Author Topic: Grid declination for UTM grid  (Read 13348 times)

boogyman

  • Full Member
  • ***
  • Posts: 192
    • View Profile
Grid declination for UTM grid
« on: July 22, 2013, 07:09:23 PM »
Hello all,

I have a couple of topomaps (scale 1:10K) on which a UTM grid is printed. The map datum is WGS84. But there is no north indication whatsoever on these maps -- ie no true north, and no magnetic north (* see footnote).

Now, I believe that it must be possible to calculate the (utm) grid declination in function of:
- latitude
- distance from the central meridian of the UTM zone
=> for the center of the map.

But I cannot find the formula to use. Anyone who could point me in the right direction?

Many thanks,
Chris.

(*) The reason why is simple: I bought them online from a "topomap poster print" service, and there is no legend printed on these maps; the only annotation is the UTM grid.

boogyman

  • Full Member
  • ***
  • Posts: 192
    • View Profile
Re: Grid declination for UTM grid
« Reply #1 on: July 23, 2013, 12:25:54 PM »
Here's an URL where I found an explanation, hope it is correct:
  http://www.ehow.com/how_7490509_calculate-utm-convergence.html

regards,
Chris.

captain paranoia

  • Sr. Member
  • ****
  • Posts: 384
    • View Profile
Re: Grid declination for UTM grid
« Reply #2 on: July 23, 2013, 12:33:02 PM »
UTM projections are quite tricky.  The Wiki entry looks like it's based on the Defence Mapping Agency Technical Manual, and neither provide explanations of the mathematics, just "programmer's guides" as to how to perform the computations...

However, searching for 'utm grid convergence' gives better results:

How to calculate UTM grid convergence and
Gazza's Geodetic Calculator

The first link says convergence = atan(sin(latitude)*tan(longitude- zone_north_meridian)), and tallies with the result from Gazza's page.

This, of course, assumes you're looking for the grid convergence (the difference between grid north and true north).

If you need magnetic declination as well, then I'm afraid you cannot calculate it mathematically, as it depends on the Earth's moving molten core.  You'll have to look it up.  Google gives a number of 'calculators'.
« Last Edit: July 23, 2013, 12:42:35 PM by captain paranoia »

Lost Soul

  • Sr. Member
  • ****
  • Posts: 265
    • View Profile
Re: Grid declination for UTM grid
« Reply #3 on: July 23, 2013, 04:48:27 PM »
I use this one for magnetic declination.  http://magnetic-declination.com/#

boogyman

  • Full Member
  • ***
  • Posts: 192
    • View Profile
Re: Grid declination for UTM grid
« Reply #4 on: July 23, 2013, 07:49:09 PM »
Many thanks for your replies !

@ CP: indeed, I meant the grid convergence. I had just found the first site that you refer: using  the correct keyword (convergence) when googling makes all the difference...  ;)

@ LS: to find the magnetic declination, I use the following site: http://www.ngdc.noaa.gov/geomag-web/#declination

Cheers,
Chris.