Attachment 11
mile) distant object will allow a comparison of magnetic declinations of points that are within a 24
meter (80 foot) wide path in the direction of the distant object; while a 24 kilometer (15 mile)
distant object will allow a comparison of points within a 73 meter (240 foot) width, or effectively,
the whole CCP site. If a distant object cannot be chosen far enough away to accurately compare
the whole sight (at no time will a distant object be closer than 8 kilometer [5 mile]), then
corrections for the eccentricity would have to be made. If the grid were laid out so its center was in
line with the distant object and an equal number of points were laid out on either side of this
centerline, then this eccentricity would automatically be corrected when the azimuths are averaged.
But the points can only be compared to other points within the allowable path width when
checking for disturbances in the declinations, unless corrections for the eccentricities are allowed
for. The average value is then computed, adjusting for eccentricities if necessary, and reported as
the site declination.
A11.6.1.2. Distant Hub Method. After the grid is laid out, additional hubs are laid out a minimum
of 90 meters (300 feet) in all four directions from the center point of the grid and designated as
"Hub N," "Hub S," "Hub E," and "Hub W." "South Azimuth Marks" are placed perpendicular to
the "Hub S," 6 meters (20 feet) apart, and coincident to the grid layout, as shown in Figure A11.1.
These azimuth marks will then be used for sighting and taking declination readings. After the grid
and azimuth marks are accurately set, the surveyor will set up and level his transit over the center
point and sight it on the "Hub S" mark and zero the vernier. The surveyor then must release the
compass needle and turn the transit to center it on the compass needle while all the time tapping the
compass to minimize friction effects. A reading will be taken here (to the nearest one minute (1')),
then deflect the compass needle with a small magnet, realign the transit with the compass and take
a third reading. These three readings are averaged to provide the declination for this spot. The
surveyor will accurately record the time to the nearest minute for the first and third reading. After
the readings are completed for the center point (which will be used for reference), the surveyor will
then set up the transit over the other points of the grid and follow the same steps as above while
sighting at the appropriate "Azimuth Mark" and determine the declination of each of these grid
points. Approximately every 20 to 30 minutes, or any time a reading turns out to be outside the
allowable 12 minutes (12') of arc, the surveyor must re-setup over the center point and take new
readings to check for diurnal changes in the declination. If readings are found to be outside the
allowable 12 minutes (12') of arc, after making corrections for diurnal changes, the surveyor will
set up at the bad point and re-check it to see if the results are repeatable. If all the readings are
within the required 12 minutes (12') after the surveyor has made diurnal corrections, he then can
average these readings and determine the site declination.
A11.6.2. Magnetic Direction Survey. This survey is to check the layout of the markings at an existing
CCP or to lay out the markings for a new CCP.
A11.6.2.1. New CCP. For new CCP , the surveyor will determine the center of the pad and mark it
with a bronze surveying marker accurately grouted in place. This point will be stamped "Center of
Calibration Pad." After the center point is located and set, the surveyor will accurately locate and
set the following control points and pavement markings in a similar manner. See Figure A11.2 for
greater detail of the control point layout.
A11.6.2.1.1. True North-South. A north and south control point will be set on a "true north-
south" line established through the center of the calibration pad marker. The north-south
control points must be located radially from the center of the compass calibration pad at a
distance of 9 meters (30 feet). These points will be stamped "NT" for the north point and "ST"
A11-3
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