FCC True North

Is there any reason this would not be geodetic north? Doesn't the FCC record location by lat-lon, thus making geodetic an appropriate interpretation?

None of the alternatives seem likely to me, who has never filled out such a form.

-Astronomic north would probably not be significantly different for this purpose from geodetic.
-Grid makes no sense unless you are giving location on UTM or SPC.
-Magnetic is out of the question, as that isn't called "true".

www.munn-reese.com/microwave/Form601Q.pdf

See pages 7 and/or 9.

I can't point you in the right direction (literally or metaphorically). But I wonder a couple of things. Assuming there is no real and universal term for it, could you simply ask the person ordering the survey what s/he expects and/or if they can provide you with definition. If they didn't write the spec, see if you can talk to the author of it.

Second, how accurate can the tower's orientation be determined? Is astronomic north close enough for the orientation of a tower, in that the difference between that and geodetic north is no more than the precision of the aim of the tower.

I'll tell you what has flown before. We use geodetic. We survey on the grid to make it easier.

We do some work where we tune the antenna's to a specific azimuth. MANY times I'm parked a 1/2 mile or more away to see the top. It would be VERY difficult to tell it's direction unless I climbed the tower (which ain't happening.)

Good luck, but geodetic = true. No biggie there.

FCC True North>Astronomic

true = astronomic...not gird.. Sorry kris, I gotta disagree..

Astronomic north is what they are looking for, especially with cowboys link and the gps sidebar note.

Foggy

I think Kris has it right...he was just a little vague on what he meant.

SPC (or UTM/LDP) will return a gamma (convergency) angle that can be EASILY converted to a Geodetic Bearing.

Geodetic North does NOT equal Astronomic North [EXACTLY], except where it DOES (which I suppose could be said of UTM/SPC/LDP Grid North too).

As far as EXACTLY what "True North" IS...well that depends on your definition. I lean towards Geodetic North, but when retracing old GLO stuff, Astronomic North would be a more "accurate" description.

Loyal

Didn't even need a surveyor for this one...

http://en.wikipedia.org/wiki/True_north

True North is...

...a really good novel

Foggy

I used geodetic north to align the few that I did.
But looking at the document that SC posted; I just have to ask.... what is auto north? Page 7. I can't believe it's what I think it is.

Geodetic North

Wiki doesn't have a page for Geodetic North, but they do have this

I edited the Wikipedia True North page to distinguish astronomical vs geodetic north, which they had apparently considered identical.

Let me know if you don't like it, or else just go edit it yourself. Wikipedia is about consensus.

Truest North

I think that one could make the argument, that the “most precise” (maybe even accurate) TRUE[st] North, would involve ITRF-2008 'day of observation' coordinates tied to the IGS Tracking Network (assuming that you are using GPS/GNSS). When the NGS rolls out be the Multi-year Adjustment later this month, then I would be inclined to use their version of ITRF/IGS08.

NAD83 Geodetic North ? WGS84 Geodetic North ? ITRF2008 Geodetic North, and so on and so forth. BUT, they are VERY [very-very] close, and the difference is not something that most surveyors have to worry about very often (EXCEPT of course when they DO).

In fact, the “North Pole” is a moving target to begin with (see the IERS EOP files), and so is North America (and every other place on Earth) for that matter. So it's like hitting a moving buffalo from the flat car of a moving train (only much easier).

I suppose that there might be a place or two on Earth where everything = everything else (all things are equal), but it probably moves around a lot, AND involves controlled substances and Tequila.

I also don't know what "auto" means on Page 7/9, but like Mighty, I think it sounds scary!

Loyal

Truest North

"Auto" sound like they are delegating it to the UNKNOWN and UNDESCRIBED software somebody put in a GPS unit.

But for their purposes, I think any version of geodetic OR astronomical north would be adequately accurate. I don't think you can measure or design an antenna radiation pattern down to arc minutes, and certainly not seconds.

Notice they seem to imply they will accept a recreational grade GPS position to 0.1 second of lat-lon, and we know most of those units don't bother with the WGS84-NAD83 conversion, much less flavors thereof, even if someone gets a long average so that they stand a decent chance of being within 0.1 second.

Bill

I agree 1000% with what you just stated above.

