Why record date on a survey?

[P A Hill & E V Goodall]

I was recently pointed to the following site, which can be used to find the 
magnetic declination of any location on Earth since 1900.  Thanks to John 
Halleck for the giving me the URL.

http://www.ngdc.noaa.gov/cgi-bin/seg/gmag/fldsnth1.pl

My government tax dollars at work (its certainly better than building bombs 
and it's a lot cheaper)! It even produces values for the whole world.

I pass this on to remind us all why for most locations in the world the 
DATE of a survey is relevent. A quick check of my city, Salt Lake City, 
Utah, reveals that the drift of the horizontal component over the last 25 
years is nearly 2 degrees which is certainly enough to show up in a cave 
plot when combining data from then with data from 2002.

Or checking somewhere completely differnt -- 40 E 43 N (somewhere near the 
Caucasus Mountains in Georgia) the change over the last 25 years has been 
about a degreee.  More than enough to make an old and new survey not line 
up very well.

Note that the values are greater in many other locations. For example
London England has had more than a 3 degree change over the last 25 years.

Good surveying to you and don't forget to write down the date, if you want 
everyone to use your data many years from now.

-Paul Hill

p.s. Apologies to those on both list I sent this to who may get this 
message twice.