Need Help with Convergence angle and True North ASAP

[pls7809]

I'm having brain flatulence right now...I have a client that wants his house staked so the front door is facing true north.

I have a bearing for the property line on a map based on CCS83, Zone 6 and the convergence angle for the property is -0°28'55".

Is the convergence applied counter-clockwise or clockwise, or am I out to lunch?

[Woodcutter]

The longitude of the central meridian for zone 6 is 116-15-00 W.

Is your location west or east of this CM?

If west, then your grid azimuth needs to rotate clockwise, if east, then counter clockwise, to get true.

But, check me on this as I winged it out pretty quick.

[Dave Karoly, PLS]

Geodetic north is to the right of grid north with a negative convergance angle.

These were designed back in the day to get from geodetic to grid north. Geod - conv. angle = grid north.

[hellsangle]

. . . before it starts gettin' foggy . . .

[pls7809]

That's right. Thanks for the help everyone.

[7702]

Gosh, not too many clients would even know what true north represents. Assuming the map info is correct, if you ignored the mapping/convergence angle, you would still be within half of a degree from the desired result. Rotate the wrong direction and you're off almost a degree.

It helps me to keep the relationship of True North (geodetic) and Grid North straight by remembering that the two are equal only along the central meridian for the zone. If your position is not exactly on the central meridian then the meridional lines (true north) will be converging on the central meridian. This concept is fairly easy to visualize and is a check to make sure things were rotated the right direction. When using the relationship of Grid north = true north - the convergence angle, one must remember to keep the proper sign for the convergence angle, since it is negative when west of the central meridian.

Since your staking for the structure will only be as good as the map and monuments you are relying on, I would probably be inclined to take a real close look at the info provided, including the convergence angle. If the map covers a large area, you might be well served to calc your own convergence angle, specific to the longitude of the actual construction site.

I staked a structure once so that the long axis was aligned in a true north-south direction. Like Mr. hellsangle, suggested, I did so using Polaris observations. Plug in the Laplace correction(deflection from the vertical) and you get something very close to true north.

A morning shot on good ol' Sol is always an option, too. Of course, it all depends on how close to true north you need to be.

[land butcher]

Asians are very particular on which way their house faces and won't buy one that's below the street.
One house in my area, where everything is on the irvine 45, was built diagonally across the lot so the front door faces north.

[MDL]

Thanks LB
I also have a mirror on the front door to deflect the evil sprits.

Mike

[land butcher]

Thanks LB
I also have a mirror on the front door to deflect the evil sprits.

Mike


LOL. Does it work?


BTW, thats all stuff Riki learned when in Real Estate.