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2012 Thread "Geodetic North Line" ?

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(@rlshound)
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Hello, while researching I found an interesting thread from 2012 about laying out a longitudinal line along a county border in Arizona. I work in Arizona so it was very pertinent. I have an estimate on the table that requires the establishment of a geodetic north line. I've been thinking about different ways to stake this line ....The project is west of the central meridian with a negative convergence of 0-04-22, if I stake two points along grid north then calculate for the offset? Shoot a static session?... in the previous post "latitudinal display" was mentioned. Could someone please explain that? I'm looking for two methods to check the results against each in a simple format. As always your advice would be appreciated, Thanks, Paul

 
Posted : September 1, 2016 12:53 pm
MightyMoe
(@mightymoe)
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rlshound, post: 389174, member: 6800 wrote: Hello, while researching I found an interesting thread from 2012 about laying out a longitudinal line along a county border in Arizona. I work in Arizona so it was very pertinent. I have an estimate on the table that requires the establishment of a geodetic north line. I've been thinking about different ways to stake this line ....The project is west of the central meridian with a negative convergence of 0-04-22, if I stake two points along grid north then calculate for the offset? Shoot a static session?... in the previous post "latitudinal display" was mentioned. Could someone please explain that? I'm looking for two methods to check the results against each in a simple format. As always your advice would be appreciated, Thanks, Paul

I've had to do it for a boundary, it was a bit difficult because I couldn't get a good response from the imperial decree just what line (realization and epoch) was to be staked.

But from there it's pretty simple, create a Transverse Mercator LDP with an origin point on the longitude (you need to know what flavor) and proceed along an easting that you chose.

But understand, the time when the line was created and whatever created the line (deed/statute) will probably dictate just where the line will go, a NAD27 line is 150- 200' from a NAD83 line in my area, and NAD83(86) will not be where NAD83(93) ect. will be, so using current CORS/OPUS may be incorrect.

 
Posted : September 1, 2016 1:13 pm
(@rlshound)
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Thanks Moe,
I'm reviewing how to create a LDP. The line just has to be geodetic north, it is not recreating an existing record boundary line. Thanks for the advice on how and when as it pertains to an existing line and how that compares to opus. Keep you posted, Thanks Again, Paul

 
Posted : September 1, 2016 2:24 pm
loyal
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Welllll, as Mighty Moe pointed out above, it makes a difference (maybe trivial maybe not), depending on the datum and/or reference frame that you base your Longitude on.

Unless your client has specifically requested a particular DATUM, then I would be inclined to use the Latest IGS Latitude/Longitude on your "start Point" (with a [say] 4 hour OPUS solution), and use the IGS de jour position on that point as the origin of the Transverse Mercator (or Lambert) LDP. So long as you HOLD that Lat/Lon during your survey, you will be "setting out" the best available "geodetic North Line" possible. Remember, the current Broadcast Orbits are essentially in the IGS reference frame, and ALL of the IGS orbits are. This "line" will be "closer" to the next NAD than NAD83 will be.

Loyal

 
Posted : September 1, 2016 2:39 pm
(@tom-adams)
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Can't you just use the Lat/Long and move north without letting the longitudinal spherical coordinate change? Why would you have it convert to plane coordinates then figure out how to get it back tor geodetic north? (Or am I being ignorant? I'm not an expert)

 
Posted : September 1, 2016 2:45 pm

bill93
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I'm with Tom. Once you establish a starting point, forget projections and just hold the longitude.

 
Posted : September 1, 2016 3:06 pm
loyal
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Tom Adams, post: 389205, member: 7285 wrote: Can't you just use the Lat/Long and move north without letting the longitudinal spherical coordinate change? Why would you have it convert to plane coordinates then figure out how to get it back tor geodetic north? (Or am I being ignorant? I'm not an expert)

YES...the problem might be in the implementation of that. Using "most" RTK software, that would be the path of least resistance. Other than that, it would not be so easy. Now another "azimuth mark" to the start point would make a Total Station solution okay, UNTIL you got "over the hill" on the North Line. Things might get a little squirrely after that. An LDP would make a total station traverse somewhat easier to deal with, even though you would need something to close into from time to time.

