True North vs. Geodetic North
c&j
you prefer; this is different from what is usually called 'grid' north, which is
north relative to the mapping projection being used in the area of interest.
For example, we often use the UTM mapping projection for a reference frame. The
UTM projection, like all map projections (Lambert, Stereographic, etc.) all try
to project the surface of the earth, a curved surface, onto a plane surface (the
map). This cannot be done without some degree of distortion occuring in the
features being mapped. On any one point on the map, there is a north that is
straight up the page aligned with the grid marks (the grid north), true north
can be visualized as a line of sight on the map which is actually a curved
surface (much like great circle routes you often see on these Mercator world
maps) that deviates away from grid north up to a degree or so to the left or
right, this angle is called the convergence, and is different for any point on
the map, but it can be calculated.
Geodetic north in my opinion is the same as true...in that geodetic north refers
to north by way of the geoid (the shape of the earth) which is the north
geographic pole....and true north refers to the same direction astronomically
(ie. the direction to Polaris, the 'north' star)
J. Keith Maxwell
Minimum
Technical Standards (I got caught at the last minute having to take this
course before the end of the year. We have to take 6 hours every 5 years -
and I had forgotten to do it before renewal this year.) Anyway, the subject
came up about "to what do you reference your bearings." The instructor and
others said that true north was different from geodetic north (by a few
seconds in his area - North Alabama). I have always understood true north
to be the same as geodetic north which is also sometimes referred to as
astronomic north. He said there was some long formula that converted
between the two, but that he usually called NGS when he wanted to know.
Any shed light or references would be helpful.
Keith Maxwell
David A Wahlström
the difference and I know I can call my friend Tom for the details);
however, I believe the use of the term "true" is incorrect, or at best,
ambiguous. This, I believe, can be supported by case law!
Dave Wahlstrom
J. Keith Maxwell <jke...@maxwell-eng.com> wrote in article
<75322v$m1h$1...@camel18.mindspring.com>...
Shelby H. Griggs, PLS
Senne, Inc.
The correct terms should be Geodetic Az and Astronomic Az.
Geodetic Az = Astronomic Az + LaPlace Correction
Grid Az = Geodetic Az - Convergence angle + 2nd term
Except for mountainous areas, the LaPlace correction is realatively small.
Also, for all but high order surveys, the 2nd term can be neglected.
LaPlace Azimuth Correction: At any point on the earth's surface there are at
least two types of azimuths: astronomic azimuth and geodetic azimuths. The
former, obtained from observations on celestial bodies, is dependent on the
direction of gravity at the location which may not coincide with the normal
to the spheroid. Gravitational anomalies are caused by irregularities of the
physical surface of the earth and the variable densities beneath the
surface. On the other hand, geodetic triangulation networks are referred to
the spheroid and are more or less independent of gravity direction
variations. if the LaPlace correction of a station is known, it can be
applied to thre astronomic azimuth obtained by observation to get geodetic
azimuth. How accurately this can be done depends on the availability of
LaPlace data in a given location. In the western mountains of the United
States, the correction may be as large as 20"; in the eastern United States
the correction will not usually not exceed 7".
The NGS has a program called DEFLEC96 which can be used to estimate the
LaPlace correction at any location in the United States. See the following
link for information on DEFLEC96 http://www.ngs.noaa.gov/DEFLEC/deflec.html.
In MHO no surveyor should be without the above referenced handbook. It costs
about $6.00 and has all kinds of useful information.
Best regards,
--
Shelby H. Griggs, PLS
OrbiTech, LLC
GPS Surveying, Consulting and Training
P.O. Box 5305
Bend, OR 97708-5305
Voice / FAX: (541) 383-2715
Mobile: (541) 912-8530
E-mail: orbi...@transport.com
df...@bellsouth.net
>
> Could someone help me out. I attended a seminar this weekend on our states
> Minimum
> Technical Standards (I got caught at the last minute having to take this
> course before the end of the year. We have to take 6 hours every 5 years -
> and I had forgotten to do it before renewal this year.) Anyway, the subject
> came up about "to what do you reference your bearings." The instructor and
> others said that true north was different from geodetic north (by a few
> seconds in his area - North Alabama). I have always understood true north
> to be the same as geodetic north which is also sometimes referred to as
> astronomic north. He said there was some long formula that converted
> between the two, but that he usually called NGS when he wanted to know.
