True north from magnetic north

I have a base line established in state plane coordinates. I set up at one end backsight the other end and turned an angle to magnetic north. I obtained the declination for the date of observation and added it to the angle to obtain true north. I am approximately 15 degrees different than an observation made years ago. The change in declination over these years would not amount to 15 degrees. What am i not seeing here? If anyone has done this procedure please help.
I looked at a USGS quad map and the old angle is close to the angle obtained from measuring on the map.
Thanks,

Question No.1:

Did you factor in the gamma angle (convergence) between SPC Grid North, and Geodetic North?

15 DEGREES is HUGE (the gamma angle won't account for anywhere near this much),

Question No.2:

Are you sure you applied the declination/variance angle in the right direction (this is sometimes less than intuitive).

Loyal

Question 2:

Did you occupy the same station on the same baseline as the observation done years ago?  If not, you could have a big chunk of iron pulling the needle off at the new station.

With that much involved, my best guesses are:

1. Applied the angle correction in the wrong direction
2. Local attraction

Could be either, could be neither.  Time to assume, test assumptions, change assumptions, test new assumptions, repeat as necessary until you come up with a viable explination.

Larry P

More specific facts would help focus the advice.  Where, how many years, what values do you have, how was the "observation" made?  As noted, if you want true north you have to start with the SPC convergence angle correction, but it won't be anywhere near as larg as your discrepancy so there is something else wrong.

If you don't have the old value of declination, you can get an estimate from the NGS historical declination calculator at
http://www.ngdc.noaa.gov/geomag/magfield.shtml

The change can be quite large in the western US over many  decades.  Here, it has been 10 degrees over the last 160 years since many of our land surveys.  For you in San Antonio it gives a little less than 6 degrees change in the last century.
1910    9° 9' E
1940    9° 53' E
1975    8° 15' E
1985    7° 24' E
2010    5° 9' E

It is interesting to note that applying the declination the wrong way in the 1980's would make about 15 degreees diffence.

Let us know more about the problem, and whether you get it resolved.

Thank you everyone for your helpful information, and i apologize for taking so long to get back on here!
The site is 52 acres of open land upon which are four radio antennas.
I tested for local attraction by sighting at opposite ends of our base line and checking the compas bearing. This showed about a degree difference so i assumed that wasn't the probelm, as i was looking for 15 degrees.

Well it turns out it has to be local attraction. 

We did a Polaris observation Friday night and found the 15 degrees! All is now right with the world. Thanks again.

"Just Curious, Did You Observe Polaris From Both Ends?"

Probably not- the other end is kinda hot.