Loyal

 
Posted : September 1, 2016 3:07 pm
(@tom-adams)
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Loyal, post: 389212, member: 228 wrote: YES...the problem might be in the implementation of that. Using "most" RTK software, that would be the path of least resistance. Other than that, it would not be so easy. Now another "azimuth mark" to the start point would make a Total Station solution okay, UNTIL you got "over the hill" on the North Line. Things might get a little squirrely after that. An LDP would make a total station traverse somewhat easier to deal with, even though you would need something to close into from time to time.

Loyal

Could you do a static survey to points that are approximate north from you first one, then calculate a "move" to get on the same longitude? That way, you could pick your approximate point locations where you can see to the top of the hill or whatever. If you only need two ends of a line (obviously I don't know the scope of the job) you could set one where you want it and calculate only one "move".

 
Posted : September 1, 2016 3:27 pm
loyal
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Tom Adams, post: 389215, member: 7285 wrote: Could you do a static survey to points that are approximate north from you first one, then calculate a "move" to get on the same longitude? That way, you could pick your approximate point locations where you can see to the top of the hill or whatever. If you only need two ends of a line (obviously I don't know the scope of the job) you could set one where you want it and calculate only one "move".

You bet, and the end result "should" be better than RTK (although not much). In fact, that would be the "best" solution from an accuracy standpoint IMO.

Loyal

 
Posted : September 1, 2016 3:34 pm
(@rlshound)
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Hello Loyal, Tom,
Thanks for your explanations. I'm far from being any kind of expert and just trying to learn. Attached is a report that a former surveyor in this office received in the same area from several years ago. Loyal your saying if I hold the IGS position highlighted that will be the best "setting out" option...and Tom if I stake to that easting as I proceed then that will accomplish the intent?

Attached files

20160901153222.pdf (79.8 KB) 

 
Posted : September 1, 2016 3:43 pm

(@tom-adams)
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rlshound, post: 389220, member: 6800 wrote: Hello Loyal, Tom,
Thanks for your explanations. I'm far from being any kind of expert and just trying to learn. Attached is a report that a former surveyor in this office received in the same area from several years ago. Loyal your saying if I hold the IGS position highlighted that will be the best "setting out" option...and Tom if I stake to that easting as I proceed then that will accomplish the intent?

I'm just chatting, listen to Loyal for real advice. I don't know what you mean by stake out the "easting" that sounds like a northing/easting plane coordinate system. I am saying stake out the "Longitude West." with your gps machine thingy. If the first point is @ W104å¼29'12.3456789" then stake out that same spherical coordinate, and not the "easting" plane coordinate. Your "geodetic north" is going to follow the sphere and not a plane (if I said that right)

 
Posted : September 1, 2016 3:59 pm
loyal
(@loyal)
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rlshound, post: 389220, member: 6800 wrote: Hello Loyal, Tom,
Thanks for your explanations. I'm far from being any kind of expert and just trying to learn. Attached is a report that a former surveyor in this office received in the same area from several years ago. Loyal your saying if I hold the IGS position highlighted that will be the best "setting out" option...and Tom if I stake to that easting as I proceed then that will accomplish the intent?

Well, sort of!

Bear in mind that THAT igs08 position (2014.07695) was valid 2å?+ years ago, it would be several centimeters different TODAY. That's the downside of playing in the dynamic system playpen. IGS14 will be "online" (and used by OPUS) sometime soon, and although that doesn't "solve" the velocity issue, it will be the "latest greatest" (at least for a while). As a practical matter, the IGS velocity (and the rotation of the North American Plate around a Euler point in South America), isn't going to ADD UP to a significant rotation of your "LINE" anytime soon.

So long as you properly DOCUMENT what you do (IGS, NAD, whatever), the next guy should be able to carry on.

Loyal

 
Posted : September 1, 2016 4:02 pm
bill93
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Holding the SPC or UTM easting as you move northward will get you a grid north. For geodetic north you hold longitude in whichever system you are measuring it.

 
Posted : September 1, 2016 4:06 pm
(@bajaor)
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How long is the line and to what level of accuracy do you need to or expect to stake it?

 
Posted : September 1, 2016 5:06 pm
aliquot
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You are making this way to complecated. If you are using RTK just move north and hold the longitude steady. If you need to care about the difference between NAD27 North and NAD83 North you will need a more accurate measuring method than RTK.