>
> Any shed light or references would be helpful.
>
> Keith Maxwell
I believe the term "true" is ambiguous and should not be used.
DL Ferguson
http://personal.lig.bellsouth.net/d/f/dferg
SamSurveyr
usually always based our bearings on the magnetic north at the time the survey
was done. This was common in rural WV where we worked. Each year the college
would do a star or sun shot (or both) and calculate the magnetic declination on
the same two points. Then we would plot our magnetic north arrow on our plats
and denote the declination as of that date. There are very few geodetic
monuments in the state that a surveyor could use to orient to true north, so
magnetic was common.
I always thought that there should be a shift to using "true" north, so
that all surveys could correspond to each other. But then how would we ever
get to use the information we learned in school about declination and apply it
in the field to prove up the older surveys!! As I have seen here lately, there
is so much technology that is is no "surveying" anymore, it's just punching a
keyboard and running a program and plotting some points that most people have
no idea the reason they do what they do. Anyway, enough rambling.
Sam
Hunter Nash
>that all surveys could correspond to each other.
I am having the same argument here in New Mexico. We use a GPS and it
has "North" figured out to... well it's really close. The problem is
that when we retrace a survey, the guys that did the sun shot in 1888
were close, but.... we're closer. My boss (an old timer) argues that
we must always rotate our bearings to the prior survey. the title
company also agrees that the new survey legal description must
reasonablly match the existing deed.
My argument is that if we can be more precise, why not?
SwampStomper
line?
--
Charles German, RLS
Land Surveying is all I've ever done
and I'll continue until I go home.
Tom
although you were close, they are closer? If you have many years worth of records
using a particular basis of bearing, is it prudent to make a change that has the
possiblity of clouding the title to the underlying property? Most of our surveys
are based on plane state coordinates in the field. When the data comes in and I
have a legal description that describes a certain monumented line as haveing a so
and so bearing, I rotate to that basis. To do otherwise means attempting to
explain on the plat why the drawing shows one bearing and the legal describes
another.
Kent McMillan
>I am having the same argument here in New Mexico. We use a GPS and it
>has "North" figured out to... well it's really close. The problem is
>that when we retrace a survey, the guys that did the sun shot in 1888
>were close, but.... we're closer. My boss (an old timer) argues that
>we must always rotate our bearings to the prior survey. the title
>company also agrees that the new survey legal description must
>reasonablly match the existing deed.
>
>My argument is that if we can be more precise, why not?
>
I'd agree with you that an important result of resurveying is modernizing
boundary descriptions. In my own practice in Texas I use Grid North of
the State Plane Coordinate System as a uniform basis of bearings and make
no apologies about it.
As for dealing with land title insurance companies, the simplest solution
is to write up an explanation of why modern resurveys are able to more
accurately measure directions and distances than previous surveyors could,
and to explain the difference between relative directions and absolute
directions. The sentence "Upon the ground the lines are the same." is one
to think about including. You'll find that once the insurers have seen the
explanation a few times it will finally sink in that there is no great
problem.
If you want to use some monumented line as a bearing basis, then best
practice would be to also note the actual bearing of the line for which a
bearing recited in some previous survey description was used. This makes
the North direction of the resurvey reproducible without the continued
existence of certain monuments.
Best regards,
Kent McMillan, RPLS
Austin TX
SwampStomper
It is common practice here to state the "Basis of North" on the plat. A
notation at the north arrow should state, for example: "Basis of north
is an Iron found at the NE corner of Elm Street and an axle found at the
north corner of lot 215 and a plat of survey by John doe dated 12/12/12
showing these corners.
More often shortened to "Basis of north is existing corners and a plat
of survey by...."
Charles M. German, RLS
40 years of stomping swamps.
MacSurvey1
my survey equipment, I like to be as close to the location of the boundary line
as possible. If I know that the bearing of the original survey is true north,
as obtained from astronomic observations or what have you, I feel alot more
comfortable determining true north myself and running from a known point along
or to and along the line through the ______ swamp.