Some of the nuances between the various “datums” and realizations thereof are [for the most part] pretty esoteric. As SURVEYORS, we should be aware of them, and have a reasonable grasp of the magnitude of these variations, but in the final analysis, the rotational differences are trivial until you start processing Static GPS vectors.

Loyal

I actually did it, around 1960. I was just a kid but I remember it.

I was out with my dad and another surveyor to determine "true north" for the FCC.

It was for the antenna layout of my town's first radio station, AM of course.

It all was about determining north from Polaris, which we observed at either the north or south elongation, by non-quartz clock. We didn't have any hyper accurate clocks back then and we weren't listening to WWV.

I kind of remember that wherever Polaris was on a given night was probably good enough for the FCC. The instrument was probably a K&E Paragon, or a Gurley. Don't remember for sure and likewise not important to the FCC.

Loyal

The only thing I could come up with to explain "auto" north is automobile north. Park under the tower; point the truck with compass display to say west, then line up the device with the truck. Bingo you have a transmitter pointing west!

Truest North

> But for their purposes, I think any version of geodetic OR astronomical north would be adequately accurate. I don't think you can measure or design an antenna radiation pattern down to arc minutes, and certainly not seconds.

That's what has me a bit concerned. My client is asking for two alignments: one for the existing tower cross-section, which is a triangle about 3 feet on a side; the other is for a 3-foot-long antenna boom to be installed some 200 feet above ground. The requested accuracy for both, reportedly taken from FCC requirements, is 1 degree. That's about 0.05 foot in 3 feet.

I plan to run static sessions on 2 or 3 points around the tower while I locate the tower legs with a total station from a couple of closer-in points. Then I'll tie in the GPS points. I'll run the GPS through OPUS-RS and process the local vectors, then adjust the whole shebang.

The trick is going to be finding a suitable location for a point from which I'll be able to see the boom well enough to provide alignment instructions to the riggers during the install. My neck is sore just thinking about it.

Loyal

LOL...good one (and probably not far from the "truth")

As Bill (and others) have pointed out, the actual alignment (or measurement of the existing alignment) of the tower/antenna is going to have plenty of slop in it from the get go. I'm pretty sure that ANY [decent] Astronomic OR Geodetic Bearing would be just dandy.

The "trick" (as always) is making sure what the client ACTUALLY wants (or needs), and then providing that information with credible meta-data and other supporting documentation.

Loyal

I consider Wikipedia as garbage.

In terms of "True North" with respect to deflections from the Conventional International Origin (CIO), take a gander at the following from the U.S. Naval Observatory. I get one of these each week, and a similar one from Paris:

PREDICTIONS:
The following formulas will not reproduce the predictions given below,
but may be used to extend the predictions beyond the end of this table.

x = 0.0862 + 0.0195 cos A + 0.1211 sin A - 0.0517 cos C - 0.0070 sin C
y = 0.3348 + 0.1106 cos A - 0.0170 sin A - 0.0070 cos C + 0.0517 sin C
UT1-UTC = -0.2917 - 0.00068 (MJD - 55757) - (UT2-UT1)

where A = 2*pi*(MJD-55749)/365.25 and C = 2*pi*(MJD-55749)/435.

TAI-UTC(MJD 55750) = 34.0
The accuracy may be estimated from the expressions:
S x,y = 0.00068 (MJD-55749)**0.80 S t = 0.00025 (MJD-55749)**0.75
Estimated accuracies are: Predictions 10 d 20 d 30 d 40 d
Polar coord's 0.004 0.007 0.010 0.013
UT1-UTC 0.0014 0.0024 0.0032 0.0040

MJD x(arcsec) y(arcsec) UT1-UTC(sec)
2011 7 8 55750 0.0573 0.4417 -0.29248
2011 7 9 55751 0.0596 0.4424 -0.29270
2011 7 10 55752 0.0618 0.4429 -0.29271
2011 7 11 55753 0.0638 0.4433 -0.29251
2011 7 12 55754 0.0657 0.4437 -0.29217
2011 7 13 55755 0.0675 0.4440 -0.29174
2011 7 14 55756 0.0693 0.4442 -0.29130 (This goes on and on.)

This is only significant if you need to get "True North" determined to SECOND-ORDER ACCURACY. This is useless info if your azimuth requires Third-Order accuracy or lower.