True and N compass

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True and N compass

Postby gindling » Sep 13, 2013 8:07 pm

Hello all. I was wondering how folks get the correct angle between True North and Magnetic North for the compass rose on a cave map? Im using the data from Compass and I couldn't find a way to make a compass rose on that program to import into my drawing program. Is there a table or something I can put in my GPS point and get the true and magnetic north angles?
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Re: True and N compass

Postby Tlaloc » Sep 13, 2013 8:49 pm

It's on the USGS topo map of the quad where your cave is. You can view or download them at the USGS site.
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Re: True and N compass

Postby Scott McCrea » Sep 13, 2013 8:52 pm

Here's where you can calculate the declination for any location: http://www.ngdc.noaa.gov/geomag-web/#declination

re: topo map declination: declination changes. Get the current info to be sure.
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Re: True and N compass

Postby gindling » Sep 14, 2013 9:08 am

Thanks again!
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Re: True and N compass

Postby Bob Thrun » Sep 14, 2013 2:47 pm

The NOAA site uses the IGRF magnetic models that cover the whole earth. It smooths through some local variaations. You should verify its numbers by a surface survey. I have found the IGRF numbers accurate for my caving area. A declination map from an older book, say pre-1950, shows a lot more wiggles in the contours than does a current map.

The code and coefficients used for the IGRF models (They are for five or ten year intervals.) can be downloaded from the NGDC site. The declination calculations are based on the NGDC code.
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Re: True and N compass

Postby driggs » Sep 16, 2013 8:35 am

Compass comes with extensive and searchable documentation (also available online). See relevant topics: Survey Declination Overview, Compass Declination Settings

Calculating the magnetic declination for a survey depends on knowing both the location and the date, since the earth's magnetic field changes constantly with time. To let Compass calculate magnetic declination for each of your surveys, you must have a Project file (with .MAK extension) and set its Base Location. You must enter a date in each survey header, and hit the Calculate Declination button. You can "sanity check" it by making sure it agrees within a few tenths of a degree with an online declination calculator or USGS topo map (assuming the topo map is relatively recent).
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Re: True and N compass

Postby Shane S » Sep 16, 2013 11:53 am

Unless you are surveying you usually don't have to be all that accurate to navigate a cave map. Often when I find landmarks in a cave I can orient my direction with right and left fairly well but I usually will take a compas alone. But here is a chart that should guesstimate an approximation fairly close.
http://www.rei.com/learn/expert-advice/ ... ation.html
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