Can't understand how it comes to be that a licensed land surveyor does not no
how to use bearings, how to obtain true north, how to determine an adjustment
from an 1820 bearing to now, etc.
Have GEODIMETER 600 ROBOTIC will travel wire Bob Mac Farlane:
macsu...@aol.com
Russell Yuille
MacSurvey1 wrote in message <19981215172708...@ng37.aol.com>...
J. Keith Maxwell
but haven't studied it in depth (obviously). Thanks for taking your time to
respond to my request. This is why I subscribe to this newsgroup. Every
once in a while you get a genuine response and learn something in the
process.
Keith Maxwell
Shelby H. Griggs, PLS wrote in message ...
>From the SOkkia 1998 Celestial Observation Handbook by Elgin, Knowles &
>Senne, Inc.
>........
Matthew A. Graham
perpetuate previous work. I completely disagree with the notion of
rewriting a legal description because you can do it better than it was
done 50 years ago. The problem with this as I see it, is that in or 5
or 6 "generations" down the road, you may wind up with a description
that doesn't necessarily match the intent of the original description,
even if it is more accurate. And the next guy always thinks he is "more
right".
Just my 2 cents worth.
Matthew A. Graham, R.L.S.
SamSurveyr wrote:
>
> Exactly. You orient best you can, run your traverse line around the property,
> tieing in all pertinent points, close the traverse, calculate your data and
> determine your new bearings from there. If all your doing is one of those
> pesky mortgage surveys, then fine, use the same bearings and make a note. but
> if your going to do a property survey or retracemnet survey, put the recent
> bearings on it. Why do a recent survey and then orient to 1908???
> What you are supposed to be doing is updating, not backtracking. Who
> cares if the title co. can't figure out what's going on? Let them get
> educated. Your not in the business to make it easy for everyone to understand
> surveying.....like they say...if it were easy, then everyone would do it!
> The problem I see is most people just throw an arrow on their plat and
> don't tell you what it is! Isn't there a minimum standard for plats anymore??
> Let someone know if it's true, magnetic, grid, etc. As I stated, in WV you
> had to show a true north arrow and the magnetic arrow and the angle of
> declination on the date it was calculated, if you used magnetic bearings, which
> most did. This was mandatory.
> Some of the plats I see, I can't hardly figure out, so I know the clients are
> clueless!
> And as far as matching the older deeds, why ? That's why you write a
> description(as I have been saying all along), so that a new deed, if needed,
> can be written from your description.
> Maybe I'm too much of an idealist...........
> Sam
>
> p.s. I'm looking at a plat tonight where the bearings don't all run clockwise.
> Now when the lawyer goes to make up a deed, he's going to go three lines ok,
> then the last one is going to go the wrong direction. And these are licensed
> surveyors that do contract work for our co. Yikes!
SamSurveyr
>how to use bearings, how to obtain true north, how to determine an adjustment
>from an 1820 bearing to now, etc.
Exactly. You orient best you can, run your traverse line around the property,
MacSurvey1
For you whom are critical of my spelling, I'll give you this, I may not be the
best at English, grammer, etc. but anything such as maps and proposed
decscriptions that go out of this office are in fact grammetricly correct in
every manner, including spelling and we do not rely on spell checking to check
out the of and offs if you know what I mean.
Sorry but some replys can sure tick you off, I was not meaning to be derogitory
to the originally thread, just state my posisiton. THATS WHAT WE ARE ALL HERE
FOR ISN'T IT. ALTHOUGH I'M PUSHING 60, I STILL INTEND TO LEARN SOMETHING FROM
MY PEERS EACH DAY. GOD BLESS AND HAPPY HOLIDAYS.
please excuse my spelling and grammer but I'm trying to get some other things
done also.
MacSurvey1
SAID....................................................
MacSurvey1
should reflect interpretation of description, adjoiners description, evidence,
etc. Then if the existing description contains serious errors, missing
courses, etc. a new description should be prepared. One of the ways it is done
around here is the old description remains followed by the statement to the
effect. "And more particularly described according to a recent survey as
follow:"
With that I'm on to Georgia for 3 weeks vacation. An perhaps to visit Le
Powers(allsurveyors,com) to personally get back the $499.95 he owes me.
Russell Yuille
too.
All the best, Mac.
