 1:50K paper map. (I'll use 82G.08 - Beaver Mines as an example, but you can use any map. Look in the lower left corner, and you'll see a faint blue number which is the full UTM:

  683000 E.

Now look in the lower right corner, and you see 717000.

  717000 E.

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

  Leading Digit
  For Easting
  East of the transition 6
  West of the transition 7