Sounding Better!
Meridian Convergence – How it Affects Us
By Jerry Knisley
Recently a customer asked a question in regards to calibrating a LIDAR system at their office
and then traveling 40 miles away to survey. They noticed a small error in the data when they
did a new calibration on site. The customer then asked if meridian convergence could be the
culprit.
This article is intended to explain in general what Meridian Convergence is and how it applies
to working in HYPACK®.
First, here are a few terms to understand:
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True north is the direction towards the North Pole,
Magnetic north is the direction a compass-needle points to, and
Grid north is the direction indicated by the vertical grid-lines on an Ordinance Survey
map;
Magnetic declination tells you the direction of magnetic north, measured from true
north, and
Convergence tells you the direction of grid north, measured from true north.
FIGURE 1.
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In HYPACK®, you can display both Projection Grid Lines and Latitude and Longitude lines. In
Figure 1 there are both Latitude/Longitude lines as well as XY Grid lines. This is an extreme
situation as the example is at the origin point of the state plane.
The True North Azimuth is about 2 degrees off of the Grid North Azimuth.
A location in the middle of the state plane used in the next section was 230 Nautical Miles
away.
FIGURE 2. 230 NM Between the Grid Origin and the Survey Location
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0
N
au
tic
al
M
ile
s
Survey Location
Grid Origin
Example 2: As you can see in Figure 3, the grid alignment is perfect at the center of the grid.
The Grid North and the True North line up with our excavator reaching out along the axis.
FIGURE 3. Grid North and True North Aligned
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For reference purposes a parallel line was created 40 nautical miles to the east of the center
of the grid.
FIGURE 4. Reference Line
In Figure 5 and Figure 6, there is clearly a difference in Grid North versus the True North. The
excavator has moved from one site in the original example to one that is 40 miles away. A
line was generated in the same geodesy using Grid North as the reference.
FIGURE 5. Difference Between Grid North and True North
Measuring the error between the excavator bucket center and the
Longitude line shows a difference in position of roughly 0.2 feet.
FIGURE 6. Difference from Bucket to Longitude Line = 0.2 feet
In Figure 5, the ruler (yellow line) was used to trace the Longitude line
to determine the difference between the Grid North and True North.
With the movement 40 miles from the center of the grid, the heading
out of the GPS (True North) would put the excavator off by 0.5
degrees from Grid North.
The conclusion from this example is that, yes meridian convergence is
something that everyone should be aware of and take into account
when precise measurements are required.
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