Matthew A. Graham
my point. I am simply saying that a legal description should be left
alone, unless, as you point out, there is a gross error. Leaving a
description as is does not mean it should be surveyed with the same
methods with which it was originally performed. I am merely saying that
a description should be left intact. One can and should record their
results of survey showing the current evidence and boundaries to the
best of their ability at the time. I don't believe that necessitates the
changing of a recorded document. A survey done after the original deed
is recorded should not change the original documentation but merely
reflect that surveyor's opinion of the boundary based on the original
deed and available evidence.
Matt
SamSurveyr wrote:
> Matthew, you evidently have not done much deed research. The books are full of
> errors! Any prudent surveyor would run the title back to it's origin. don't
> tell me you want to use that description for YOUR survey. Each surveyor should
> fully describe his own survey in the manner in which we were taught to write
> them. That's what you get paid to do! Are you going to tell me your going to
> spend a week retracing a tract of land, plot up a plat and then dig up an
> eighty year old description and put with it? Didn't you set new corners?? Was
> that oak tree called for still there?? Did you mark any line trees or corner
> trees that you would like to tell anyone about?? You have to describe your
> survey as you left it for others to follow.
> Just because you write a new description, does not mean the others are
> gone forever. I know descriptions are passed from one owner to another, but
> that is only if there is no survey between transactions. Each time a survey is
> done on a plot of land a new description should be written, unless there are
> absolutely no changes in the land, corners, terrain, etc.
> Maybe it's because I did a lot of rural boundary work, but do you know
> how frustrating it is if you just get a simple metes and bounds deed without a
> "description"?? It calls for a stone at the end of a line....well, dangit, is
> it on this point, on the north side of the creek, on top of the ridge, under
> the cliff, near a large white oak, etc. It's always helpful to the next
> surveyor or landowner to be able to find the corners by the DESCRIPTION. ie.
> N 34-12-30 E at 423 feet crossing a small run, in all 476.29 feet to a 1-inch
> by 36-inch iron pipe (set), on the top of the point 15 feet below a large rock,
> from which a 17-inch Red Oak bears S 34-56-20 E, thence down the point, etc.
> etc. DESCRIBE your survey!
> And your darn right you can do it better!! What's the need to do
> anything at all?...just give the guy a copy of the old survey along with the
> old description. After all it's the original!!!! Can't get any better than
> that can it?? There is a reason they get you to survey their land, you know.
> They want an updated plat and description, believe me. And the only reason
> they may not ask for it is they don't know any better! You have to give them
> what you were trained to give them. People do not understand surveying in the
> least, it is up to you to provide them with the total package....if you are
> reputable. Geesh....nuff said.
> Sorry I get so worked up guys....bear with me.....I'm really a nicer guy
> than I sound here! lol Thanks, Sam
Kantoku
from Las Vegas, NM. No one is served, except maybe the title or real estate
company, in rotating to old record bearings. Holding bearing to the extent
your boss is doing, gives them too much weight. If your doing 1888 era
surveys then your probably working with old GLO surveys. Take solars. I'd
do it for each 1/2 mile of line.
If you have to do any double proportioning or grant/broken bdy adjustments
you should to be on a astronomic basis of bearing. Can't make cardinal
offsets unless you know where astro north is. Even if the old field notes
tell you the Deputy Surveyor was using a non solar type compass. Original
monuments control not bearings. I see the thing you mentioned nearly every
day along with using metes and bounds to retrace a aliquot parcel. Who do
you work for?
Garland Burnett, RLS, New Mexico
Hunter Nash wrote in message <3676334a...@news.campuscw.net>...
>> I always thought that there should be a shift to using "true" north,
so
>>that all surveys could correspond to each other.
>
SamSurveyr
>perpetuate previous work. I completely disagree with the notion of
>rewriting a legal description because you can do it better than it was
>done 50 years ago. The problem with this as I see it, is that in or 5
>or 6 "generations" down the road, you may wind up with a description
>that doesn't necessarily match the intent of the original description,
Matthew, you evidently have not done much deed research. The books are full of
Kent McMillan
>I am simply saying that a legal description should be left
>alone, unless, as you point out, there is a gross error. Leaving a
>description as is does not mean it should be surveyed with the same
>methods with which it was originally performed. I am merely saying that
>a description should be left intact. One can and should record their
>results of survey showing the current evidence and boundaries to the
>best of their ability at the time. I don't believe that necessitates the
>changing of a recorded document. A survey done after the original deed
>is recorded should not change the original documentation but merely
>reflect that surveyor's opinion of the boundary based on the original
>deed and available evidence.