 
Posted : September 1, 2016 5:54 pm

thebionicman
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aliquot, post: 389252, member: 2486 wrote: You are making this way to complecated. If you are using RTK just move north and hold the longitude steady. If you need to care about the difference between NAD27 North and NAD83 North you will need a more accurate measuring method than RTK.

Bingo. We are talking about direction, not position. The realization has no practical effect.

 
Posted : September 1, 2016 6:22 pm
(@scott-zelenak)
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I'd just set up on the desired point and take line on Alpha Ursae Minoris at culmination.
But I guess I'm a Luddite, so....

 
Posted : September 2, 2016 5:11 am
(@shawn-billings)
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There are so many ways to do this, it's hard to determine one method. It will depend on your equipment and your capabilities.

If you have RTK:
Collect point (fixed solution) at origin stake. Create localization at origin point with Geodetic North as the orientation and 0,0 as the origin grid coordinates. Head Northerly to the desired distance for the azimuth stake. Watch coordinate values and stake with Easting as close to 0 as possible. Note, it doesn't have to be zero, you could use 5000,5000 or any origin value you prefer. The directional error, expressed angularly, will be directly related to the easting error of the two RTK points (origin and azimuth stake) and the distance between them. Some data collectors will show the geographic coordinates in real time (latitude, longitude) and some do not. If your data collector does express geographic coordinates, then you can simply note the longitude at the origin point, head North and stake the azimuth point at the same longitude.

If you have only Post Processing:
Set receiver on origin stake, then set second receiver on distant, inter-visible point. Collect data for necessary time for fixed vector solution. Process data. In processor, create coordinate system with origin stake as origin of new system. Calculate inverse between origin and second point. Return to field with total station or theodolite and set up on origin, backsight second point and turn the angle to North. If you can't set up a coordinate system in the processor, use Grid. In your case, the convergence angle is -00å¡04'22". So the geodetic bearing from Origin to Second is going to be Grid Azimuth (inverse) - 00å¡04'22" (counter-clockwise rotation). Using this geodetic bearing, for Origin to Second, return to field, set up total station or theodolite and turn to geodetic North.

If you have a Total Station and Celestial Ephemeris:
Setup total station on Origin point and perform celestial observation. For years, I've used time from an $100 Garmin e-Trex and position as well. It's served me very well. Polaris will provide the best accuracy due to it's low apparent movement (makes time less critical) and it's small size (improves pointing accuracy). Other stars can be used but give up accuracy due to apparent motion. The sun in generally more convenient, but requires a solar filter when used with a total station. A theodolite can be used without filter using a projection method (hold a white card under the focal lens and watch the projection of sun and cross-hairs intersect.

If you have a gyro-scope:
Least likely because most mortal surveyors don't have one. I've never even seen one, but from what I've read, they produce results easily, but require about 20 minutes to do so and have an angular accuracy of about 20 arc seconds. This is old information and I don't know if the specs have improved for these devices with newer technology.

Regarding GPS solutions:
All of the normal rules apply. You need open skies at the origin and azimuth point (and second point for post processed solution) as well as at the base (if you are using a base for RTK and not RTN). I would recommend with the post processing route that once you stake the azimuth point from the origin, you set up your static gear on the origin point and the new azimuth point and collect necessary data for a good vector and make sure that this new vector is North once post processed.

 
Posted : September 2, 2016 5:37 am
MightyMoe
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If you have a pre-determined longitude to stake along then you need to find that line of longitude, if you a point with an unknown longitude then you need to figure out what the longitude on the point is. Either way I would still wish to have my geodetic north and grid north have a deflection of 0 along that line, which means you are staking along an easting and along the longitude.
GPS software I've used has been able to do that since at least 1996 in the field, or predetermined in the office

I can always put located points along the line into a state coordinate system (some call that grid??) later.

If you want to keep it on SPC grid through the whole experience then I would locate the starting point, find the longitude, go north/south to the other end, and navigate to the desired longitude, locate a point there, then stake between the two points, most RTK units will display the longitude where you are at all times if you push the correct button.

 
Posted : September 2, 2016 7:01 am
john-hamilton
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Take a polaris observation to a reference mark. Turn the angle to north (astronomic or geodetic if you use the laplace correction). Then use the double centering procedure (do people still remember how to do that?) to run the line north. At the north end, take a check observation to Polaris.

 
Posted : September 2, 2016 7:12 am

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