>
I wonder whether there is some miscommunication arising from regional
differences.
By the phrase "legal description" I would think that you mean both the
general description of the property such as "that same land conveyed
by Willis Greenhaw et ux to Samantha Redford by Warranty Deed dated
March 29, 1936 recorded in Volume 736 at Page 34 of the Deed Records of
Travis County, Texas" and the more particular description of the property
by metes and bounds.
If you mean that it is a good practice to continue to refer to the original
instrument that created the boundaries of the property, i.e. in the above
example the original conveyance from Greenhaw to Redford, even though several
mesne conveyances have been since, I think I'd agree (although I personally
prefer to mention also the most recent conveyance as a courtesy to future
title searchers).
On the other hand, what purpose is served by reproducing a 1936 metes and
bounds description in a 1999 conveyance? As for the original intent, does
not the general description establish that to your satisfaction? Even a
good quality 1936 vintage set of field notes (without gross error) will
almost always be deficient in significant ways: monuments will have been
destroyed and the metes will seldom be described with an accuracy consistent
with modern practice. Modernization is the obvious cure for this.
I don't know about others, but I consider the general description of a tract
and certain presumptions arising from the circumstances of the conveyance (that
Texas courts have recognized about strips and gores, for example) to be sufficient
to overcome a number of failings of more recent resurveys of a particular tract
that may have been used as a basis for drafting a description for a conveyance.
Barring something truly remarkable, the calls for adjoiner with public roads,
with the lines of senior adjoining tracts, and/or the calls for monuments will
be surely held as superior to erroneous metes.
Is the difficulty that in some other regions metes and bounds descriptions are
more metes than bounds? If so, I'd agree that a new description by metes without
the correlated recitals of bounds isn't much help to anyone.
SwampStomper
> >
> >My argument is that if we can be more precise, why not?
> >
Exactly! Damn the title co.s and lawyers. Let them get a surveyors
license if they want the plat changed.
Jerry Wahl
instead of to the group.
I am not 100 percent sure that True North is clearly defined. As
previous posts have touched upon. There is true north as defined by
astronomic observations alone, (astronomic north) and geodetic north.
They are usually very close to the same. As a practical matter the
difference has not been worth mentioning but with GPS it has begun to
become relevant.
For Public Land surveys performed by the BLM and it's predecessor GLO
since roughly 1895 have used solar observations as the primary method of
orienting their surveys. Compass adjusted to true based upon polaris
observations were used before and some after that, but local conditions
and practices varied. Just look at a map of Arkansas for example.
The current Manual has a few relevant clauses 1) Section 2-17
"The direction of each line of the public land surveys is determined
with reference to the true meridian as defined by the axis of the
earth's totation. Bearings are stated in terms of angular measure
referred to the true north or south".
Strictly speaking I interpret this as geodetic basis. And if we take
this as a definition of true we have something to start with.
However on the next page of the Manual Section 2-19 defines acceptable
practice (circa 1973) for determination of this value, and this includes
sun, polaris and other astronomic methods (uncorrected by a laplace) as
well as geodetic methods such as the control network and gyros which are
probably geodetic. It should be remembered that the issue of the small
difference was irrelevant for that level of work and the accuracy
attainable by the general methods used up until recently with GPS and
this is why the methods are mixed.
A pre GPS Cadastral Survey would not likely be much better than a
minute, and going back in time downhill from there.
A BLM Cadastral retracement will report their new retraced values (If
they differ from the previous by a significant amount) for the line as
the bearing is a "measurement" not just an assumed thing to be rotated
all over the place as is more common in urban surveying.
If you have a legitimate "basis" for your bearings, you should not be
afraid to report using it. It CAN be "based on the 1898 record bearing
for the east line of Section 15 of North" or it can be "based upon
polaris observations on the east line of Section 15" However you can
only define this basis onece and use an actual recovered line of the
previous record. OR it is perfectly legit to base your bearings upon
NAD83 or "grid based upon GPS control", etc. and whatever.
If you rotate ALL your lines to the 1888 survey how would you close.
Makes no sense except as a basis of bearing. The title company may need
re-educating and a consideration as to who is the professional
responsibile for the description. I once worked for a company which
secumbed to pressure from a city to conform our bearings to the existent
plats on 3 sides. Unfortunately these all had different basis and thus
the parcel would not now close by 18 feet.
If the client wants to dictate the bearing and distances, I am not sure
why they are hiring a surveyor.
- jerry wahl
Hunter Nash wrote:
>
> > I always thought that there should be a shift to using "true" north, so
> >that all surveys could correspond to each other.
>
> I am having the same argument here in New Mexico. We use a GPS and it
> has "North" figured out to... well it's really close. The problem is
> that when we retrace a survey, the guys that did the sun shot in 1888
> were close, but.... we're closer. My boss (an old timer) argues that
> we must always rotate our bearings to the prior survey. the title
> company also agrees that the new survey legal description must
> reasonablly match the existing deed.
>
Kantoku
Part of the problem, at least in New Mexico, is the title co. and others
wanting to see the same bearings and distances on resurveys as what apprear
on the deed your handed. It is not as bad as it use to be but their is
still a mind set in this direction from some folks. You also see things
like a nice aliqoute legal from a patent turning into a M&B desc that does
not even come close to the patented deed lines.
That really drives the realestate and title folks nuts when the adjoiner is
the Fed.
SwampStomper wrote in message <3678FE7B...@darientel.net>...
>
>
>
>> >
>> >My argument is that if we can be more precise, why not?
>> >
>
>
SamSurveyr
description should accompany that survey. The description should be in a metes
and bounds format with bearings, distances, monument description, etc. so that
your client may retrace your survey on the ground. And if he decides to sell
the property 20 years from now, he can furnish his attorney with a proper
description from which a deed may be drawn up. I have run into hundreds of
deeds that have lines going the wrong way, running backwards, etc. because
someone looked only at a plat and wrote the deed going the wrong way on a plat.
Don't assume that ANYONE has the knowledge to look at a plat and write a deed
from it! That is why you write a description.
I'm not upset with any one, but I see surveys occasionally with a nice
plat and only a general description, and I don't think it does justice to the
client or the profession.
Anyway, ya'll take care.........Sam
Vincent J Matthews
and astronomic north. The difference between astronomic and geodetic is the
affect of gravity of your observation which is the laplace correction.
Hope this helps
Vince
J. Keith Maxwell wrote in message <75322v$m1h$1...@camel18.mindspring.com>...
Bertho Boman
understand that on a practical basis for surveying the detailed
definitions does not make much difference but I am surprised that it
is not a black and white well defined issue. To further cloud it, I
have some more questions and comments.
I understand that if we define the Earth spin-axis "today" as geodetic
north and then use astronomical observations to measure the spin axis,
there will be a difference based on local gravity.
Here is where it gets muddy. Does not the Earth's spin axis slowly
move? I also believe that it has a wobble on it. Now where is
geodetic north?
This issue sounds similar to the old problem of defining time. The
Earth spin rate is slowly changing and the definition of the second
eventually was defined as fraction of Earth rotational speed at
January 1, 1900. Is there a similar exact definition of north based
on the spin axis at a given time?
On the next issue, my memory is really weak. In astronomy, there is
a "north-south" axis defined and it is easy to confuse it with the
Earth's axis when astronomical north is used as the definition. There
is a large long term difference between the two, one with a 26,000
year cycle the other one with an 18.6 year cycle. In other words,
astronomical north is not necessarily referring to the earth spin axis
or measurements of it , it can refer to a point on the celestial
sphere.
It seems to me that we should have an absolute definition of the
Earth's North-South axis that is used for all measurements and never
changes. If the spin axis would move, surveys would have to be
updated (theoretically). It makes sense that there is a time fixed
absolute definition and then all others are referenced to it, by
different methods, magnetic, grid, astronomical measurements of the
current spin axis and so on. This issues get real when the Earth's
plate movements are surveyed and analyzed.
There are all kinds of interesting movements and changes in the
Earth's spin axis as discussed in "GEODESY" by Petr Vanicek & Edward
Krakiwsky.
Unfortunately, I did not find an absolute definition of north in that
book.
Take Care,
Bertho Boman
Vinland Corporation
On Sat, 19 Dec 1998 06:45:15 -0500, "Vincent J Matthews"
<sve...@darientel.net> wrote:
>In the past the word true was used to show the difference between magnetic
>and astronomic north. The difference between astronomic and geodetic is the
>affect of gravity of your observation which is the laplace correction.
>Hope this helps
>Vince
>J. Keith Maxwell wrote in message <75322v$m1h$1...@camel18.mindspring.com>...
----> snip
Kent McMillan
>Here is where it gets muddy. Does not the Earth's spin axis slowly
>move? I also believe that it has a wobble on it. Now where is
>geodetic north?
>
>This issue sounds similar to the old problem of defining time. The
>Earth spin rate is slowly changing and the definition of the second
>eventually was defined as fraction of Earth rotational speed at
>January 1, 1900. Is there a similar exact definition of north based
>on the spin axis at a given time?
Bertho:
Yes the Earth wobbles slightly as it spins. The wobble is taken into
account in high accuracy astronomic observations for azimuth by reducing
the derived azimuth to the mean pole of epoch, an average position of the
axis of rotation that is fixed by international agreement. The effect is
so slight, however, that it is negligible for land surveying purposes,
i.e. azimuth uncertainties less than 1" are not significant.
Possibly a reader can provide an address for the entitity publishing the
changing earth orientation parameters.
Bertho Boman
An internationally defined, time fixed, non-moving, reference spin
axis.
Bertho Boman
=========================================
KSnippins
Not only is it not flat..... but the earth "wobbles" on its axis of rotation
because of gravitational attractions. True North involves many variables,
check out the International Earth Rotation Service (IERS) at:
http://hpiers.obspm.fr/
The measurements of the Earth's rotation are under the form of time series of
the so-called Earth Orientation Parameters (EOP) . Universal time (UT1), polar
motion and the celestial motion of the pole (precession/nutation) are
determined by VLBI. The satellite-geodesy techniques, GPS, SLR and DORIS,
determine polar motion and the variations of universal time.
Obviously, the level of detail required to determine "true north" is directly
related to the requirements of the task at hand.
~kurt
-----------------------------------------------------------------------------
Kurt B. Wurm, P.L.S.
University of Maine
Dept. of Spatial Information Science and Engineering
Kent McMillan
Bertho:
Actually, I consider "True North" to be a term of art which when used by
land surveyors means Astronomic North. For most practical applications,
the difference between Astronomic North and Geodetic North is not worth
worrying about. For example, where I sit, the difference is about 3 arc
seconds in azimuth.
By the way, the Earth Orientation Department of the U.S. Naval Observatory
has an interesting web page
http://maia.usno.mavy.mil/eo/whatiseop.html
that, among other things shows a plot of the polar motion over the last
several years. You can see how complex the motion of the pole is.
Best regards,
Kent McMillan, RPLS
Austin TX
Robert Lindsay
Stars do move over time in the celestial sphere seen from Earth, the axis of
spin of the Earth wobbles and wanders with time, the Earths magnetic field moves
over time, and the continents themselves wander over time. The best answer
today, for the average person needing accurate positioning, is GPS and Glonass
type systems.
On the more humorous side, in a few hundred years, continental drift will slide
all the monuments we set out from under our carefully observed coordinates. So,
let's not get all twisted over what North truly is and just make sure that we
have good ties to the geodetic control grid, previous/adjacent surveys and
annotate our plans of survey clearly and accurately so that those who follow us
can follow our work.
Robert Lindsay
Bertho Boman wrote:
>
> This whole issue of "true North" appears to be rather muddy. I
> understand that on a practical basis for surveying the detailed
> definitions does not make much difference but I am surprised that it
> is not a black and white well defined issue. To further cloud it, I
> have some more questions and comments.
>
> I understand that if we define the Earth spin-axis "today" as geodetic
> north and then use astronomical observations to measure the spin axis,
> there will be a difference based on local gravity.
>
> Here is where it gets muddy. Does not the Earth's spin axis slowly
> move? I also believe that it has a wobble on it. Now where is
> geodetic north?
>
> This issue sounds similar to the old problem of defining time. The
> Earth spin rate is slowly changing and the definition of the second
> eventually was defined as fraction of Earth rotational speed at
> January 1, 1900. Is there a similar exact definition of north based
> on the spin axis at a given time?
>
SamSurveyr
>let's not get all twisted over what North truly is and just make sure that we
>have good ties to the geodetic control grid, previous/adjacent surveys and
>annotate our plans of survey clearly and accurately so that those who follow
>us
Good point, Robert. All this accuracy doesn't mean squat except to the
adjoining landowners who may be scrutinizing your work!
Sam
c&j
> The problem is that there is no permanent feature to hang that alignment on,
> Stars do move over time in the celestial sphere seen from Earth, the axis of
> spin of the Earth wobbles and wanders with time, the Earths magnetic field moves
> over time, and the continents themselves wander over time. The best answer
> today, for the average person needing accurate positioning, is GPS and Glonass
> type systems.
>
> On the more humorous side, in a few hundred years, continental drift will slide
> all the monuments we set out from under our carefully observed coordinates. So,
> let's not get all twisted over what North truly is and just make sure that we
> have good ties to the geodetic control grid, previous/adjacent surveys and
> annotate our plans of survey clearly and accurately so that those who follow us
> can follow our work.
Yeah, someone said one time (I think Einstein) that there was no absolute frame of
reference...(remember the 'luminiferous ether' theory?)
KSnippins
> The problem is that there is no permanent feature to hang that alignment
on,[etc]
There is a big monument right near the home of a Mr. S. Claus, painted with
white & red candy cane stripes -- perhaps one of the most famous monuments
ever, it can be seen in nearly all of the holiday tv specials. I don't have
the full description with rm's etc. but it must be really close to the reindeer
corral. ;-) Seasons greetings group (sorry to waste bandwidth, but all
this talk about the north pole...)
~kurt
--------------------------------------
Kurt B. Wurm, P.L.S.
University of Maine
Department of Spatial Information Science and Engineering
Jack Savlan
difference between the elipsoid and the geoid. The mass anomalies
within the earth produce corresponding gravity anomalies which deflect
the plumbline away from the true vertical.
geodetic azimuth = astronomical azimuth - (deflection of the vertical in
the plane of the prime vertical)(tan(geodetic latitude))
When we perform astronomical observations, and solve the PZS triangle,
astronomical azimuth is our result.
With respect I thank Clair E. Ewing and Michael M. Mitchell for writing
a great geodesy text, and Oregon Tech for an excellent surveying
program.
Jack A. Savlan, PS
Edward M. Reading
>snip<
> With respect I thank Clair E. Ewing and Michael M. Mitchell for writing
> a great geodesy text, and Oregon Tech for an excellent surveying
> program.
>
> Jack A. Savlan, PS
GO Hustling OWLS !!!!
Edward M. Reading, PLS
Bertho Boman
where the North Pole is.
Merry Christmas, Happy Holidays and a Happy New Year!
Bertho Boman
Kent McMillan
That's an easy one, Bertho. Since the pole varies within a circle of
about 8m radius (on a time scale of decades, not geologic time) and
since Santa Claus Industries, A.G. must be well over a hectare in area,
centered on the mean pole, the instantanteous pole is always within the
perimeter fence of the facility.
ron bissell
When my nephew ask where the North Pole is, I do the architects thing
and wave my thumb between East and West and say "About there."
Merry Christmas and a very Happy New year to you all.
May your troubles be few and you Joys many.
Terry Maw
"Edward M. Reading" wrote:
I'll add a third to the owls
Terry R. Maw, PLS (72)
Terry Maw
And all along I thought that north was the direction that the
compass's's's(sp) people hung on their windshields in the pickup trucks
pointed to...
Terry
Terry Maw
And all along I thought that north was the direction that the
compass's's's(sp) people hung on their windshields in the pickup trucks
pointed to...so sad - another myth destroyed.
John B.
>And all along I thought that north was the direction that the
>compass's's's(sp) people hung on their windshields in the pickup trucks
>pointed to...
No, that's called the engine block.